V. P. Markov.

Systematic Procedure for the Qualitative Analysis of Anions.

The usual procedure of qualitative analysis is divided into two parts, very different in character.

The determination of cations constitutes an orderly system of successive separation, from the substance under investigation, first of groups of cations and then of individual cations. Having isolated a given cation, we are able to carry out with it all the particular reactions, which is of enormous importance for the student of this subject.

The determination of anions presents an entirely different picture. Here, as a rule, they are not separated from one another—not even groups of anions, to say nothing of individual anions. The groups of acids according to Bunsen’s classification remain, for the student, purely theoretical; in practice he never has them before him. Anions are determined from separate portions of the substance, each of which rarely serves for the determination of more than 2–3 acids. The simultaneous presence of anions interferes with the performance of the particular reactions; some of them prove impossible to carry out, others are obscured by the presence of related anions, and as a result, when the work is completed, there remains a certain uncertainty as to the accuracy of the determination of anions, in contrast to the determination of cations.

For instructional purposes it would be important to change this state of affairs by applying to the determination of anions the principles underlying the determination of cations, namely:

1. By means of the successive precipitation of one portion of the substance with group reagents, to divide the anions into groups.

2. Within each group, to separate the anions from one another and obtain them in a form convenient for carrying out particular reactions.

3. Both during the preparation of the substance for analysis and during the precipitation of groups of anions with group reagents, and, as far as possible, also during the isolation of individual anions, to avoid conditions under which alteration of the ions or their complete destruction is possible.

4. If it proves impossible to isolate a given anion in an unchanged form, then its conversion into another ion and its isolation in a new form (as, for example, \(\mathrm{ClO_3'}\) in the form of \(\mathrm{Cl'}\), or \(\mathrm{AsO_3'''}\) in the form of \(\mathrm{AsO_4'''}\)) should be carried out under conditions that exclude the possibility of error:

a) the anion into which the ion under investigation is converted must first have been isolated completely,

b) no other ions capable of giving the same final anion must be present.

With regard to the 4th point, it may be noted that even in the determination of cations one has to accept a change in their valence (\(\mathrm{Sn^{\bullet\bullet}}\) and \(\mathrm{Sn^{\bullet\bullet\bullet\bullet}}\), \(\mathrm{Fe^{\bullet\bullet}}\) and \(\mathrm{Fe^{\bullet\bullet\bullet}}\), etc.).

The advisability of carrying out a systematic procedure for the analysis of anions was always felt from the pedagogical standpoint, and from time to time more or less successful schemes of work were published, namely:

1. In 1900, R. Abegg and W. Gerz¹, in a brief article, gave a scheme for the systematic qualitative analysis of anions, based on the use of salts of calcium, barium, zinc, and silver as group reagents.

2. In 1909, Milobędzki² gave a scheme of analysis in which salts of magnesium, strontium, zinc, lead, and silver were used as group reagents.

3. In 1912, A. Noyes³ proposed a method for determining volatile acids by fractional distillation with phosphoric acid.

4. In 1923, Z. Karaoglanow and M. Dimitrow⁴ published a method based partly on the volatility of acids, and partly on the successive precipitation of anions in the form of salts of barium and calcium, lead and silver, in neutral, acidic, or alkaline medium.

Besides the works mentioned, which endeavor to provide a scheme for determining, as far as possible, all commonly encountered anions, there is a very considerable number of works concerning one or another group of anions.

It is somewhat strange that relatively very little attention is paid to works of a general character. First, there are few of them; and second, they usually do not provoke any particularly lively discussion and criticism that would reveal their merits and shortcomings. An exception is the work of Abegg and Gerz, which immediately after its appearance was subjected to devastating criticism by W. Fresenius⁵, who demonstrated the complete unsuitability of the proposed method. Meanwhile, the other methods also have their weak points, which have prevented them from entering practice.

The second method for determining anions in order of publication—Milobędzki’s—consists of the following:

1. Part of the substance under investigation serves for determining $\mathrm{ClO'}$ and others, by odor upon acidification and by other indications.

2. Part of the solution is precipitated with magnesia mixture. The precipitated ions are $\mathrm{AsO_3'''}$, $\mathrm{AsO_4'''}$, $\mathrm{PO_4'''}$, and $\mathrm{PO_3'''}$, which are then determined after dissolving the precipitate.

3. Part of the solution under investigation is precipitated with $\mathrm{Ag_2SO_4}$. In solution: $\mathrm{NO_3'}$, $\mathrm{NO_2'}$, $\mathrm{ClO_3'}$, and $\mathrm{CH_3COO'}$; the latter are detected by their particular reactions.

4. The remainder of the solution is precipitated with $\mathrm{Sr(NO_3)_2}$ on heating. After several hours it is filtered. In the precipitate: $\mathrm{PO_4'''}$, $\mathrm{AsO_4'''}$, $\mathrm{PO_3'''}$, $\mathrm{AsO_3'''}$, $\mathrm{SO_3''}$, $\mathrm{CO_3''}$, $\mathrm{C_2O_4''}$, $\mathrm{Fl'}$, and $\mathrm{SO_4''}$; in the filtrate—the rest. The precipitate is examined for $\mathrm{SO_3''}$, $\mathrm{CO_3''}$, $\mathrm{C_2O_4''}$, $\mathrm{Fl'}$, and $\mathrm{SO_4''}$ by separate tests.

5. The filtrate from $\mathrm{Sr(NO_3)_2}$ is divided into two or three parts:

a) $\mathrm{Br'}$ and $\mathrm{J'}$ are detected; if $\mathrm{J'}$ is present, it is precipitated with $\mathrm{Pb(NO_3)_2}$; in the precipitate, $\mathrm{S''}$ is detected by the black color of the precipitate; in the filtrate, $\mathrm{CNS'}$ is detected;

b) precipitate with a solution of $\mathrm{Zn(NO_3)_2}$. In the precipitate, after dissolving in $\mathrm{NaOH}$, the ions $\mathrm{CN'}$, $\mathrm{FeCy_6'''}$, and $\mathrm{FeCy_6''''}$ are detected.

1. The filtrate from $\mathrm{Zn(NO_3)_2}$ is divided into 3 parts:

a) acidify with $\mathrm{HNO_3}$—determine $\mathrm{S_2O_3''}$ (evolution of $\mathrm{SO_2}$ and $\mathrm{S}$),

b) detect $\mathrm{CNS'}$,

c) precipitate with a solution of $\mathrm{AgNO_3}$; the precipitate is treated with $\mathrm{NH_4OH}$; $\mathrm{AgCl}$ and $\mathrm{AgBr}$ pass into solution. From the ammoniacal solution, $\mathrm{Cl'}$ and $\mathrm{Br'}$ are determined.

The analysis is carried out not with one and the same sample of substance, but with four separate ones. Within each group, the determination of anions is performed for the most part by separate tests, without isolating them. The whole

The method thus occupies an intermediate position between the method of individual tests and the method of systematic analysis, the features of which I indicated at the beginning.

Further, there are still the following shortcomings:

1) Silicic acid and chromic acid are completely excluded from consideration.

2) No attention is paid to the fact that the precipitation of $\mathrm{AsO_3^{'''}}$ by a magnesia mixture is incomplete, and with a very high content of $\mathrm{NH_4^{\prime}}$ does not occur at all¹. Since $\mathrm{AsO_3^{'''}}$ is also precipitated poorly by means of $\mathrm{Sr(NO_3)_2}$ and $\mathrm{Zn(NO_3)_2}$, its admixture may interfere with the partial reactions for other anions.

3) On treatment of the precipitate of silver salts with ammonia, not only $\mathrm{AgBr}$ and $\mathrm{AgCl}$ dissolve, but also $\mathrm{AgCNS}$, likewise precipitated by the action of $\mathrm{HNO_3}$ as a white precipitate very similar to $\mathrm{AgCl}$.

Noyes’s method is applicable only to volatile acids and consists, in general outline, of the following:

2 grams of the substance under examination are heated with a mixture of 25 cu. cm of water and 10 cu. cm of phosphoric acid (85%). The first 10 cu. cm of distillate are collected in barium water; in them the more volatile acids are detected: carbonic, hydrosulfuric, sulfurous, hydrocyanic, nitric and, in part, hydrohalic acids. Then the receiver is replaced by a new one with 35 cu. cm of water and the distillation is continued until white fumes appear. In the distillate are detected: the remainder of the hydrohalic acids, hydrorhodanic acid, carbon dioxide (as a product of the decomposition of oxalic acid), hydrogen sulfide, and hydrocyanic acid (from the complex acids $\mathrm{FeCy_6^{'''}}$ and $\mathrm{FeCy_6^{''''}}$). Then 5–10 g of copper filings are added to the residue in the flask and it is again distilled. The appearance of sulfurous acid in the distillate is a sign of the presence of sulfuric acid.

In the original substance, phosphoric and boric acids are determined.

It is clear that this method is not applicable if ions are present that are readily decomposed ($\mathrm{S_2O_3^{''}}$), or that readily react with one another in an acid medium ($\mathrm{J^{\prime}}$ and $\mathrm{NO_2^{\prime}}$, $\mathrm{J^{\prime}}$ and $\mathrm{ClO_3^{\prime}}$, $\mathrm{CrO_4^{''}}$ and $\mathrm{S_2O_3^{\prime}}$, etc.). Nonvolatile acids also cannot be determined. It therefore has no general significance.

The method of Karaoglanov and Dmitriev has one point in common with Noyes’s method: the acids distilled over on heating with acetic acid form the 1st group. The residue from distillation is precipitated successively:

1) With a mixture of $\mathrm{Ba(CH_3COO)_2}$ and $\mathrm{Ca(CH_3COO)_2}$ from an acetic-acid solution:
$\mathrm{SO_4^{''}}$, $\mathrm{SiFl_6^{''}}$, $\mathrm{CrO_4^{''}}$, $\mathrm{Fl^{\prime}}$, $\mathrm{C_2O_4^{''}}$, $\mathrm{JO_3^{\prime}}$ and $\mathrm{C_4H_4O_6^{''}}$ . . . . . . . (III gr.).

2) With a mixture of $\mathrm{Ba(CH_3COO)_2}$ and $\mathrm{Ca(CH_3COO)_2}$ from a neutral solution:
$\mathrm{PO_4^{'''}}$, $\mathrm{AsO_4^{'''}}$ and $\mathrm{SiO_3^{''}}$ . . . . . . . . . . . . . . . . . (IV gr.).

3) With $\mathrm{Pb(CH_3COO)_2}$ in neutral solution: $\mathrm{J^{\prime}}$, $\mathrm{FeCy_6^{''''}}$, $\mathrm{AsO_3^{'''}}$, $\mathrm{C_4H_4O_6^{''}}$ (V gr.).

4) With $\mathrm{Pb(CH_3COO)_2}$ in the presence of $\mathrm{NH_4OH}$: $\mathrm{FeCy_6^{'''}}$ and $\mathrm{AsO_3^{'''}}$ (VI gr.).

5) With $\mathrm{AgNO_3}$ in the presence of $\mathrm{HNO_3}$: $\mathrm{Cl}$, $\mathrm{Br}$, $\mathrm{CNS^{\prime}}$ . . . . . (VII gr.).

In the residue are determined $\mathrm{ClO_3^{\prime}}$, $\mathrm{ClO_4^{\prime}}$ and $\mathrm{BrO_3^{\prime}}$ . . . . . (VIII gr.).

Nitric and boric acids form group II and are determined in the test of the residue after driving off the volatile acids.

In this method the procedures of qualitative analysis of cations are applied most consistently. The acids are successively precipitated by group reagents and can then be isolated (in the work, nevertheless, chiefly—

separate samples in this way). The drawback is the same as in Noyes’s method: the work is carried out in the presence of acid, even if weak, as a result of which errors may occur. Further, it is not easy to identify \(SO_3'\) and \(S_2O_3''\) when they are present together, etc. Somewhat strange is the statement that \(S_2O_3''\), upon the action of concentrated \(H_2SO_4\), gives a milky turbidity without evolution of \(SO_2\).

In undertaking the present work I set myself the goal of avoiding, as far as possible, the shortcomings of the methods described above. As group reagents I adopted the salts of magnesium, barium, zinc, lead, and silver, thereby dividing the anions into 6 groups, namely:

Group I. Acids precipitated by magnesium salts: \(SiO_3''\), \(Fl'\), \(PO_4'''\), \(AsO_4'''\), and partly \(AsO_3'''\).

Group II. Acids not precipitated by magnesium salts, and not precipitated by barium salts in the presence of ammonium salts from hot solution: \(CO_3''\), \(SO_4''\), \(SO_3''\), \(CrO_4''\).

Group III. Acids not precipitated by the preceding reagents, but precipitated by zinc salts:

1st subgroup: precipitated by \(ZnCO_3\): \(FeCy_6''''\) and \(S''\).

2nd subgroup: not precipitated by \(ZnCO_3\), but precipitated by \(Zn(NO_3)_2\): \(FeCy_6'''\) and \(CN'\).

Group IV. Acids not precipitated by the preceding reagents, but precipitated by lead salts: \(AsO_3'''\), \(S_2O_3''\), \(J'\), \(BO_3'''\).

Group V. Acids not precipitated by the preceding reagents, but precipitated by silver salts: \(Cl'\), \(Br'\), \((J')\), \(CNS'\).

Group VI. Those remaining, not precipitated by the preceding reagents: \(NO_3'\), \(NO_2'\), \(ClO_3'\), \((BO_3''')\).

As is evident from the list of anions, I have confined myself to the most commonly encountered anions, entirely excluding organic acids (with the exception of \(CN'\) and \(CNS'\)). The application of this method to the most important organic acids will be the subject of further work.

