INDIRECT EXTRACTION-COLORIMETRIC METHOD FOR THE DETERMINATION OF TRACE AMOUNTS OF PHOSPHORUS IN HIGH-PURITY TIN

Yu. L. Lel’chuk, V. B. Sokolovich, G. A. Detkova

(Presented by the scientific-methodological seminar of the Chemical-Technological Faculty)

In the photometric determination of phosphorus, use is usually made of yellow phosphomolybdic heteropoly acid, heteropoly acids in which molybdenum ions are partially replaced by vanadium or tungsten, and also the blue reduction products of heteropoly acids.

The greatest sensitivity is characteristic of methods based on the colorimetry of phosphomolybdenum blue extracted with organic solvents. However, even in this case, as follows from the literature data and as verified by our experiments, the sensitivity does not exceed \(10^{-7}\%\).

Heteropoly acids belong to complex compounds. The chemistry of their formation and a number of their most important properties have not yet been sufficiently studied. The composition of phosphomolybdic heteropoly acid is expressed by the formulas

\[ \mathrm{H}_3[\mathrm{P}(\mathrm{Mo}_3\mathrm{O}_{10})_4]\cdot n\mathrm{H}_2\mathrm{O}\ [1]\ \text{or}\ \mathrm{H}_7[\mathrm{P}(\mathrm{Mo}_2\mathrm{O}_7)_6]\cdot x\mathrm{H}_2\mathrm{O}\ [2] \]

The composition of phosphomolybdenum blue has not been precisely established. It is believed that phosphomolybdenum blue is a complex combination of oxygen compounds of hexavalent molybdenum and molybdenum in lower oxidation states with phosphoric acid [3].

From all the formulas proposed for phosphomolybdic heteropoly acid and phosphomolybdenum blue, it follows that 12 molybdenum ions coordinate with one phosphorus ion.

I. P. Onufrienok [4], in order to increase the sensitivity of determining traces of arsenic, proposed colorimetrically measuring colored compounds that can be formed by molybdenum contained in arsenomolybdenum blue, in particular the rhodanide complex of molybdenum.

We decided to increase the sensitivity of determining phosphorus in high-purity tin by the same method, i.e., by decomposing phosphomolybdenum blue, binding the liberated molybdenum into the molybdenum thiocyanate complex, and colorimetrically measuring it.

For the reduction of hexavalent molybdenum to pentavalent molybdenum, stannous chloride [5] is most often used. However, the molybdenum thiocyanate complex has a more stable color if reducing agents weaker than stannous chloride are used, for example potassium iodide, thiourea, ascorbic acid, acetone, etc.

Tananaev and Lokhvitskaya [6] indicate that rhodanide ion can reduce hexavalent molybdenum to pentavalent molybdenum in an acidic medium, and also emphasize the dependence of the stability of the color of molybdenum rhodanide on the acidity of the medium.

Reznik, Gansburg, and Sachko [7] propose an accelerated variant of the rhodanide method for determining molybdenum, based on the reaction between molybdenum and rhodanide without a reducing agent in the presence of copper sulfate in a hydrochloric acid medium.

At a molybdenum content on the order of \(10^{-3}\%\) and below, the sensitivity of the determination and the stability of the rhodanide complex can be increased by extracting molybdenum rhodanide with organic solvents.

We decided, after destruction of phosphomolybdenum blue, to convert the molybdenum contained in it into the rhodanide complex according to Reznik’s procedure [7], using a visual extraction-colorimetric end point.

The optimum conditions for formation of the blue were first established (reducing agent, pH, temperature, etc.). It was found that the best reducing agent in an aqueous medium is sulfurous hydrazine; the medium is 0.3–0.6 normal with respect to sulfuric acid; heating is carried out at a temperature of about \(100^\circ\mathrm{C}\) in a boiling water bath. The presence of traces of tin interferes. The color of the blue weakens and becomes unstable. At a higher tin content the solutions become turbid. Under the described conditions, the blues of phosphomolybdic and arsenomolybdic heteropoly acids are formed. Siliceous heteropoly acids and blues are not obtained. Isoamyl alcohol was chosen as the extractant for the rhodanide complex.