Precipitation is everywhere carried out in a neutral or weakly alkaline medium.

Conditions for the Coexistence of Certain Anions.

The question of the possibility of the coexistence of ions is usually decided in the sense that strong oxidizing agents (\(CrO_4''\), \(FeCy_6'''\), etc.) and reducing agents (\(S'\), \(S_2O_3''\), \(SO_3''\)) cannot be present simultaneously in solution.

When one is dealing with some natural product, this decision fully corresponds to reality. The immense intervals of time that have elapsed since the formation of the rock give all reactions full opportunity to proceed to completion, however slowly they may proceed.

With an artificial mixture the matter is different. Its time of existence is usually not long; some reactions do not have time to proceed to any noticeable extent, and we may observe the joint presence of such apparently incompatible ions as \(CrO_4''\) and \(S_2O_3''\). In analyzing a mixture of this kind one must work cautiously, so as not to accelerate reactions, which would prevent the accurate determination of the ions originally present.

To clarify this question, at least qualitatively, a series of experiments was carried out.

\(CrO_4''\).

Experiment No. 1.
(Solutions everywhere \(1/1\) N.)

Six test tubes were taken with mixtures: 1) 5 c.c. \(K_2CrO_4\) and 5 c.c. \(Na_2S\); 2) 5 c.c. soln. \(K_2CrO_4\) and 5 c.c. \(Na_2S_2O_3\); 3) 5 c.c. soln. \(K_2CrO_4\) and 5 c.c. soln. \(Na_2SO_3\); 4) 5 c.c. \(K_2CrO_4\) and 5 c.c. \(Na_3AsO_3\); 5) 5 c.c. \(K_2CrO_4\) and 5 c.c. \(K_4FeCy_6\); 6) 5 c.c. \(K_2CrO_4\) and 5 c.c. water.

At room temperature (15°—18° C.) a change in coloration over time was observed.

Test tube No. 6 served for comparison.

Nos. of test tubes	30 min.	1 hour	3 hours	18 hours	24 hours	Estimate of reaction rate
1.	Doubtful.	Slight change.	Yellow-green.	Dark green.	Dark green ppt.	Rather slow.
2.	No.	Doubtful.	Slight.	Light yellow-green.	Light yellow-green.	Very slow.
3.	Doubtful.	Slight.	Clearly green.	Yellow-green.	Yellow-green.	Slow.
4.	Clearly green.	Dark green.	Dark green.	Dark green.	Dark green.	Rapid.
5.	No.	No.	No.	No.	No.	No.

Experiment No. 2.

The same mixtures, but at the temperature of a water bath (90°—100°).

Nos. of test tubes	1 min.	5 min.	30 min.	2 hours	6 hours	Estimate
1.	No.	Dark-green turbidity.	Dark-green ppt.	—	—	Very rapid.
2.	»	No.	Turbidity.	Turbidity.	Slight ppt.	Very slow.
3.	»	»	Slight precipitate, pale green.	Slight precipitate, pale green.	Abundant yellow-green precipitate.	Slow.
4.	Green.	Dark green.	Dark green.	—	—	Very rapid.
5.	No.	No.	No.	Turbidity.	Slight ppt.	Very slow.

Experiment No. 3.

Two samples were taken: No. 1) 5 cubic cm \(K_2CrO_4 + 5\) c.c. \(Na_2SO_3 + 0.5\) g \((NH_4)_2SO_4\), and No. 2) 5 c.c. \(K_2CrO_4 + 5\) c.c. \(Na_2S_2O_3 + 0.5\) g \((NH_4)_2SO_4\).

Observation at 90°—100°.

Nos. of test tubes	15 minutes	30 minutes	Estimate
1	Green liquid and precipitate.	Green liquid and precipitate.	Rapid.
2	Turbid brown liquid.	Turbid liquid, brown precipitate.	Rapid.

The effect of adding ammonium salts was very strong.

To assess the effect of the joint presence of ammonium salts and sodium carbonate, the following experiment was set up.

Experiment No. 4

Test tubes were taken with the mixture:

No. 1) 5 c.c. \(K_2CrO_4\) + 5 c.c. \((NH_4)_2S\) + 15 c.c. \(Na_2CO_3\) + 5 c.c. \(NH_4Cl\)
2) ” + 5 c.c. \(Na_2S_2O_3\) + 15 c.c. \(Na_2CO_3\) + 5 c.c. \(NH_4Cl\)
3) ” + 5 c.c. \(Na_2SO_3\) + ” + ”
4) ” + 5 c.c. \(Na_3AsO_3\) + ” + ”

Heating on a water bath:

No. of test tube	3 min.	15 min.	1 hour	2 hours	4 hours	Evaluation
1.	Precipitate.	Tubular precip.	Layer of precipitate 1 cm. thick.	—	—	Very rapid.
2.	None.	None.	None.	None.	Doubtful.	None.
3.	”	”	Slight green precip.	Considerable green-blue precip.	Abundant green-blue precipitate.	Rather rapid.
4.	Greenish.	Green solution	Dark-green solution, green precipitate.	Considerable flocculent precipitate.	Drop of green precipitate.	Rapid.

On the basis of this experiment we may conclude that, in the presence of ammonium salts, the ion \(CrO_4''\) is incompatible with the ions \(S''\), \(AsO_3'''\), and \(SO_3''\), even with the simultaneous presence of a considerable excess of \(Na_2CO_3\). As for the ion \(S_2O_3''\), under these conditions it is compatible with \(CrO_4''\).

Experiment No. 5

10 c.c. of a solution of \(Na_3AsO_4\) + 10 c.c. \(Na_2SO_3\) and 80 c.c. of water were taken. The mixture was heated on a water bath. At known intervals of time a sample (5 c.c.) was taken, the magnesia mixture with a high content of \(NH_4Cl\) was added to precipitate \(AsO_4'''\), and the filtrate, after acidification, was tested for the presence of \(AsO_3'''\) by passing \(H_2S\).

30 min.	1 hour	3 hours	6 hours	Evaluation
None.	None.	Doubtful.	Slight precipitate.	Very slow.

Experiment No. 6

10 c.c. \(Na_3AsO_4\) + 10 c.c. \(Na_2S_2O_3\) and 80 c.c. of water were taken. The mixture was heated on a water bath. Samples of 5 c.c. were precipitated with magnesia mixture to remove unchanged \(AsO_4'''\); the filtrate was treated with excess bromine water to convert the formed \(AsO_3'''\) into \(AsO_4'''\), and the precipitation of \(MgNH_4AsO_4\) was observed.

30 min.	1 hour	3 hours	6 hours	Evaluation
Traces?	On standing, a clear precipitate.	After 5 min., a clear precipitate.	Immediately, a distinct precipitate.	Rather slow.

Experiment No. 7.

Mixtures taken:

1) 20 c.c. \(Na_3AsO_4\) + 20 c.c. \(Na_2CO_3\) + 5 c.c. \(Na_2SO_3\) + 10 c.c. \(NH_4Cl\) + 45 c.c. water.
2) 20 c.c. \(Na_3AsO_4\) + 20 c.c. \(Na_2CO_3\) + 5 c.c. \(Na_2S_2O_3\) + 10 c.c. \(NH_4Cl\) + 45 c.c. water.

Heated on a water bath. The samples taken were examined as in Experiments Nos. 5 and 6.

Nos.	30 min.	2 hours	5 hours	Assessment
1	No.	Turbidity.	Turbidity.	No.
2	No.	Doubtful	Traces of precipitate.	Very slow.

Thus, in the presence of an excess of \(Na_2CO_3\), the ion \(AsO_4'''\) proves to be compatible with \(S_2O_3'\) and \(SO_3''\).

\(FeCy_6'''\).

Experiment No. 8.

Two mixtures were taken: 1) 5 c.c. of a solution of \(K_3FeCy_6\) + 10 c.c. of a solution of \(Na_2CO_3\) and 5 c.c. of a solution of \(Na_2S\), and
2) as above, but with the addition of 0.5 g \(NH_4Cl\).

Both samples were heated on a water bath.

Nos.	5 min.	15 min.	Assessment of reaction rate.
1	Precipitate.	Very abundant precipitate.	Very rapid.
2	Considerable precipitate.	» » »	Very rapid.

Experiment No. 9.

Mixtures taken:

1) 5 c.c. of a solution of \(K_3FeCy_6\) + 5 c.c. \(Na_2SO_3\) + 10 c.c. water.
2) 5 c.c. of a solution of \(K_3FeCy_6\) + 5 c.c. \(Na_2SO_3\) + 10 c.c. \(Na_2CO_3\).
3) 5 c.c. of a solution of \(K_3FeCy_6\) + 5 c.c. \(Na_2SO_3\) + 10 c.c. \(Na_2CO_3\) + 0.5 g \(NH_4Cl\).

Heating on a water bath. Initial color—yellow with a greenish tint.

Sample Nos.	5 min.	15 min.	1 hour	Assessment
1	Liquid turbid, color yellow-green, odor.	Abundant green precipitate, odor.	Excess \(SO_3''\) precipitated with \(BaCl_2\).	Very rapid.
2	Almost colorless.	Almost colorless, no precipitate.	Filtrate test for \(FeCy_6'''\)—negative.	»
3	Pale yellow.	Almost colorless, weak green precipitate.	Filtrate test for \(FeCy_6'''\)—intense.	»

Experiment No. 10.

Mixtures taken:

1) 5 c.c. of a solution of $K_3FeCy_6$ + 5 c.c. of $Na_2S_2O_3$ + 10 c.c. of water.
2) 5 c.c. of a solution of $K_3FeCy_6$ + 5 c.c. of $Na_2S_2O_3$ + 10 c.c. of $Na_2CO_3$
3) 5 c.c. of a solution of $K_3FeCy_6$ + 5 c.c. of $Na_2S_2O_3$ + 10 c.c. of $Na_2CO_3$ + 0.5 g $NH_4Cl$.

Heating on a water bath:

No. of sample	5 min.	15 min.	1 min.	Evaluation.
1	Dark-green liquid and precipitate.	Dark-green precipitate; liquid colorless.	—	Very rapid.
2	None.	Greenish liquid; no precipitate.	Colorless liquid; weak yellow precipitate.	Very rapid.
3	None.	Considerably paler coloration.	Almost colorless.	Very rapid.

The experiments cited, of course, are not distinguished by particular precision and do not cover all possible combinations of ions, but they are sufficient to accept the following propositions:

1) In a neutral, and still more in an alkaline, medium, the simultaneous presence for a more or less prolonged time of such anions as enter into vigorous reaction in an acid medium is possible.

2) Ammonium salts, when heated, facilitate the interaction of ions owing to their hydrolysis.

3) The addition of a considerable amount of carbonates, by preventing the accumulation of $H^\cdot$ ions and promoting the rapid removal of ammonia on boiling, weakens or even entirely eliminates the influence of the presence of $NH_4$.

4) Some combinations of ions ($CrO_4^{\prime\prime}$ and $AsO_3^{\prime\prime\prime}$, $S^{\prime\prime}$ or $SO_3^{\prime\prime}$; $FeCy_6^{\prime\prime\prime}$ and $S^{\prime\prime}$, etc.) are unstable even in the presence of a considerable amount of $Na_2SO_3$.

Preliminary tests.

In the determination of anions, as in the determination of cations, we cannot avoid introducing some ion that is itself subject to determination. Of the cations we introduce $NH_4$ in precipitating with ammonium sulfide and ammonium carbonate; likewise we shall be compelled to introduce some anion in the form of salts of Mg, Ba, Zn, Pb, and Ag, serving as group reagents.

Since it is desirable to introduce only one anion, it should be chosen so that the salts of all the above-mentioned metals are soluble in water, and, on the other hand, so that it can be readily detected from a test of the original substance. It is obvious that the choice is possible only between the ions: $NO_3^\prime$ and $CH_3COO^\prime$.

Of these, the ion $CH_3COO^\prime$ is less suitable because, in the presence of considerable amounts of it, $CN^\prime$ is very incompletely precipitated in the form of $Zn(CN)_2$, owing to the formation of a complex salt 7), and the cyanide ion will be present in considerable quantity in the precipitate after the action of $AgNO_3$, where one will have to deal a second time with its separation and determination. The presence of $CH_3COO^\prime$ also affects the completeness of precipitation of some other anions, though more weakly.

7) Abegg. Handbuch der Anorg. Chemie B. II Abt. 2. S.337.

Therefore, the most suitable is the introduction of the ion \(NO_3'\), all the more so since its detection does not present any particular difficulty.

The test for \(NO_3'\) must necessarily be preceded by a test for \(NO_2'\) and, if the result is positive, by removal of \(NO_2'\).

The third ion which usually requires determination from a separate sample of the substance being analyzed is \(CO_3''\). In the general case the substance under investigation may contain cations of all groups. The cations of the higher groups must be removed, which is possible only by boiling with a solution of soda or by fusion with it; in doing so the ion \(CO_3''\) is introduced.

1. Determination of \(NO_3'\) and \(NO_2'\).

This is carried out by means of \(FeSO_4 + H_2SO_4\) and \(KJ + CH_3COOH\).

The sample must be free: a) from cations capable of giving precipitates with \(SO_4''\); b) from anions giving precipitates with \(Fe\cdots\); c) from anions capable, like \(NO_3'\) and \(NO_2'\), of giving a brown coloration under the action of \(FeSO_4 + H_2SO_4\).

For this purpose, to a sample of the aqueous extract of the original substance we add a solution of \(Na_2CO_3\) until precipitation ceases (ions of heavy metals are removed), or else, what is simpler and better, we take a sample from the soda extract.

After filtering off the precipitate, to the filtrate we add a small excess of \(Pb(CH_3COO)_2\), whereby \(FeCy_6'''\), \(SO_3''\), \(S_2O_3''\), \(J'\), \(AsO_3'''\), \(S''\), and others are removed; we filter off the precipitate and add an excess of \(ZnSO_4\) to the filtrate. In this way \(FeCy_6'''\) and \(Pb\cdot\cdot\) are removed (the latter is harmful because of the formation of \(PbSO_4\)). To a portion of the prepared sample we add a solution of \(KJ\) and a little \(CH_3COOH\).

The immediate liberation of \(J_2\) (brown coloration of the solution, blue coloration with starch) indicates the presence of \(NO_2'\). In this case we get rid of its presence by one of the following methods:

1) We boil for a prolonged time the neutralized sample with an excess of \((NH_4)_2SO_4\), or

2) To the sample we add a considerable quantity of urea (8 g per 10 c.c.), and add the mixture in small portions to dilute sulfuric acid. ^8)

After removal of \(NO_2'\), to the liquid we add a solution of \(FeSO_4\) and, along the wall of the test tube, concentrated sulfuric acid. The appearance of a distinct brown ring indicates the presence of \(NO_3'\). A very weakly expressed brown ring cannot serve as proof of the presence of \(NO_3'\), in view of the formation of traces of it during the decomposition of \(NO_2'\).

2. Determination of \(CO_3''\).

In the usual test by the action of an acid, the presence of other anions that are decomposed under the action of acid likewise with evolution of gas interferes: \(SO_3''\), \(S_2O_3''\), \(S''\), \(CN'\), \(NO_2'\). The test of the evolved gas for clouding a drop of \(Ba(OH)_2\) solution likewise does not attain the aim, since \(SO_3'\) gives the same result. The presence of \(CO_3''\) can be reliably proved in the following way:

To a sample of the original substance we add dilute \(H_2SO_4\).

a) A gas is evolved which has no odor and clouds \(Ba(OH)_2\); \(CO_3''\) is present.

b) A gas is evolved which has an odor and clouds a drop of \(Ba(OH)_2\).

In the latter case we heat a new portion of the substance with \(H_2SO_4\) in a test tube with a delivery tube. The evolved gases are absorbed by a solution of \(Ba(OH)_2\); if no precipitate appears, \(CO_3''\) is not present. If a precipitate appears, we wash it by decantation with water, add an excess of bromine water to convert \(SO_3'\) into \(SO_4''\), and again act with \(H_2SO_4\). If gas again evolves, clouding \(Ba(OH)_2\), then \(CO_3''\) is present.

^8) Vetter. Qualitative Analysis, p. 353 (ed. 1909).

Preparation of the Substance for Analysis

A substance can be subjected to analysis for anions only when it contains no cations other than the alkali metals and is completely soluble in water. Usually this is not the case, and we must, first, bring all the anions into solution, and second, precipitate all undesirable cations. The most radical means of achieving these aims is fusion with soda, in which the cations of all groups except the alkali-metal group remain, after leaching the melt with water, as a precipitate of carbonates, oxides, or free metals, while the anions pass into solution in the form of sodium salts.

A less reliable means is prolonged boiling of the substance with a concentrated solution of soda, since in this case some compounds—for example, silver halide salts, calcium fluoride, and others—remain unchanged, and the corresponding anions may not be detected.

But it would be a mistake, in order to save labor, to proceed at once to fusing the substance with soda. The high temperature greatly promotes reactions among the ions present in the substance under investigation, and after fusion we may find ions that were not there previously, and conversely. If, for example, the ion \( \mathrm{Cr}^{\prime\prime\prime} \) and \( \mathrm{NO}_3^{\prime} \) were present, then after fusion with soda we shall find the ions \( \mathrm{CrO}_4^{\prime\prime} \) and \( \mathrm{NO}_2^{\prime} \); the ion \( \mathrm{S}_2\mathrm{O}_3^{\prime\prime} \), on fusion, is destroyed, forming, among other things, the ion \( \mathrm{S}^{\prime\prime} \); \( \mathrm{ClO}_3^{\prime} \) passes first into \( \mathrm{ClO}_4^{\prime} \) and \( \mathrm{Cl}^{\prime} \), and then into \( \mathrm{Cl}^{\prime} \). Many similar examples can be found.

It is therefore necessary to carry out both operations in succession. First, by boiling with a concentrated solution of \( \mathrm{Na}_2\mathrm{CO}_3 \) for about \(1\)—\(1\frac{1}{2}\) hours, we bring almost all the anions into solution, filter off the precipitate of the unchanged part of the substance and of the carbonates of heavy metals, and examine the filtrate for anions. We fuse the precipitate with \( \mathrm{Na}_2\mathrm{CO}_3 \), leach it with hot water, filter, and wash with soda solution. The filtrate is again examined for anions. If here we find the newly formed anion \( \mathrm{CrO}_4^{\prime\prime} \), we shall have to regard it as not having existed in the original substance, since otherwise it would have been detected in the soda extract.

The complication of the work due to the separate examination of the soda extract and the melt is not great, because in the melt we have to deal with only a few anions, and their determination proceeds quickly.

In preparing the soda extract, complete removal of ammonium salts occurs at the same time, owing to the volatility of ammonium carbonate. This is of great importance, since the presence of ammonium salts is harmful in the precipitation of the 1st group of anions.

Experiment No. 11

A solution of 5 g \( \mathrm{NH}_4\mathrm{Cl} \) and 10 g \( \mathrm{Na}_2\mathrm{CO}_3 \) in 200 cubic cm of water was taken and boiled over an open flame.

Sample No.	Reaction to \( \mathrm{NH}_4 \): 30 min.	Reaction to \( \mathrm{NH}_4 \): 45 min.	Reaction to \( \mathrm{NH}_4 \): 1 hour	Reaction to \( \mathrm{NH}_4 \): 1 h 15 min.
1	Strong.	Noticeably weakened.	None.	—
2	”	Noticeably weakened.	Weak.	None.

If, in preparing the soda extract, too little \(Na_2CO_3\) has been taken, it may happen that there will not be enough of it to remove the cations of the heavy metals and ammonium. In view of this, after boiling for some time one should take a sample of the solution and determine whether it contains an excess of \(Na_2CO_3\), by its effervescence with a weak acid and by its strongly alkaline reaction.

Determination of anions in the soda extract and in the fusion.

Limits of accuracy of precipitation by group and special reagents.

Even in quantitative determinations, not to mention qualitative ones, we often have to make use of precipitation reactions in which the precipitate cannot be regarded as completely insoluble in water. The following table gives figures for the solubility of certain compounds often encountered in qualitative and quantitative analysis:

Substance	T.	Solubility, grams per liter	Solubility, gram-equivalents per liter	Author
\(KHC_4H_4O_6\)	20	5,7	\(\dfrac{1}{13,7}\)	Alluard. Lieb. Annal 133.292
\(Na_2H_2Sb_2O_7\)	”	2,9	\(\dfrac{1}{69}\)	Moissan. Traité de Chimie minérale 111.383.
\(K_2PtCl_6\)	”	9,0	\(\dfrac{1}{27}\)	Bunsen and Kirchhoff. Pogg. Ann. 113.373.
\(MgNH_4PO_4 \cdot 6H_2O\)	”	0,033	\(\dfrac{1}{3700}\)	Menshutkin. Analytical Chemistry, 1913.
\((NH_4)_2CaFeCy_6\)	”	2,7	\(\dfrac{1}{57}\)	J. Brown. Journ. Amer. Chem. Society, 1907.
\(CaCO_3\)	”	0,013	\(\dfrac{1}{4000}\)	Holleman. Zeitsch. für phys. Chemie 12.125.
\(CaC_2O_4\).	18°	0,0056	\(\dfrac{1}{11400}\)	Landolt-Bernstein Phys.-chem. Tabellen.
\(SrSO_4\).	18°	0,114	\(\dfrac{1}{800}\)	”
\(PbSO_4\)	20°	0,042	\(\dfrac{1}{3600}\)	Böttger. Zeitsch. f. phys. Chemie. 46.604.
	20°	0,14	\(\dfrac{1}{1000}\)	Menshutkin. Analytical Chemistry, 1913.

These figures show that the limiting concentration of the precipitated ion in quantitative determinations varies within wide limits, in some cases reaching

\[ \frac{1}{1000} \;—\; \frac{1}{800}\, E. \]

Qualitative determinations in gen—

still do not require the same accuracy as quantitative ones, and therefore I considered the permissible concentration limit in group separations to be equal to \(1 - \frac{1}{500}\) E,

and, in the isolation of anions, \(\frac{1}{250}\) E.

In those cases where there were no literature data concerning solubility, or where the figures differed greatly, the limiting concentration was determined in two ways:

1. A mixture of the precipitating reagent and the salt of the anion under examination was taken, and the concentration of the anion under examination in the mixture was observed at which a still perceptible turbidity was obtained.

2. In a sample of the solution after precipitation of the anion under examination, its concentration was determined by the gravimetric or volumetric method.

The first method, of course, cannot give exact values of the limiting concentration of a given ion, but for the approximate estimate required in the present case it is satisfactory.

I. GROUP.

Acids precipitated in the form of magnesium salts: \(\mathrm{Fl}'\), \(\mathrm{SiO_3}''\), \(\mathrm{PO_4}'''\), \(\mathrm{AsO_4}'''(\mathrm{AsO_3}''')\).

1. Hydrofluoric acid. \(\mathrm{HFl}\).

1) \(\mathrm{Mg(NO_3)_2}\), in the absence of ammonium salts or in the presence of a small amount of them, precipitates amorphous, semitransparent \(\mathrm{MgFl_2}\), difficult to filter.

Sensitivity of the reaction:

Experiment No. 12.

5 c.c. of \(\mathrm{Mg(NO_3)_2}\) solution, \(\mathrm{NH_4Fl}\) (variable amount), were taken, and the whole was made up with water to 10 c.c.

Concentration of \(\mathrm{Fl}'\) (gram-equiv./L)

\(0.01\) E.	\(0.003\) E.	\(0.001\) E.	\(0.0002\) E.	\(0.0000\) (control).
Immediately a strong turbidity; on standing, flakes.	Turbidity on heating; flakes.	On heating, turbidity.	None.	None.

Limiting concentration \(\mathrm{Fl}' = < \frac{1}{1000}\) E. The precipitate obtained:

1. Soluble in mineral acids in the cold.
2. Almost insoluble in \(\mathrm{CH_3COOH}\), either in the cold or on heating.

The reaction may serve for separation from other anions of Group 1.

Experiment No. 13.

A solution containing 0.1817 g of \(\mathrm{Fl}'\) was taken and precipitated with a slight excess of \(\mathrm{Mg(NO_3)_2}\) solution. The precipitate was washed with water, treated with dilute \(\mathrm{CH_3COOH}\), filtered again (to facilitate filtration, filter fibers were added), washed with hot water, and gently ignited.

Weight of \(\mathrm{MgFl_2} = 0.2913\) g \(= 0.1785\) g \(\mathrm{Fl}'\).

1. The precipitate dissolves rather readily in concentrated solutions of ammonium salts. Owing to this, the precipitation of $\mathrm{Fl}'$ in the form of $\mathrm{MgFl}_2$ becomes incomplete.

Experiment No. 14.

5 cc of a magnesium mixture of the following composition were taken: $\mathrm{Mg(NO_3)_2}$ $1/1$ E, $\mathrm{NH_4OH}$ $1/1$ E and $\mathrm{NH_4}$ $1/1$ E. $\mathrm{NH_4Fl}$ was added and the volume was made up to 10 cc with water.

Concentration of $\mathrm{Fl}'$.

0.01 E.	0.003 E.
Turbidity at once; on heating, a precipitate.	No turbidity; on heating, no precipitate.

Note. A small amount of ammonium salts, only slightly increasing the solubility of $\mathrm{MgFl}_2$, is useful in that the precipitate obtained is more easily filtered.

4) $\mathrm{Ca(NO_3)_2}$ gives a gelatinous precipitate of $\mathrm{CaFl}_2$. The properties are the same as those of $\mathrm{MgFl}_2$.

5) Under the action of concentrated $\mathrm{H_2SO_4}$ and slight heating, $\mathrm{HFl}$ is evolved, which etches glass.

The experiment is carried out in a platinum or lead dish covered with waxed glass bearing a design scratched through to the glass. After removal of the wax, a matte design remains.

2. Silicic acid. $\mathrm{H_2SiO_3}$

1. $\mathrm{Mg(NO_3)_2}$, in the absence of ammonium salts, precipitates almost quantitatively.

Experiment No. 15.

10 cc of a solution containing 0.0677 g of $\mathrm{SiO_2}$ were precipitated with slight heating by 30 cc of $\mathrm{Mg(NO_3)_2}$ solution (E); the precipitate was filtered off and washed. The filtrate from the precipitate and the first 10 cc of wash waters were collected separately—“filtrate I”; the next 50 cc of wash waters constituted filtrate II.

The $\mathrm{SiO_2}$ content in the filtrates was determined by the usual method: repeated evaporation in concentrated $\mathrm{HCl}$.

	I	II	III wash with $\mathrm{CH_3COONH_4}$	Concentration of $\mathrm{SiO_3}^{\prime\prime}$ in filtrate I
Test 1	0.0015 g	0.0018 g	0.0025 g	$\dfrac{1}{1070}$ E
Test 2	0.0017 g	0.0020 g		$\dfrac{1}{941}$ E

1. $\mathrm{Mg(NO_3)_2}$ in the presence of ammonium salts precipitates considerably less completely.