The method we propose is based on decomposition of the weighed portion of the tin under analysis with hydrochloric acid, hydrobromic acid, and bromine; removal of tin and arsenic; conversion of silicon into the weakly active precipitate \( \mathrm{SiO_2} \); conversion of phosphorus into phosphomolybdenum blue; extraction of the complex with isoamyl alcohol; re-extraction and destruction of it with ammonia; conversion of the liberated molybdenum into the rhodanide complex; extraction of the complex with isoamyl alcohol; and comparison of the color intensity of the extract with a standard scale imitated by a mixture of potassium dichromate and cobalt chloride solutions.

The determination is interfered with by tin and, among the impurities usually encountered in high-purity tin, by arsenic, silicon, and trivalent iron. Tin is removed in the form of tin chloride or bromide by threefold evaporation with HCl and HBr. Arsenic is reduced by hydrobromic acid to the trivalent state and is removed in the form of arsenic chloride or bromide. During evaporation with acids, silicon passes into the weakly active precipitate \( \mathrm{SiO_2} \). The interfering action of trivalent iron is eliminated by reducing it to the divalent state; the reduction is catalyzed by cuprous ions.

The method makes it possible to determine \(0.01\ \mu\mathrm{g}\) of phosphorus in high-purity tin. The sensitivity of the method with a gram weighed portion of tin is \(1 : 10^{-6}\%\).

The relative error of the determination is \(\pm 10\text{–}20\%\).

Decomposition of the weighed portion of the tin under analysis and removal of tin and arsenic in five parallel samples takes about 6 hours. The extraction-colorimetric end point takes about 4 hours.

Necessary reagents and solutions

1. Distilled water, GOST 6709-53, redistilled in a quartz apparatus. Stored in a polyethylene vessel.

1. Hydrobromic acid, analytical grade, GOST 2062-43, and bromine are additionally purified by threefold distillation in a quartz apparatus. The distilled hydrobromic acid is stored in a polyethylene vessel, and the bromine in a quartz bottle.

2. Sulfuric acid, chemically pure, passing the Savalle test, GOST 4204-48. A \(5N\) solution is prepared by mixing 28 ml of concentrated acid with 172 ml of bidistillate. A \(0.5N\) solution is prepared by diluting the \(5N\) solution. To the latter solution, 0.1 g of ascorbic acid is added for every 100 ml of solution.

3. Aqueous ammonia, analytical grade, GOST 3760-47.

4. Isoamyl alcohol, analytical grade, GOST 5830-51. Distilled in a quartz apparatus.

5. Isopiestic hydrochloric acid. To obtain such an acid, concentrated hydrochloric acid, analytical grade, is poured into a large desiccator, below the level of the fluoroplastic support. A wide polyethylene or fluoroplastic dish filled with bidistillate is placed on the support, and the desiccator is tightly closed with a lid. Saturation of the water with HCl vapors lasts 3–4 days. The normality of the acid obtained in this way, purified from traces of metals, is approximately 9–10.

6. Ammonium molybdate, 1% solution. Prepared by dissolving 1 g of ammonium molybdate purified by recrystallization in 100 ml of \(5N\) sulfuric acid.

Purification of ammonium molybdate from traces of phosphorus and arsenic is carried out as follows: 30 g of analytical-grade salt are dissolved in the minimum amount of bidistillate with addition of concentrated ammonia until a faint odor appears. After dissolution of the bulk of the salt, disregarding the remaining precipitate, 1 g of magnesium nitrate is added to the solution, dissolved in 5 ml of bidistillate. The solution is heated and kept at 40°C for 20 min, 1–2 ml of concentrated ammonia are added, and it is allowed to stand for 12 hours.

Table 1

Checking the accuracy of phosphorus determination in standard solutions

The solution is filtered, slightly evaporated, cooled, and, with stirring, concentrated ammonia is added until strong turbidity appears.

...of the solution. The precipitated ammonium molybdate crystals are allowed to settle, then filtered and washed 2 times with concentrated ammonia, transferred to a beaker, dissolved in a minimum amount of water, filtered into a clean beaker, and ammonium molybdate crystals are again isolated by adding concentrated ammonia and ethyl alcohol. The separated crystals are filtered through a glass filter, washed with ethyl alcohol, dried, and stored in a weighing bottle.