Experiment No. 16.

10 c.c. of a solution containing 0.1961 g of SiO₂ were precipitated with 35 c.c. of a magnesia mixture of the composition: Mg(NO₃)₂—$\frac{1}{3}$ E; NH₄NO₃—$\frac{1}{1}$ E and NH₄OH—$\frac{1}{2}$ E; the precipitate was washed with 60 c.c. of water with a small addition of the magnesia mixture.

SiO₂ content

	Amount of SiO₂ in the precipitate	In solution	Total	Concentration of SiO₃″ in the filtrate
Sample 1	0.1887 g	0.0067 g	0.1954 g	$\frac{1}{475}$ E
Sample 2	0.1892 g	0.0057 g	0.1949 g	$\frac{1}{560}$ E

Experiment No. 17.

10 c.c. of a solution containing 0.1961 g of SiO₂ were precipitated with 30 c.c. of a magnesia mixture of the composition: Mg(NO₃)₂—$\frac{1}{2}$ E; NH₄NO₃—$\frac{1}{2}$ E and NH₄OH—$\frac{1}{4}$ E. The precipitate was washed with 60 c.c. of water.

In the filtrate (100 c.c.) was found—0.0136 g SiO₂; concentration of SiO₃″ in the solution = $\frac{1}{890}$ E.

The precipitate MgSiO₃ is readily soluble in weak CH₃COOH; after ignition it is insoluble.

3). Ammonium salts in a neutral or weakly alkaline medium separate gelatinous silicic acid.
Precipitation is not complete.

Experiment No. 18.

10 c.c. of a solution of Na₂SiO₃ $\left(\sim \frac{1}{1}E\right)$ were precipitated with 25 c.c. of (NH₄)₂CO₃ (E).

After standing for 2 hours at 40°, the mixture was left overnight at 15°. The separated H₂SiO₃ was filtered off. 10 cubic centimeters of the filtrate were evaporated twice with concentrated HCl. Weight of ignited SiO₂ = 0.0022 g.

Thus the conc. of SiO₃″ = $\frac{1}{136}$.

4) An ammoniacal solution of zinc oxide, when boiled until NH₃ is removed, quantitatively precipitates SiO₃″ in the form of the zinc salt.

Experiment No. 19.

5 c.c. of a $\frac{1}{1}$ E solution of Na₂SiO₃ and 20 c.c. of a solution of Zn(OH)₂ were heated until NH₃ was completely removed. The solution was filtered; 25 c.c. of it were evaporated (twice) with concentrated HCl.

Weight of $\mathrm{SiO_2}=0.0007$ g; concentration of $\mathrm{SiO_3^{\prime\prime}}$ in solution =

\[ =\frac{2\times 0.0007\times 1000}{25\times 60}\sim \frac{1}{1000}\ \mathrm{E}. \]

The successive application of reactions 3 and 4 serves for the quantitative determination of $\mathrm{SiO_3^{\prime\prime}}$ in the presence of $\mathrm{F^\prime}$.

5) The usual quantitative method for separating $\mathrm{SiO_3^{\prime\prime}}$—by repeated evaporation with concentrated $\mathrm{HCl}$ in the form of $\mathrm{SiO_2}$, insoluble in water and acids.

3. Phosphoric acid. $\mathrm{H_3PO_4}$.

1) $\mathrm{Mg(NO_3)_2}$, in the absence of ammonia and ammonium salts, gives an amorphous precipitate of $\mathrm{Mg_3(PO_4)_2}$ or $\mathrm{MgHPO_4}$. Precipitation is very incomplete owing to the fairly considerable solubility of the indicated salts (1 part in 600 parts of water).

2) $\mathrm{Mg(NO_3)_2}$, in the presence of ammonium salts and a considerable quantity of $\mathrm{NH_4OH}$, precipitates $\mathrm{PO_4^{\prime\prime\prime}}$ quantitatively, in the form of the crystalline precipitate $\mathrm{MgNH_4PO_4}$.

The precipitates $\mathrm{Mg_3(PO_4)_2}$, $\mathrm{MgHPO_4}$, and $\mathrm{MgNH_4PO_4}$ are readily soluble in weak acetic acid even after ignition.

3) An ammoniacal solution of zinc oxide, on heating, gives a voluminous precipitate. On cooling the precipitate dissolves again. After boiling until the $\mathrm{NH_3}$ has been completely removed, almost all the phosphoric acid remains in the precipitate together with excess $\mathrm{Zn(OH)_2}$.

Experiment No. 20.

5 cc of a solution of $\mathrm{Na_2HPO_4}$ $\left(\frac{1}{1}\ \mathrm{E}\right)$ and 15 cc of an ammoniacal solution of zinc oxide $\left(\frac{1}{2}\ \mathrm{E}\right)$ were boiled until the $\mathrm{NH_3}$ had been completely removed. The precipitate was filtered off. 10 cubic centimeters of the filtrate, with molybdenum mixture, gave on heating a yellowish opalescence and after 2 hours an insignificant precipitate. Dissolved in $\mathrm{NH_4OH}$ and precipitated with $\mathrm{Mg(NO_3)_2}$, it gave only traces of a precipitate of $\mathrm{MgNH_4PO_4}$.

4) Molybdenum mixture quantitatively precipitates, in the form of a bright-yellow crystalline precipitate, $(\mathrm{NH_4})_3\mathrm{PO_4}\cdot 12\mathrm{MoO_3}$, insoluble in $\mathrm{HNO_3}$ but soluble in alkalis.

5) $\mathrm{AgNO_3}$ gives a yellow precipitate of $\mathrm{Ag_3PO_4}$, soluble in acids (more difficultly in acetic acid) and in ammonia.

4. Arsenic acid $\mathrm{H_3AsO_4}$.

1) $\mathrm{Mg(NO_3)_2}$ reacts just as with $\mathrm{PO_4^{\prime\prime\prime}}$.

2) $\mathrm{H_2S}$ in a strongly acid medium precipitates $\mathrm{As_2S_5}$; in a weakly acid medium, $\mathrm{As_2S_3}$. In the latter case, reduction of $\mathrm{As^{\cdots\cdots}}$ to $\mathrm{As^{\cdots}}$ initially takes place.

3) $\mathrm{SnCl_2}$ in an acid medium liberates free As; in this, $\mathrm{H_3AsO_3}$ occurs as an intermediate product.

4) $\mathrm{AgNO_3}$ gives a chocolate-brown precipitate of $\mathrm{Ag_3AsO_4}$, soluble in acids and ammonia.

5) Molybdenum mixture reacts as with $\mathrm{PO_4}$, but the resulting precipitate separates more slowly; on boiling, rapidly. Its color is yellow.

6) An ammoniacal solution of zinc oxide, on boiling until the $\mathrm{NH_3}$ has been removed, precipitates $\mathrm{AsO_3^{\prime\prime\prime}}$, but less completely than $\mathrm{PO_4^{\prime\prime\prime}}$.

5. Arsenious acid.

1) \(Mg(NO_3)_2\), in the absence of \(NH_4OH\), gives a precipitate only in rather concentrated solutions.

Experiment No. 21.

10 cc of a \(\left(\dfrac{1}{1}E\right)\) solution of \(K_3AsO_3 + 5\) cc of \(Na_2CO_3\) \(\left(\dfrac{1}{1}E\right)\) were precipitated with 30 cc of a \(\left(\dfrac{1}{1}E\right)\) solution of \(Mg(NO_3)_2\). A sample of the filtrate, 10 cc, was neutralized with \(H_2SO_4\) and titrated with a solution of \(J\) \(\left(\dfrac{1}{25}E\right)\) in the presence of \(NaHCO_3\); 9.65 cc of \(J\) were consumed.

\[ \text{Conc. } AsO_3'''=\frac{1.5 \times 9.65}{10 \times 25}=\frac{1}{17.3}\,E \]

2) \(Mg(NO_3)_2\), in the presence of \(NH_4OH\) and in the absence of ammonium salts, precipitates \(AsO_3'''\) to a considerable extent.

Experiment No. 22.

5 cc of a \(\left(\dfrac{1}{1}E\right)\) solution of \(K_3AsO_3 + 2\) cc \(NH_4OH\) \((d=0.910)\) were precipitated with 10 cubic centimeters of \(Mg(NO_3)_2 + 13\) cc of water. The liquid was heated to \(100^\circ\), cooled, and filtered. A sample (10 cc) was boiled with an excess of \(Na_2CO_3\) until complete removal of \(NH_4\), filtered, neutralized with \(H_2SO_4\), and titrated with a solution of \(J\) \(\left(\dfrac{1}{213.3}E\right)\) in the presence of \(NaHCO_3\).

7.2 cubic centimeters of \(J\) were required.

\[ \text{Concentration } AsO_3'''=\frac{1.5 \times 7.2}{213.4 \times 10}=\frac{1}{197.6} \]

3) \(Mg(NO_3)_2\), in the presence of a large amount of ammonium salts and ammonia, precipitates only from concentrated solutions.

Experiment No. 23.

5 cc of a solution of \(K_3AsO_3 + 10\) cc of a solution (5 \(E\)) of \(NH_4Cl + 2\) cc of \(NH_4OH\) \((d=0.910)\) were precipitated with 10 cc of a solution of \(Mg(NO_3)_2\) \(\left(\dfrac{1}{1}E\right)\), 3 cc of water. The mixture was heated to \(100^\circ\), cooled, and filtered. A sample of the filtrate (10 cc) was boiled with an excess of \(Na_2CO_3\) until removal of \(NH_4\), neutralized with \(H_2SO_4\), and titrated with a solution of \(J\) \(\left(\dfrac{1}{10.67}E\right)\) in the presence of \(NaHCO_3\).

10.9 cc of \(J\) were required.

\[ \text{Concentration } AsO_3'''=\frac{1.5 \times 10.9}{10.67 \times 10}=\frac{1}{6.52}\,E. \]

9) Luce and Swinne, Zh. R. F. Kh. O., 1909, 1488, and also J. Brunnich and F. Smith, Zeitsch. f. angew. Chemie 1910, 292.

4) \( \mathrm{Ba(NO_3)_2} \), in the presence of a large quantity of ammonium salts and ammonia, precipitates only from very concentrated solutions.

Experiment No. 24.

\[ 10\ \text{cc of } \mathrm{Na_3AsO_3}\left(\frac{1}{1}E\right) + 5\ \text{cc of } \mathrm{Na_2CO_3}\left(\frac{1}{1}E\right) + 5\ \text{cc of } \mathrm{NH_4Cl}\left(\frac{1}{1}E\right) + 5\ \text{cc of } \mathrm{NH_4OH}\ (d\,0.94) \]

were precipitated with 30 cc of \( \mathrm{Ba(NO_3)_2} \). The filtrate, in the presence of an excess of \( \mathrm{NaHCO_3} \), was titrated with a solution of \( \mathrm{J}\left(\frac{1}{25}E\right) \). The work was carried out as with \( \mathrm{Mg(NO_3)_2} \).

\[ 10\ \text{cc filtr.} = 21.95\ \text{cc } \mathrm{J}. \]

\[ \text{Concentration of } \mathrm{AsO_3'''} = \frac{1.5 \times 21.95}{10 \times 25} = \frac{1}{7.6}E. \]

5) \( \mathrm{Zn(NO_3)_2} \) precipitates only from rather concentrated solutions.

Experiment No. 25.

\[ 10\ \text{cc of a solution of } \mathrm{K_3AsO_3}\left(\frac{1}{1}E\right) + 5\ \text{cc of a solution of } \mathrm{Na_2CO_3}\ \frac{1}{1}E. \]

were precipitated with 30 cc of a solution of \( \mathrm{Zn(NO_3)_2} \). 10 cc of the filtrate were tested for \( \mathrm{AsO_3'''} \), as above.

\[ \text{There were required } 9.65\ \text{cc } \mathrm{J}\left(\frac{1}{25}E\right). \]

\[ \text{Concentration of } \mathrm{AsO_3'''} = \frac{1.5 \times 9.65}{10 \times 25} = \frac{1}{17.3}E. \]

6) \( \mathrm{Pb(NO_3)_2} \), in the absence of ammonia, precipitates very incompletely; in the presence of an excess of \( \mathrm{NH_4OH} \), the precipitation is sufficiently complete.

Experiment No. 26.

To 10 cc of a solution of \( \mathrm{K_3AsO_3}\left(\frac{1}{1}E\right) \) there were added 20 cc of a solution of

\[ \mathrm{Pb(NO_3)_2}\left(\frac{1}{1}E\right) \]

and 10 cc of strong \( \mathrm{NH_4OH} \). The precipitate was filtered off.

A sample of the filtrate (20 cc) was heated until the \( \mathrm{NH_3} \) had been completely expelled and titrated with a solution of \( \mathrm{J}\left(\frac{1}{125}E\right) \) in the presence of \( \mathrm{NaHCO_3} \); there were required 2.1 cc of \( \mathrm{J} \), which corresponds to the concentration

\[ \mathrm{AsO_3'''} = \frac{2.1 \times 1.5}{125 \times 20} = \frac{1}{800}E. \]

7) An ammoniacal solution of zinc oxide, after boiling until the ammonia has been completely removed, leaves a precipitate, sparingly soluble in water, of \( \mathrm{Zn_3(AsO_3)_2} \) together with excess \( \mathrm{Zn(OH)_2} \).

Experiment No. 27.

5 cc of \(Na_3AsO_3\), \(\frac{1}{1}E\) + 10 cc of a solution of \(Zn(OH)_2\), boiled until the reaction for \(NH_3\) disappears; 5 cc of the filtrate required 1.50 cc of \(J\left(\frac{1}{125}E\right)\).

\[ \text{Concentration of } AsO_3''' = \frac{1.5 \times 1.5}{125 \times 5} = \frac{1}{278}E. \]

8) Hydrogen sulfide in an acid medium immediately gives a precipitate of \(As_2S_3\).
9) \(AgNO_3\) gives a yellow precipitate of \(Ag_3AsO_3\), soluble in acids and ammonia.
10) \(SnCl_2\) reduces \(AsO_3'''\) to metallic As.

Conditions for the Precipitation of Anions of the 1st Group.

Considering the particular reactions of the anions included in this group, we see that the conditions for the complete separation of the anions of this group from the subsequent groups are different.

For the precipitation of \(Fl'\) and, in part, \(SiO_3''\), the absence of ammonium salts and ammonia is required; for \(PO_4'''\) and \(AsO_4'''\), on the contrary, the presence of a considerable quantity of them is necessary. This circumstance compels the precipitation to be carried out in 2 operations.

The prepared problem (soda extract or aqueous solution of sodium fusion) is first precipitated, without acidifying it, with an excess of \(Mg(NO_3)_2\).

The precipitate, \(MgFl_2\), \(MgSiO_3\), and part of \(Mg_3(PO_4)_2\), \(Mg_3(AsO_4)_2\), \(Mg_3(AsO_3)_2\), and \(MgCO_3\), is filtered off and washed with water containing a small amount of \(Mg(NO_3)_2\).

To the filtrate we add 1–2 grams of solid ammonium nitrate (so as not to dilute the solution) and precipitate the remainder of \(PO_4'''\) and \(AsO_4'''\) with ammonia in the form of \(MgNH_4PO_4\) and \(MgNH_4AsO_4\).

The precipitates are examined for anions of the 1st group. The remaining anions remain in the filtrate.

Separation of the Anions of the 1st Group from One Another.

Both precipitates of magnesium salts obtained in the separation of the anions of the 1st group may be subjected to various treatments in order to isolate the individual anions.

1) If the question is not the detection of very small quantities of \(Fl'\) and \(SiO_3\), the course of the separation may be as follows:

The precipitates are treated with weak \(CH_3COOH\), filtered, and the remaining insoluble \(MgFl_2\) is washed with hot water. To facilitate washing of the gelatinous precipitate \(MgFl_2\), it is useful before filtration to add either paper pulp (the filter used for separating the precipitate of the 1st group of anions), or else an acid-insoluble indifferent precipitate, for example, a well-washed precipitate of \(BaSO_4\).

With the precipitate \(MgFl_2\) we carry out the glass-etching reaction.

The filtrate from \(MgFl_2\) is twice evaporated on a water bath with concentrated hydrochloric acid to a powdery state. All silicic acid is separated in the form of \(SiO_2\). It is moistened with \(HCl\), diluted with hot water, and filtered.

On the filter—\(SiO_2\).
In the solution—\(AsO_4'''\), \(PO_4'''\), and \(AsO_3'''\).

To the solution (not too concentrated) I add a considerable amount of $\mathrm{NH_4Cl}$, then, with heating, slowly $\mathrm{NH_4OH}$ until a precipitate forms, and then another $1/5$–$1/3$ of the volume of the whole liquid of strong ammonia.

Solution: $\mathrm{AsO_3^{\prime\prime\prime}}$—we carry out individual reactions.

Precipitate: $\mathrm{MgNH_4AsO_4}$ and $\mathrm{MgNH_4PO_4}$.

I dissolve in weak $\mathrm{HCl}$, add $\mathrm{Na_2SO_3}$ until there is a strong odor of $\mathrm{SO_2}$, and pass hydrogen sulfide through:

Precipitate: $\mathrm{As_2S_3}$ from $\mathrm{AsO_4^{\prime\prime\prime}}$.

Filtrate: $\mathrm{PO_4^{\prime\prime\prime}}$—we carry out individual reactions.

2) The following course of analysis may be used, which is usually practiced in the quantitative determination of $\mathrm{F^\prime}$ and $\mathrm{SiO_3^{\prime\prime}}$ when both are present.

We convert the mixture of magnesium salts again into sodium salts by fusion with $\mathrm{Na_2CO_3}$ or by boiling with a solution of soda. We filter. Carefully neutralize the filtrate, add $(\mathrm{NH_4})_2\mathrm{CO_3}$, heat for some time to $40^\circ$, and let stand overnight.

Filter off the separated $\mathrm{H_2SiO_3}$.

To the filtrate (in a porcelain dish) add phenolphthalein and, cautiously, dilute $\mathrm{H_2SO_4}$ until decolorization; boil. The pink color that reappears is again made to disappear by adding $\mathrm{H_2SO_4}$, and the liquid is heated again. This must be repeated until the disappearance of the pink color is no longer caused by the addition of only 1–2 cu. cm of weak $\mathrm{H_2SO_4}$.

To the averaged filtrate add an excess of ammoniacal zinc oxide solution and boil until the ammonia is completely removed. In the precipitate there is a residue of $\mathrm{H_2SiO_3}$, then almost all $\mathrm{PO_4^{\prime\prime\prime}}$, $\mathrm{AsO_4^{\prime\prime\prime}}$, and $\mathrm{AsO_3^{\prime\prime\prime}}$ in the form of Zn salts; the residue of these is removed with $\mathrm{AgNO_3}$; in the filtrate, $\mathrm{F^\prime}$, on which we carry out individual reactions.

We wash the precipitate, evaporate twice to dryness with strong $\mathrm{HCl}$, add $\mathrm{HCl}$, dilute with water, and filter off the precipitate of $\mathrm{H_2SiO_3}$. The solution containing $\mathrm{PO_4^{\prime\prime\prime}}$, $\mathrm{AsO_4^{\prime\prime\prime}}$, and $\mathrm{AsO_3^{\prime\prime\prime}}$ is examined as above.

II group of anions.

Anions precipitated from a hot solution by barium salts in the presence of ammonium salts and ammonia: $\mathrm{SO_4^{\prime\prime}}$, $\mathrm{SO_3^{\prime\prime}}$, $\mathrm{CrO_4^{\prime\prime}}$.

1. Sulfuric acid. $\mathrm{H_2SO_3}$.

1) $\mathrm{Ba(NO_3)_2}$ precipitates in the form of $\mathrm{BaSO_4}$, insoluble in weak acids.

2) In a reducing flame $\mathrm{BaSO_4}$ is converted into $\mathrm{BaS}$, which gives off hydrogen sulfide with acids.

3) $\mathrm{AgNO_3}$ gives no precipitate in dilute solutions.

2. Sulfurous acid. $\mathrm{H_2SO_3}$.

1) $\mathrm{Ba(NO_3)_2}$ precipitates in the form of $\mathrm{BaSO_3}$, soluble in dilute acids with evolution of gaseous $\mathrm{SO_2}$.

2) By the action of oxidizing agents, $\mathrm{SO_3^{\prime\prime}}$ is converted into $\mathrm{SO_4^{\prime\prime}}$.

3) $\mathrm{AgNO_3}$ gives a precipitate of $\mathrm{Ag_2SO_3}$, soluble in ammonia and in acids (except acetic acid). On boiling with water, $\mathrm{Ag_2SO_3}$ slowly passes into $\mathrm{Ag_2SO_4}$ and metallic $\mathrm{Ag}$ with evolution of $\mathrm{SO_2}$.

3. Chromic acid. \(H_2CrO_4\).

1) \(Ba(NO_3)_2\) precipitates quantitatively a yellow precipitate of \(BaCrO_4\), insoluble in acetic acid, but soluble in \(HCl\) and \(HNO_3\).

2) \(AgNO_3\) gives a red precipitate of \(Ag_2CrO_4\), readily soluble in hot concentrated ammonia and only very slightly in dilute cold ammonia. Soluble on heating in weak \(HNO_3\).

3) By the action of reducing agents it is readily converted into compounds of the chromium oxide series.

4) In an acid medium it liberates free \(J\) from \(KJ\).

Sulfurous and chromic acids may be considered incompatible, since even in an alkaline medium they react slowly with one another, especially in the presence of ammonium salts.

Precipitation of the anions of Group II and their isolation.

The filtrate after precipitation of Group I, containing ammonium salts and ammonia, is moderately heated and precipitated with an excess of \(Ba(OH)_2\); we filter hot.

In the precipitate are \(BaSO_4\) and \(BaSO_3\), or \(BaCrO_4\); the precipitate is washed and dissolved in \(HNO_3\).

The precipitate \(BaSO_4\) is reduced to \(BaS\) and treated with acid. The evolution of \(H_2S\) proves the presence of \(SO_4''\).

Filtrate:
a) colorless, has the odor of \(SO_2\) — \(SO_3''\).

b) yellow — \(CrO_4''\).

In both cases we remove \(Ba\) with an excess of \(Na_2CO_3\) on heating and carry out the specific reactions for the corresponding anion.

III. Group of anions.

Acids precipitated by zinc salts: \(FeCy_6'''\), \(FeCy_6''''\), \(CN'\), \(S''\).

1. Ferrocyanic acid. \(H_3FeCy_6\).

1) \(Mg(NO_3)_2\) and \(Ba(NO_3)_2\) do not precipitate.

2) \(ZnCO_3\) (freshly precipitated) does not precipitate.

Experiment No. 28.

To 5 cu. cm of a solution of \(K_3FeCy_6\) \(\left(\frac{1}{2}E\right)\) an excess of freshly precipitated \(ZnCO_3\) was added; the precipitate was filtered off and thoroughly washed. The filtrate gives an intense reaction for \(FeCy_6'''\). The precipitate was dissolved in \(HCl\); the test for \(FeCy_6'''\) gives a negative result.

3) \(Zn(NO_3)_2\) gives an orange precipitate of \(Zn_3(FeCy_6)_2\), which:

a) is insoluble in dilute \(CH_3COOH\).
b) is soluble on heating in weak \(HCl\).
c) is soluble in concentrated \(NH_4OH\).
d) is soluble in a solution of \(KOH\).

Precipitation is quantitative; the filtrate gives no reaction for \(FeCy_6'''\).

4) \(FeSO_4\) in an acid medium gives a blue precipitate of \(Fe_3(FeCy_6)_2\).

5) \(AgNO_3\) gives an orange precipitate \(Ag_3FeCy_6\), which:

a) is readily soluble in \(NH_4OH\).
b) is insoluble in \(HNO_3\).
c) under the action of \(KOH\) in the cold is converted into \(Ag_2O\) and \(K_3FeCy_6\). \(^{10}\)
d) Weak acids do not liberate \(HCN\).

Ferrocyanohydric acid. \(H_4FeCy_6\).

1) \(Mg(NO_3)_2\) in the cold and \(Ba(NO_3)_2\) do not precipitate (the latter only in very strong solutions).

2) \(ZnCO_3\) (freshly precipitated) completely precipitates \(FeCy_6''''\) in the form of \(Zn_2FeCy_6\).

Experiment No. 29.

We precipitate a solution of \(Zn(NO_3)_2\) with an excess of \(Na_2CO_3\). The precipitate is washed slightly with water, so as not to introduce an excessive amount of soluble salts; thorough washing is unnecessary, since the presence of \(Na_2CO_3\) does not interfere with the reaction. One may directly add a mixture of \(Zn(NO_3)_2\) and \(Na_2CO_3\). The precipitate \(ZnCO_3\) obtained is added in excess to a test portion of a solution of \(K_4FeCy_6\). Filter.

I acidify the filtrate with \(HCl\) and test for the presence of \(FeCy_6''''\) by means of \(FeCl_3\)—there is no reaction.

3) \(Zn(NO_3)_2\) gives a white precipitate \(Zn_2FeCy_6\), which:

a) is insoluble in dilute acids
b) is insoluble in strong \(NH_4OH\)
c) is soluble in a solution of \(KOH\).

4) \(FeCl_3\) in an acid medium gives a blue precipitate \(Fe_4(FeCy_6)_3\). Alkalis (\(KOH\)) convert \(FeCy_6''''\) into solution in the form of \(K_4FeCy_6\); a brown precipitate \(Fe(OH)_3\) remains.

5) \(AgNO_3\) gives a white precipitate \(Ag_4FeCy_6\), which:

a) under the action of \(HNO_3\) is converted into an orange precipitate \(Ag_3FeCy_6\)
b) is slightly soluble in ammonia, but not in ammonium salts.
c) under the action of \(KOH\) is converted into \(Ag_2O\) and \(K_4FeCy_6\).
d) Weak acids do not liberate \(HCN\).

Hydrocyanic acid. \(HCN\).

1) \(Mg(NO_3)_2\) and \(Ba(NO_3)_2\) do not precipitate.

2) \(Zn(NO_3)_2\) precipitates a white precipitate \(Zn(OH)_2\), readily soluble in alkalis and weak acids (even acetic), with liberation of \(HCN\).

In the presence of a large quantity of acetate salts the precipitation is incomplete.

Experiment No. 30.

5 c.c. of a \(\frac{1}{2}E\) solution of \(KCN\) were precipitated with 10 c.c. of \(Zn(NO_3)_2\) \(\left(\frac{1}{2}E\right)\) +

• 50 c.c. of water. The precipitate was filtered off; in the filtrate the concentration of \(CN'\) was determined by titration with a \(\frac{1}{100}E\) solution of \(AgNO_3\) according to Liebig.