Table 2

Verification of the accuracy of phosphorus determination by the method of additions

1. Hydrazine sulfate, chemically pure grade; a 0.15% solution is prepared by dissolving 0.15 g of the reagent in 100 ml of bidistillate.

Table 3

Results of parallel determinations of phosphorus in a high-purity tin sample

1. Potassium thiocyanate, chemically pure grade, GOST 4139-48; a 50% solution is prepared by dissolving 50 g of the reagent in 100 ml of bidistillate. After heating to room temperature and settling for half an hour, the solution is filtered through glass filter No. 4.

1. Copper sulfate, chemically pure, GOST 4165-48; a 0.4% solution is prepared by dissolving 0.4 g of the salt in 100 ml of bidistillate. A 0.04% solution is prepared before use by the appropriate dilution of the 0.4% solution.

2. Potassium dichromate, chemically pure, 2% and 0.5% solutions.

3. Cobalt chloride, chemically pure, 5% and 15% solutions.
4. Standard phosphorus solutions.

The initial standard phosphorus solution with a phosphorus content of 1000 μg/ml is prepared from chemically pure \(\mathrm{Na_2HPO_4 \cdot 12H_2O}\). The percentage content of phosphorus in the reagent is first determined by the gravimetric method, and on the basis of these data the weighed portion of salt is calculated. The titer of the prepared solution is checked by the same gravimetric method, i.e., by precipitating phosphorus from a 50-ml volume of the prepared solution as \(\mathrm{MgNH_4PO_4}\) and igniting it to \(\mathrm{Mg_2P_2O_7}\); the conversion factor of \(\mathrm{Mg_2P_2O_7}\) to phosphorus is 0.2783. The initial standard solution is stored in a polyethylene vessel.

Working standard phosphorus solutions with phosphorus contents of 1 μg/ml and 0.1 μg/ml are prepared each time immediately before use by appropriate dilution of the initial standard solution with bidistillate.

Glassware and equipment

1. Quartz dishes with spouts for decomposing the weighed portion of the tin being analyzed. Diameter 5–6 cm, height 2.5–3.0 cm.
2. Quartz separatory funnels with a capacity of 20–25 ml.
3. Quartz flasks with ground-glass stoppers for storing solutions.
4. Polyethylene bottles for storing solutions and bidistillate.
5. Colorimetric cylinders with ground-glass stoppers, capacities 5 and 12 ml.
6. Laboratory pipettes graduated for 1, 2, and 5 ml.
7. A stand covered with Teflon, on which the weighed portion of tin is decomposed and the tin, arsenic, and excess acids are removed at a temperature of 80–90°C.
8. Electric lamp for drying, 500 watts.
9. Thermometer for monitoring the temperature on the stand.
10. A cabinet made of organic glass, in which decomposition of the weighed portion of the tin being analyzed and removal of tin and arsenic are carried out. Cabinet width 26 cm; height above the surface of the Teflon-covered stand, 32 cm.
11. Volumetric flasks of 25, 50, 100, and 1000 ml.

Before use, the glassware is washed with chromic mixture, steamed, and rinsed with distillate and bidistillate.

Special attention must be paid to ensuring that colorimetric cylinders of the same capacity have the same internal diameter. It is preferable for the small cylinders to be quartz.

Preparation of tin samples for analysis

When high-purity tin samples are taken, the tin must be protected from contact with the fingers by a thin polyethylene film.

For convenience in taking the sample and to accelerate decomposition of the weighed portion, the tin to be analyzed is converted into thin sheets. For this purpose, into a tall porcelain beaker washed with chromic mixture, water purified ...

with hydrochloric acid and again with water; 5–10 g of tin are placed in it, and the beaker is heated on an electric hot plate to a temperature somewhat above the melting point of tin. The molten tin is poured from a height of 20 cm, dropwise, onto a clean white glazed tile.