For 25 c.c. of the filtrate, \(1/1.05\) c.c. of \(AgNO_3\) was used,
\(2/1.14\) c.c. „

\(^{10}\) Gmelin—Kraut. Handbuch der anorg. Chemie. Vol. V, Abt. 2, 226.
\(^{11}\) Gmelin—Kraut. Handbuch der anorg. Chemie. Vol. V, Abt. II, 225–226.

\[ \text{Concentration of } \mathrm{CN}' = \begin{cases} \dfrac{1.05 \times 2}{100 \times 25} = \dfrac{1}{1190}\,\mathrm{E},\\[6pt] \dfrac{1.14 \times 2}{100 \times 25} = \dfrac{1}{1096}\,\mathrm{E}, \end{cases} \qquad \text{mean} = \dfrac{1}{1143}\,\mathrm{E}. \]

Experiment No. 31.

5 cc of a KCN solution \(\left(\dfrac{1}{1}\mathrm{E}\right)\) were precipitated with 10 cc of \(\mathrm{Zn(CH_3COO)_2}\left(\dfrac{1}{1}\mathrm{E}\right)\) + 10 cc of \(\mathrm{CH_3COONH_4}\left(\dfrac{1}{1}\mathrm{E}\right)\) + 40 cc of water. The filtrate was titrated as before.

For 25 cc of filtrate there were used:

1) 3.25 cc of \(\mathrm{AgNO_3}\); concentration

\[ = \dfrac{3.25 \times 2}{100 \times 25} = \dfrac{1}{385}\,\mathrm{E} \]

2) 3.28 cc of \(\mathrm{AgNO_3}\); concentration

\[ = \dfrac{3.28 \times 2}{100 \times 25} = \dfrac{1}{381}\,\mathrm{E} \]

\[ \text{Mean} = \dfrac{1}{383}\,\mathrm{E} \]

1. \(\mathrm{FeSO_4} + \mathrm{FeCl_3}\) give a blue precipitate. For the reaction to be successful, it must always be carried out in the following manner:

To the test portion add \(\mathrm{NaOH}\) or \(\mathrm{KOH}\) until an alkaline reaction is obtained, then a very small amount of \(\mathrm{FeSO_4}\); boil for 5 minutes, acidify with \(\mathrm{HCl}\), and add \(\mathrm{FeCl_3}\)—a blue precipitate of \(\mathrm{Fe_4(FeCy_6)_3}\).

1. \(\mathrm{ZnCO_3}\) does not precipitate.

2. \(\mathrm{AgNO_3}\) gives a white precipitate of \(\mathrm{AgCN}\). The first portions of \(\mathrm{AgNO_3}\) do not give a precipitate, since \(\mathrm{AgCN}\), with \(\mathrm{CN}'\) present in excess, gives the complex ion \([\mathrm{Ag(CN)_2}]'\). After adding half an equivalent amount of \(\mathrm{AgNO_3}\) relative to the \(\mathrm{CN}'\) content, a precipitate appears according to the reaction:

\[ [\mathrm{Ag(CN)_2}]' + \mathrm{Ag}^{\cdot} = 2\,\mathrm{AgCN}. \]

1. On evaporation on a water bath of the liquid under examination (the reaction must be alkaline) with ammonium polysulfide, the ion \(\mathrm{CNS}'\) is formed, readily detected by the appearance of a blood-red coloration upon acidification and addition of \(\mathrm{FeCl_3}\) (formation of \(\mathrm{Fe(CNS)_3}\)).

On evaporation over an open flame, the \(\mathrm{NH_4CNS}\) initially formed is converted, because of overheating, into thiourea

\[ \mathrm{S}=\mathrm{C} \begin{matrix} \mathrm{NH_2}\\ \mathrm{NH_2} \end{matrix} \]

and the indicated reaction with \(\mathrm{FeCl_3}\) is not obtained.

Hydrogen sulfide acid \(\mathrm{H_2S}\)

1) \(\mathrm{Mg(NO_3)_2}\) and \(\mathrm{Ba(NO_3)_2}\) do not precipitate.

2) Freshly precipitated \(\mathrm{ZnCO_3}\) precipitates \(\mathrm{S}''\) quantitatively as \(\mathrm{ZnS}\).

Experiment No. 32.

3 cu. cm of a \(\dfrac{1}{1}\mathrm{E}\) solution of \(\mathrm{Na_2S}\) were precipitated by means of \(\mathrm{ZnCO_3}\), obtained by mixing 4 cc of a \(\dfrac{1}{1}\) solution of \(\mathrm{Zn(NO_3)_2}\) and 10 cc of \(\mathrm{Na_2CO_3}\) (\(\mathrm{ZnCO_3}\) was not separated from the excess \(\mathrm{Na_2CO_3}\)); the filtrate was slightly acidified and tested for the presence of \(\mathrm{S}''\) by means of \(\mathrm{Pb(NO_3)_2}\).

The presence of \(S''\) was not detected

I treat the precipitate with a 20% KOH solution. In the (acidified) solution I make a test for \(S''\); the reaction is absent or very weak.

I treat the precipitate with dilute HCl—the evolution of \(H_2S\) is intense.

3) \(Zn(NO_3)_2\) quantitatively precipitates \(S''\) both in the absence and in the presence of ammonium salts and ammonia. \(ZnS\) is insoluble in \(CH_3COOH\) and KOH.

Experiment No. 33.

Mixture taken.	Concentration of \(S''\) in the mixture.	Concentration of \(S''\) in the mixture.	Concentration of \(S''\) in the mixture.
Mixture taken.	0.001	0.0005	0.0002
\(Na_2S + Zn(NO_3)_2\)	Turbidity at once; on heating, a precipitate.	On heating, turbidity.	On heating, turbidity.
\(Na_2S + Zn(NO_3)_2 + CH_3COONH_4(E)\)	»	»	»
\(Na_2S + Zn(NO_3)_2 + CH_3COONH_4(E) + NH_4OH\left(\dfrac{E}{2}\right)\)	Opalescence at once; on heating, turbidity.	On heating, opalescence.	None.

Thus the limiting concentration of \(ZnS\) is 0.0005 E, and most likely \(= 0.0002\) E.

The filtrate from \(ZnS\) gives neither a black precipitate nor turbidity, nor in general any dark coloration upon addition of \(Pb(NO_3)_2\).

Precipitation of the anions of Group III and their separation from one another.

The anions of Group III could all have been precipitated together by means of \(Zn(NO_3)_2\), but, in treating the precipitate obtained in order to isolate the anions entering into it, we encounter the unpleasant phenomenon that the precipitate of Zn salts, under the action of ammonia or acids, rapidly turns blue, especially if \(FeCy_6'''\) and \(FeCy_6''''\) are present simultaneously. It seems to me possible that in this case the following processes take place under the action of HCl: \(Zn_3(FeCy_6)_2\) dissolves to a slight extent in HCl; the ion \(FeCy_6'''\), although in insignificant amount, forms \(Fe^{\cdots}\) ions according to the reaction

\[ FeCy_6''' + 6HCl = Fe^{\cdots} + 6HCN + 6Cl' \tag{1} \]

the \(Fe^{\cdots}\) formed immediately reacts with \(Zn_2FeCy_6\):

\[ 4Fe^{\cdots} + 3Zn_2FeCy_6 = Fe_4(FeCy_6)_3 + 6Zn^{\cdots}. \]

In view of the removal of \(Fe^{\cdots}\), reaction (1) proceeds further.

If, however, the separation of \(FeCy_6'''\) and \(FeCy_6''''\) is carried out by means of \(NH_4OH\), it is possible that the destruction of the \(FeCy_6'''\) ion that has passed into solution occurs because of processes originating in the low stability of \((NH_4)_3FeCy_6\) ¹²).

The influence of the removal of the \(Fe^{\cdots}\) ion should act here as an accelerator as well. After a whole series of attempts to find a completely reliable method for treating the precipitate of the anions of Group III, I was compelled to seek a method that would make it possible first to precipitate \(FeCy_6''''\) completely, and only then \(FeCy_6'''\) and \(CN'\). This was accomplished by using either freshly precipitated \(ZnCO_3\). A series of experiments showed that by this method a very complete separation of \(FeCy_6''''\) is achieved. It then turned out that there is no need to have pure \(ZnCO_3\); it can be replaced with equal success by a mixture of a zinc salt with an excess of soda solution. The use of washed \(ZnCO_3\) has the sole purpose of avoiding the introduction of an excessive amount of sodium salts.

¹²) Moissan. Traité de Chimie minérale, vol. IV, 422.

Experiment No. 34.

2 cubic cm of a solution containing: 0.092 g \(K_4FeCy_6\) and 0.003 g \(KCN\) were precipitated with 3 cubic cm of \(Na_2CO_3\) solution \(\left(\frac{1}{1}E\right)\) and 2 cubic cm \(Zn(NO_3)_2\) \(\left(\frac{1}{1}E\right)\). The precipitate was filtered off and washed with a weak soda solution; amount of filtrate = 50 cubic cm.

Filtrate: test for \(FeCy_6''''\) — none.
        » for \(CN'\) — good.

Since for the test for \(CN'\) one-half of the entire filtrate was used, the amount of \(CN'\) in the test was 0.0015 g.

Precipitate: reaction for \(FeCy_6''''\) — intense.

Experiment No. 35.

2 cubic cm of a solution containing 0.0046 g \(K_4FeCy_6\) and 0.065 g \(KCN\) was precipitated with 3 cubic cm of \(Na_2CO_3\) solution \(\frac{1}{1}E\) + 2 cubic cm of \(Zn(NO_3)_2\) solution \(\left(\frac{1}{1}E\right)\).

The precipitate was filtered off and washed with \(Na_2CO_3\). Amount of filtrate 50 c. c.

Filtrate: reaction for \(FeCy_6''''\) — none,
        » for \(CN'\) — intense.

Precipitate: treated with \(CH_3COOH\) (weak); the filtrate was examined for \(CN'\) and \(FeCy_6''''\) — none.

The residue was dissolved in \(KOH\), ferric chloride \(FeCl_3\) was added, and the solution was acidified. Reaction for \(FeCy_6''''\) — intense.

Experiment No. 36.

3 cubic cm of a solution containing 0.092 g \(K_4FeCy_6\), 0.0011 g \(K_3FeCy_6\), and 0.0032 g \(KCN\) were precipitated with a mixture of 5 c. c. \(Na_2CO_3\) \(\frac{1}{1}\) and 3 c. c. \(Zn(NO_3)_2\) \(\frac{1}{1}E\). The precipitate was washed with \(Na_2CO_3\) solution. We have precipitate I and filtrate I (amount 50 c. c.).

Precipitate I is treated with weak \(CH_3COOH\), and the solution is tested for the presence of \(CN'\) — none.

The precipitate is dissolved in \(KOH\), and tests are made:

    for \(FeCy_6'''\) — none.
    for \(FeCy_6''''\) — intense.

Filtrate I: I make a test for \(FeCy_6''''\) — none. I precipitate with an excess of \(Zn(NO_3)_2\) (5 c. c.), filter. I have precipitate II and filtrate II.

Filtrate II is tested for \(FeCy_6''''\) — none,
        » for \(CN'\) — negligible traces.

Precipitate II is treated with \(CH_3COOH\) (weak) + a little \(Zn(NO_3)_2\); the resulting solution III is tested for \(FeCy_6'''\) — none;
        » » \(CN'\) — distinct.

Precipitate III (invisible on the filter) was dissolved in 5% KOH; the test for \(\mathrm{FeCy}_6''''\) is clear.

These experiments provide sufficient proof of the suitability of the method for the qualitative separation of \(\mathrm{FeCy}_6'''\), \(\mathrm{CN}'\), and \(\mathrm{FeCy}_6''''\). A certain difficulty consists in determining the necessary amount of \(\mathrm{ZnCO}_3\) for precipitating \(\mathrm{FeCy}_6''''\) and \(\mathrm{S}''\). It is inconvenient to test the completeness of precipitation from the filtrate by acidifying it and adding \(\mathrm{FeCl}_3\) or \(\mathrm{Pb(NO_3)_2}\), owing to the possibility of the presence of anions which oxidize \(\mathrm{FeCy}_6''''\) to \(\mathrm{FeCy}_6'''\) in acid medium, or conversely. I performed this test as follows: to the filtrate I added a little more \(\mathrm{ZnCO}_3\), washed the precipitate, and treated one portion of the precipitate with \(\mathrm{FeCl}_3 + \mathrm{HCl}\), and another with \(\mathrm{Pb(NO_3)_2}\). If in the first case no blue coloration appears, and in the second no black or brown coloration appears, the precipitation may be considered complete.

The separation of \(\mathrm{FeCy}_6''''\) from \(\mathrm{S}''\) in the precipitate of zinc salts presents no difficulty, since \(\mathrm{Zn_2FeCy_6''''}\), upon treatment with KOH solution, rapidly passes into solution, while \(\mathrm{ZnS}\) remains undissolved. To this it should also be added that \(\mathrm{S}''\) and \(\mathrm{FeCy}_6''''\) are incompatible even in alkaline medium (see experiment No. 8).

Thus the precipitation of the anions of Group III and their isolation should be carried out as follows:

To the filtrate from the precipitation of Group II, without cooling it so that \(\mathrm{BaS_2O_3}\) does not precipitate, add an excess of \(\mathrm{Na_2CO_3}\), boil until the evolution of \(\mathrm{NH_3}\) ceases, filter off the precipitated \(\mathrm{BaCO_3}\), and add to the filtrate washed, freshly precipitated \(\mathrm{ZnCO_3}\), or else a mixture of \(\mathrm{Zn(NO_3)_2}\) with an excess of \(\mathrm{Na_2CO_3}\); stir from time to time for 1 hour and filter.

Precipitate: \(\mathrm{ZnCO_3}\), \(\mathrm{Zn_2FeCy_6}\), \(\mathrm{ZnS}\) is treated with a solution of KOH or NaOH; \(\mathrm{FeCy_6''''}\) passes into solution, with \(\mathrm{ZnS}\) remaining in the residue.

To the filtrate after precipitation of \(\mathrm{S}''\) and \(\mathrm{FeCy_9''''}\) add an excess of \(\mathrm{Zn(NO_3)_2}\).

Precipitate: \(\mathrm{Zn(CN)_2}\), \(\mathrm{Zn_3(FeCy_6)_2}\), and \(\mathrm{ZnCO_3}\).

Add dilute \(\mathrm{CH_3COOH}\).

Does not dissolve: \(\mathrm{Zn_3(FeCy_6)_2}\).

In solution: \(\mathrm{HCN}\).

In view of the fact that hydrocyanic acid is readily volatile, it may happen that it will be carried off to a considerable extent by the evolved \(\mathrm{CO_2}\). For this reason it is better to carry out the operation of dissolving the precipitate \(\mathrm{Zn(CN)_2}\), \(\mathrm{Zn_3(FeCy_6)_2}\), and \(\mathrm{ZnCO_3}\) in a test tube with a delivery tube while heating, and to absorb the evolved \(\mathrm{HCN}\) in a weak solution of caustic alkali, with which the characteristic reactions for \(\mathrm{CN}'\) are then to be carried out.

IV GROUP.

Acids whose lead salts are insoluble: \(\mathrm{S_2O_3''}\), \(\mathrm{AsO_3'''}\), \(\mathrm{J}'\), and \(\mathrm{BO_3'''}\).

1) Thiosulfuric acid. \(\mathrm{H_2S_2O_3}\).

1) \(\mathrm{Mg(NO_3)_2}\)—does not precipitate.

2) \(\mathrm{Ba(NO_3)_2}\) does not precipitate from hot solutions. In cold solutions, after some time, a precipitate of \(\mathrm{BaS_2O_3}\) separates out, rather sparingly soluble in water

\[ \left(1 \text{ part in } 480 \text{ parts of water } = \frac{1}{60}\mathrm{E}.\right) \]

Supersaturated solutions are readily obtained.

3) \(\mathrm{Zn(NO_3)_2}\)—does not precipitate.

4) Iodine solution converts \(\mathrm{S_2O_3''}\) into \(\mathrm{S_4O_6''}\).

5) Free \(\mathrm{Cl}\) and \(\mathrm{Br}\) convert \(\mathrm{S_2O_2''}\) into \(\mathrm{SO_4''}\).

6) \(\mathrm{Pb(NO_3)_2}\) precipitates as \(\mathrm{PbS_2O_3}\); an excess of \(\mathrm{Pb''}\), the presence of ammonium salts, and a small amount of ammonia have a favorable effect.

Experiment No. 37.

1 cubic cm of a \( \frac{1}{1} E \) solution of \( \mathrm{Na_2S_2O_3} \) was precipitated with 5 cubic cm of \( \mathrm{Pb(NO_3)_2} \), with the addition of 50 c.c. of water. After 2 h the solution was filtered. The concentration of \( \mathrm{S_2O_3''} \) in the filtrate was determined by titration with iodine solution \( \left(\frac{1}{125} E\right) \).

\[ 25\ \text{c.c. filtrate} = 5.9\ \text{c.c. } J \quad \text{concn. } \mathrm{S_2O_3''} = \frac{1}{530} E \]

\[ 20\ \text{c.c.} \quad » \quad = 4.7\ \text{c.c. } J \quad » \quad » \quad = \frac{1}{532} E \]

Experiment No. 38.

1 cubic cm of a \( \frac{1}{1} E \) solution of \( \mathrm{Na_2S_2O_3} \) was precipitated with 5 cubic cm of \( \mathrm{Pb(NO_3)_2} \), with the addition of 50 c.c. of water and several drops of concentrated \( \mathrm{NH_4OH} \). The precipitate was filtered off after standing for 2 h and titrated as in the preceding experiment.

\[ 25\ \text{c.c. filtr.} = 3.7\ \text{c.c. } J \quad \text{concn. } \mathrm{S_2O_3''} = \frac{1}{845} E \]

\[ 20\ \text{c.c.} \quad » \quad = 3.21\ \text{c.c. } J \quad » \quad » \quad = \frac{1}{800} E \]

Experiment No. 39.

1 cubic cm of a \( \frac{1}{1} E \) solution of \( \mathrm{Na_2S_2O_3} \) was precipitated with 10 c.c. of \( \mathrm{Pb(NO_3)_2} \), with the addition of 50 c.c. of water, 5 drops of concentrated \( \mathrm{NH_4OH} \), and 10 c.c. of a \( \frac{2}{1} E \) solution of \( \mathrm{CH_3COONH_4} \). Filtered after 4 hours.

\[ 25\ \text{c.c. of filtrate} = 0.86\ \text{c.c. } J \quad \text{concn. } \mathrm{S_2O_3''} = \frac{1}{3610} \]

\[ 25\ \text{c.c.} \quad » \quad = 0.91\ \text{c.c. } J \quad » \quad » \quad = \frac{1}{3430} \]

7) \( \mathrm{AgNO_3} \) precipitates a white precipitate of \( \mathrm{Ag_2S_2O_3} \), which rapidly darkens because of decomposition with liberation of \( \mathrm{Ag_2S} \) even in the cold.

Hydriodic acid \( \mathrm{HI} \).

1. \( \mathrm{Pb(NO_3)_2} \) precipitates a yellow precipitate of \( \mathrm{PbI_2} \) with a silky luster, soluble on boiling and precipitating again in the form of shining plates. The precipitation is not quite good.

Solubility of \( \mathrm{PbI_2} \) in cold water \(= 0.00044\) \(^{13}\) \((0^\circ)\), \(0.00061\) at \(15^\circ\). The latter figure corresponds to the greatest concentration \(= \frac{1}{523} E\).

The presence of a considerable excess of \( \mathrm{Pb(NO_3)_2} \) diminishes the solubility.

Experiment No. 40.

3 cubic cm of a \( \frac{1}{1} E \) solution of \( \mathrm{KJ} \) were precipitated with 13 cubic cm of a solution of \( \mathrm{Pb(NO_3)_2} \), with the addition of 84 cubic cm of water. After 30 minutes the precipitate was filtered off. 25

\(^{13}\) Landolt u. Bernstein. Phys.-Chem. Tabellen.

to 100 cc of filtrate, AgNO₃ was precipitated in the presence of HNO₃. Weight of AgJ =

\[ = 0.0090\ \text{g}, \quad \text{whence the concentration of } J'' \text{ in the filtrate} = \frac{P_{\mathrm{AgJ}}\times 1000}{M_{\mathrm{AgJ}}\times 25} \]

\[ = \frac{0.0090\times 1000}{234.7\times 25} = \frac{1}{653}\ E \]

Arsenious acid \(H_3AsO_3\).

For the individual reactions, see above in group 1.

Boric acid \(H_3BO_3\).

1) \(Mg(NO_3)_2\)—does not precipitate.
2) \(Ba(NO_3)_2\)—in the presence of ammonium salts precipitates only from very concentrated solutions.
3) \(Zn(NO_3)_2\) precipitates only from solutions that are not especially dilute.
4) \(Pb(NO_3)_2\) precipitates not sufficiently well. In the absence of ammonium salts, the limiting concentration is

\[ BO_3''' = \frac{1}{300}\ E \]

Experiment No. 41.

Concentration of \(BO_3'''\).

\(\frac{1}{50}\ E\)	\(\frac{1}{100}\ E\)	\(\frac{1}{200}\ E\)	\(\frac{1}{300}\ E\)	\(\frac{1}{400}\ E\)
Strong turbidity upon heating; precipitate.	Strong opalescence; upon heating—precipitate.	Clear opalescence upon intensified heating.	Traces of opalescence.	None.

5) In the presence of ammonium salts the solubility is greatly increased.

Experiment No. 42.

To a solution containing \(BO_3'''\) and \(CH_3COONH_4\), an equal volume of \(Pb(NO_3)_2\) was added. The content of \(Pb(NO_3)_2\) and \(CH_3COONH_4\) in the mixture is equal to \(\frac{1}{2}E\).

Concentration of \(BO_3'''\).

0.2 E.	0.1 E.	0.05 E.
Abundant precipitate.	The precipitate, upon shaking, dissolves almost completely. Only turbidity remained.	Slight turbidity.

It should be borne in mind that the solution of \((CH_3COO)_2Pb\) itself gives turbidity on standing. The limiting concentration of \(BO_3'''\) is therefore close to \(\frac{1}{20}E\).

6) When concentrated \(H_2SO_4\) acts on a mixture of the dry salt and alcohol (preferably wood alcohol), boric ethyl or, respectively, boric methyl ester is formed, which burns with a green flame.

Precipitation of the anions of group IV and their separation from one another.

If our aim is to precipitate \(BO_3'''\) as completely as possible, then first of all we must take care to remove the ammonium salts as completely as possible. For this purpose we add an excess of \(Na_2CO_3\) to the sample and boil until the reaction for \(NH_4\) has completely disappeared; we filter off the precipitated \(ZnCO_3\) and add a solution of \(Pb(NO_3)_2\) to the still hot liquid until precipitation ceases.

We filter off the precipitated \(PbCO_3\), \(PbS_2O_3\), \(Pb_3(AsO_3)_2\), \(PbJ_2\), and \(Pb_3(BO_3)_2\); to the filtrate we add \(NH_4NO_3\), \(NH_4OH\) until a strong odor is present, and again \(Pb(NO_3)_2\); it is possibly better to cool it, and we filter again. We wash the precipitates with cold water and combine them for examination.

To separate \(J'\), we boil the precipitate with water, filter hot, and wash with hot water until the reaction for \(Pb\) ceases. In the filtrate is \(PbJ_2\);\(^{14}\) on the filter are \(PbCO_3\), \(Pb_3(BO)_2\), \(Pb_3(AsO_3)_2\), and \(PbS_2O_3\).

We boil the precipitate with a concentrated solution of \(Na_2CO_3\) in order to convert the anions into solution, and filter off \(PbCO_3\). The solution, now containing only the Na salts, may be examined in various ways:

1) We add an excess of bromine water; in this, \(AsO_3'''\) and \(S_2O_3''\) pass respectively into \(AsO_4'''\) and \(SO_4''\). By adding magnesia mixture we precipitate \(AsO_4'''\) in the form of \(MgNH_4AsO_4\). We filter, acidify the solution with \(HCl\), and add \(BaCl_2\)—\(BaSO_4\) is precipitated from \(S_2O_3''\); \(BO_3'''\) remains in solution.

2) To the solution we add an excess of ammoniacal zinc oxide solution, boil until \(NH_3\) is completely removed, cool, filter, and wash with water—some Zn salts. In the precipitate are \(AsO_3'''\) and \(BO_3''\); in the solution, \(S_2O_3''\) and a small amount of \(BO_3'''\). The precipitate, well washed with water, is dissolved in \(HCl\), and \(H_2S\) is passed through it—in the precipitate is \(As_2S_3\), in solution \(H_3BO_3\).

In view of the fact that the precipitation of \(BO_3'''\), and also its separation from the other anions of group IV, cannot be regarded as entirely satisfactory, it is more expedient to make a separate test for \(BO_3'''\), either from the original sample or from the filtrate from the precipitation of the group III anions, and then to precipitate group IV without removing the ammonium salts and after adding a little \(NH_4OH\).

V. GROUP.

Acids not precipitated by the preceding reagents, but whose Ag salts are insoluble: \(Cl'\), \(Br'\), \((J')\), \(CNS'\).

Hydrochloric acid. \(HCl\).

1. \(Mg(NO_3)_2\), \(Ba(NO_3)_2\), and \(Zn(NO_3)_2\) do not precipitate.

2. \(Pb(NO_3)_2\) precipitates only from rather concentrated solutions. Solubility of \(PbCl_2\)\(^{15}\) = 0.0091 at \(15^\circ C\), which gives the limiting conc.

\[ PbCl_2 = \frac{1}{31} E. \]

At \(100^\circ\) the solubility = 0.0334.

\(^{14}\) If the sample contained a considerable amount of \(Br'\) and \(Cl'\), then the filtrate obtained may contain, besides \(J'\), also \(Br'\) and \(Cl'\). They do not interfere with the test for \(J'\); the amounts of \(Br'\) and \(Cl'\) remaining in solution after precipitation of group IV are always quite sufficient to reveal them later.

If desired, \(Br'\), \(Cl'\), and \(J'\) may be precipitated from the solution as Ag salts and examined as the precipitate of group V.

\(^{15}\) Lictu. Amer. Chem. Journ. 25.469.

1. $\mathrm{AgNO_3}$ gives a precipitate of $\mathrm{AgCl}$, very sparingly soluble in water, which:

a) is insoluble in $\mathrm{HNO_3}$

b) is soluble in $\mathrm{KCN}$, commercial $(\mathrm{NH_4})_2\mathrm{CO_3}$, and in $\mathrm{NH_4OH}$.

c) under the action of $\mathrm{Na_3AsO_3}$ gives $\mathrm{Ag_3AsO_3}$ and $\mathrm{NaCl}$⁶. Arsenious-acid silver dissolves in an excess of $\mathrm{Na_3AsO_3}$

d) under the action of $\mathrm{K_4FeCy_6}$, $\mathrm{Ag_4FeCy_6}$ and $\mathrm{KCl}$ are obtained⁷.

e) Metallic zinc precipitates the metal Ag, and $\mathrm{ZnCl_2}$ passes into solution.

f) Sodium formate in the presence of $\mathrm{NaOH}$, on boiling for no more than 10 min., reduces $\mathrm{AgCl}$ to metal, while $\mathrm{Cl'}$ passes into solution.⁸ The concentration of $\mathrm{NaOH}$ should be about 8%; that of sodium formate, 10%.

1. A well-dried sample, when heated with dry $\mathrm{K_2Cr_2O_7}$ and concentrated $\mathrm{H_2SO_4}$, gives a distillate of $\mathrm{CrO_2Cl_2}$, which on absorption by water gives $\mathrm{H_2CrO_4}$ and $\mathrm{HCl}$.

2. $\mathrm{PbO_2 + C_3HCOOH}$ do not react on heating.

3. $\mathrm{KNO_2}$ in the presence of $\mathrm{CH_3COOH}$ does not give $\mathrm{Cl_2}$.

Hydrobromic acid $\mathrm{HBr}$.

1) $\mathrm{Mg(NO_3)_2}$, $\mathrm{Ba(NO_3)_2}$, and $\mathrm{Zn(NO_3)_2}$ do not precipitate.

1. $\mathrm{Pb(NO_3)_2}$ precipitates only from fairly concentrated solutions.

Solubility of $\mathrm{PbBr_2}$ at $15^\circ\mathrm{C}$⁹ $= 0.0073 = \dfrac{1}{50}\ \mathrm{N}$

» » at $100^\circ\mathrm{C} = 0.0475 = \dfrac{1}{7.7}$

1. $\mathrm{AgNO_3}$ gives a precipitate of $\mathrm{AgBr}$, insoluble in water, which:

a) is insoluble in acids.

b) is soluble in $\mathrm{KCN}$ and $\mathrm{NH_4OH}$ (less so than $\mathrm{AgCl}$)

c) is almost insoluble in commercial $(\mathrm{NH_4})_2\mathrm{CO_3}$.

d) is unchanged by the action of $\mathrm{Na_3AsO_3}$ and $\mathrm{K_4FeCy_6}$.

e) metallic zinc precipitates Ag, and $\mathrm{Br'}$ passes into solution in the form of $\mathrm{ZnBr_2}$.

1. Sodium formate under the conditions indicated for $\mathrm{AgCl}$ does not reduce $\mathrm{AgBr}$, but reduction partially occurs with 15% $\mathrm{NaOH}$ and prolonged boiling.

2. On distilling a dry sample with $\mathrm{K_2Cr_2O_7}$ and $\mathrm{H_2SO_4}$, there is only $\mathrm{Br_2}$ in the distillate; the compound $\mathrm{CrO_2Br_2}$ is not formed.

3. $\mathrm{PbO_2 + CH_3COOH}$, on heating, liberates $\mathrm{Br_2}$.¹⁰

4. $\mathrm{KNO_2}$ in the presence of $\mathrm{CH_3COOH}$ does not liberate $\mathrm{Br_2}$.

Hydroiodic acid $\mathrm{HJ}$.

1. $\mathrm{Mg(NO_3)_2}$, $\mathrm{Ba(NO_3)_2}$, and $\mathrm{Zn(NO_3)_2}$ do not precipitate.

2. $\mathrm{Pb(NO_3)_2}$ precipitates yellow $\mathrm{PbJ_2}$. Traces of $\mathrm{J'}$ remain in solution.

3. $\mathrm{AgNO_3}$ gives a yellow precipitate of $\mathrm{AgJ}$, insoluble in water, which:

a) is insoluble in $\mathrm{HNO_3}$.

b) is soluble in $\mathrm{KCN}$.

c) is almost insoluble in commercial $(\mathrm{NH_4})_2\mathrm{CO_3}$ and in $\mathrm{NH_4OH}$.

d) is unchanged by the action of $\mathrm{Na_3AsO_3}$ and $\mathrm{K_4FeCy_6}$.

e) Metallic Zn precipitates the metal Ag, and transfers J′ into solution in the form of ZnJ₂.

f) Sodium formate in the presence of NaOH does not reduce AgJ.¹⁸).

1. PbO₂ + CH₃COOH, on distillation, gives free iodine.
2. KNO₂ + CH₃COOH liberates free iodine.
3. CuSO₄ in the presence of S₂O″₃ or SO₃ quantitatively precipitates J′ in the form of Cu₂J₂.

Thiocyanic Acid HCNS.

1. Mg(NO₃)₂, Ba(NO₃)₂, and Zn(NO₃)₂ do not precipitate it.

2. Pb(NO₃)₂ precipitates only from rather concentrated solutions. The solubility of Pb(CNS)₂ is 0.0045 at 20° = 1/30 E.

3. Ag(NO₃) gives a curdy, water-insoluble precipitate of AgCNS, white in color, which:

a) is insoluble in HNO₃,
b) is soluble in NH₄OH.
c) is somewhat soluble in ammonium carbonate, (NH₄)₂CO₃ (less than AgCl).
d) K₄FeCy₆ partially converts it into Ag₄FeCy₆; CNS′ partially passes into solution, in the form of KCNS.

Experiment No. 43.

0.2 g AgCNS is treated with 20 c.c. of K₄FeCy₆ (1/2 E.) for 1 hour, then filtered. To the filtrate, containing an excess of K₄FeCy₆ and KCNS, Zn(NO₃)₂ is added to remove FeCy₆″″, and it is filtered again. The filtrate (giving absolutely no reaction for FeCy₆″″) gives a reaction for CNS′ with FeCl₃ and with AgNO₃ separates a white precipitate of AgCNS.

The precipitate after filtration of K₄FeCy₆ is washed with water, Zn and H₂SO₄ are added, and, after reduction of AgCNS to Ag, it is filtered. The filtrate gives a good reaction for CNS′.

e) Sodium formate and caustic soda, under the conditions indicated for AgCl, reduce AgCNS to metallic Ag. The CNS′ ion passes into solution.

1. Copper salts in the presence of a reducing agent (SO″₃ or S₂O₃) quantitatively precipitate CNS′ in the form of Cu₂(CNS)₂.¹⁸). This reaction is used by Weber and Winkelmann*) for the separation of Cl′ and CNS′.

2. FeCl₃ in an acidic medium gives an intense red coloration, owing to the formation of Fe(CNS)₃.

Precipitation of the Anions of Group V and Their Separation from One Another.

To the neutralized filtrate from the precipitate of group IV anions, add an excess of AgNO₃ and heat to boiling. The liquid is cooled, the curdy precipitate AgCl, AgBr, (AgJ), and AgCNS is filtered off and thoroughly washed with water.

The precipitate of silver salts may be treated by one of the following methods:

1) In the absence of CNS′

a) the precipitate is thoroughly stirred for several minutes with 10–15% solution of ammonium carbonate, (NH₄)₂CO₃, and then filtered; in the filtrate are AgCl and traces of AgBr. Upon addition of KBr in the presence of AgCl, a clear precipitate, AgBr, is formed.

The precipitate after the action of \((NH_4)_2CO_3\)—a mixture of \(AgBr\) and \(AgJ\)—is treated with strong ammonia. \(AgBr\) passes into solution, while \(AgJ\) remains in the precipitate.

In all the solutions, upon addition of \(HNO_3\) to an acid reaction, a precipitate of the corresponding salt separates out; this can then be treated with metallic zinc, whereby the halide ion passes into solution in the form of a Zn salt.

This method does not give a completely exact separation of \(Br'\) from \(Cl'\). In the presence of \(CNS'\), the addition of a very large quantity of \((NH_4)_2CO_3\) is required, which makes the method inconvenient.

b) The precipitate \(AgCl\), \(AgBr\), and \(AgJ\) is stirred for several minutes with a solution of \(Na_3AsO_3\) or \(K_4FeCy_6\), then filtered.

Precipitate: \(AgBr\), \(AgJ\) (and \(Ag_4FeCy_6\)), solution \(KCl\) or \(NaCl\) and an excess of the reagent taken.

To the precipitate \(AgBr\), \(AgJ\), and \(Ag_4FeCy_6\) (if \(K_4FeCy_6\) was taken), a piece of Zn and a little dilute \(H_2SO_4\) are added. After complete reduction of Ag, filter; on the filter are metallic Ag and \(Zn_2FeCy_6\); in solution are \(ZnBr_2\) and \(ZnJ_2\).

I neutralize the solution, add \(CH_3COOH\) and \(KNO_2\), and extract the liberated \(J_2\) with carbon disulfide or another solvent.

With the aqueous solution (after checking for complete removal of \(J'\)) containing \(ZnBr_2\), carry out separate reactions for \(Br'\). In the presence of \(CNS'\) this method is of little applicability, because part of the \(CNS'\) remains unreacted with \(K_4FeCy_6\).

c) The mixture \(AgCl\), \(AgBr\), and \(AgJ\) is reduced with metallic zinc in the presence of dilute \(H_2SO_4\); \(ZnCl_2\), \(ZnBr_2\), and \(ZnJ_2\) pass into solution. To the (neutralized) solution add \(KNO_2\) and \(CH_3COOH\); \(J_2\) is liberated and is removed by shaking with carbon disulfide or by boiling. \(^{21}\) In the residue, in separate portions, determine \(Br'\) by means of chlorine water and carbon disulfide, and \(Cl'\) by the formation of \(CrO_2Cl_2\) during distillation with \(K_2Cr_2O_7\) and \(H_2SO_4\).

1. In the presence of \(CNS'\): The mixture \(AgCNS\), \(AgCl\), \(AgBr\), and \(AgJ\) is heated for not more than 5–10 min. with 10–15 cc of a solution containing 8% \(NaOH\) and 10% \(HCOONa\). \(Cl'\) and \(CNS'\) pass into solution; in the precipitate are \(AgBr\), \(AgJ\), and metallic Ag (reduced from \(AgCl\) and \(AgCNS\)). The precipitate is examined by one of the preceding methods.

The filtrate from \(AgBr\) and \(AgJ\), containing \(Cl'\) and \(CNS'\) in solution, is almost neutralized with weak \(H_2SO_4\), and finally acidified with \(H_2SO_3\).

Add \(CuSO_4\) dropwise until a precipitate of \(Cu_2(CNS)_2\) still forms. Filter. To the solution containing \(Cl'\), add dilute \(HNO_3\), heat to remove \(SO_2\), and test for \(Cl'\). Dissolve the precipitate \(Cu_2CNS)_2\) in HCl and carry out the reaction for \(CNS'\).

VI GROUP

Acids not precipitated by all the preceding group reagents:

\(NO_3'\), \(NO_2'\), \(ClO_3'\)

Nitric acid \(HNO_3\) and nitrous acid \(HNO_2\).

The method of detection was indicated in carrying out the preliminary tests. Here I add the following reactions:

1. Concentrated \(H_2SO_4\) + diphenylamine give, in the presence of \(NO_2'\) or \(NO_3'\), a blue coloration.

\(^{21}\) F. A. Gooch and F. T. Brooks. Amer. Journ. of Sc. 40. 283.

1. Zinc or aluminum in an alkaline medium reduces \(NO_2'\) and \(NO_3'\) to ammonia.

2. Brucine + concentrated \(H_2SO_4\) gives a red coloration with \(NO_3'\). \(NO_2'\) does not give this reaction if the reagent is added in large excess (four times the volume of the liquid being examined).

3. With \(CoCl_2 + CH_3COOH\), \(NO_2'\) gives a yellow precipitate, \(K_3Co(NO_2)_6\), sparingly soluble in water. \(NO_3'\) does not react.

Chloric acid \(ClO_3'\).

It does not form sparingly soluble salts. It can be detected chiefly by reduction reactions to \(Cl_2\) or \(Cl'\), namely:

1. Under the action of \(H_2SO_3\), \(ClO_3'\) is converted into \(Cl'\), which, in the presence of \(AgNO_3\), gives a precipitate of \(AgCl\).

2. \(Zn + H_2SO_4\) and ferrous salts react in exactly the same way.

3. \(KJ\), in the presence of strong acids, rapidly, and in the presence of \(CH_3COOH\)—after several minutes, liberates free iodine.

4. \(HCl\) reacts with liberation of free chlorine.

Detection of anions of group VI

\(NO_2'\) and \(NO_3'\) were detected in the preliminary tests, so that only a test for \(ClO_3'\) is necessary. If \(NO_2'\) is absent, then \(ClO_3'\) can be detected by the liberation, in the presence of weak \(H_2SO_4\), of free iodine from \(KJ\). In the presence of \(NO_2'\), it is necessary to convert \(ClO_3'\) into \(Cl'\) by reduction with sulfurous acid or metallic zinc. The \(Cl'\) ion forms, with the excess \(AgNO_3\) present, a white precipitate of \(AgCl\), which can be examined for \(Cl\) as above.

1. Lutz and Swinne, J. R. F. Kh. O., 1909, p. 1488. ↩↩

2. М. Р. Ф. Х. О. 41. 1301. ↩

3. Journ. Amer. Chemic. Society 1912, 609. ↩

4. Zeitschr für Analyt. Chemie 63. 1. ↩

5. Zeitsch. für Analyt. Chemie 39. 566. ↩

6. G. Vortman. Allgemeiner Gang der qualit. chem. analyse ohne anw. H₂S. Wien 1908 S. 36. ↩

7. Chemiker Zeitung 40, 398 (1919), and also: G. B. Bonino, abstract in Chemisches Zentralblatt. 1922 II, p. 845. ↩

8. H. Weber and H. Winkelman. Journal of the Amer. Chem. Society XXXVIII, 2000. ↩

9. Lichty. Amer. Chem. Journal 25, 469. ↩

10. Vortman. Zeitschrift für analyt. Chemie 25, 172. ↩