If the tin is not overheated too much, thin flakes 0.1 to 0.2 mm thick and with an average weight of 1 g are obtained. Using plastic tweezers, they are collected in a weighing bottle with a ground-glass stopper.

From the tin flakes obtained, a sample is taken for analysis. The sample taken with plastic tweezers is placed on a watch glass, treated with two drops of hydrochloric acid, washed with bidistilled water, dried with a drying lamp in an organic-glass cabinet, and weighed on an analytical balance. The weighed sample is slid into a quartz dish for decomposition of the weighed portion. The watch glass is then weighed on an analytical balance, and the weight of tin is determined from the difference between the weight of the watch glass with tin and that of the empty glass.

Course of the Analysis

To a gram weighed portion of tin in a quartz dish with a spout, covered with a watch glass and immersed halfway in cold water, 3 ml of hydrochloric acid saturated with bromine are added; then bromine is cautiously introduced dropwise under the watch glass until the tin is completely dissolved (1.5–2.0 ml), after which 2.5 ml of hydrobromic acid are added.

After the vigorous reaction ceases, the watch glass is removed and washed, and the dish is placed under a drying lamp on a table covered with Teflon, in an organic-glass cabinet, for further decomposition of the weighed portion and removal of tin and arsenic.

The solution is evaporated to a slightly moist residue; 1 ml of hydrochloric acid (1:1) and 1 ml of hydrobromic acid are added, and the solution is evaporated to dryness at 80–90°C. To the dry residue are added 1 ml of hydrochloric acid (1:1), 5 ml of bidistilled water, 1 drop of 5 N sulfuric acid, and the solution is evaporated until the odor of hydrogen chloride has disappeared. Then, using 2.5–3 ml of hot bidistilled water, the contents of the dish are transferred to a colorimetric cylinder with a ground-glass stopper; 0.25 ml of a 1% solution of ammonium molybdate in pentanormal sulfuric acid and 0.1 ml of a 0.15% solution of hydrazine sulfate are added, the mixture is stirred, and it is placed for 10 minutes in a boiling water bath for formation of phosphomolybdenum blue.

The cylinder is then cooled and the phosphomolybdenum blue is extracted by shaking for one minute with 0.4 ml of isoamyl alcohol. The contents of the colorimetric cylinder are transferred to a 20-ml separatory funnel. The cylinder is rinsed with 1 ml of isoamyl alcohol, which is also run into the separatory funnel. The contents of the funnel are shaken for one minute, and after settling, the lower layer is discarded.

To wash the alcoholic layer free from excess ammonium molybdate, 4 ml of 0.5 N sulfuric acid, to which 0.1 g of ascorbic acid per 100 ml of solution has been added, are introduced into the funnel, and the mixture is shaken for one minute. After settling, the acid layer is run off to waste. The washing is repeated four times. The stem of the funnel is then thoroughly washed with bidistilled water to remove traces of the wash waters.

For reextraction and destruction of the phosphomolybdenum blue, the alcoholic extract is treated with 1.5 ml of bidistilled water with the addition of one drop of 25% aqueous ammonia solution, with one-minute shaking.

shaking. Re-extraction is repeated three times, collecting the aqueous ammonia re-extracts, after the layers have separated, in a colorimetric cylinder of 12 ml capacity. The volume of the re-extracts must not exceed five ml.

To form the rhodanide–molybdenum complex, 2.8 ml of 10 N chemically pure hydrochloric acid, 0.2 ml of a freshly prepared 0.04% solution of copper sulfate, and 2 ml of a 50% solution of potassium rhodanide are added to the aqueous-ammonia re-extract in the colorimetric cylinder; the volume is brought to 10 ml with bidistillate and mixed. After 10 minutes, 1 ml of isoamyl alcohol is added to the colorimetric cylinder and, with one minute of shaking, the rhodanide–molybdenum complex is extracted.

After standing for 30 minutes, the color of the alcohol layer is compared with the standard scale. The color of the alcoholic solution of the rhodanide–molybdenum complex is stable for about 3 hours. A blank test is also carried out in parallel.

Preparation of the standard scale

From the initial standard phosphorus solution containing 1000 μg of phosphorus in 1 ml, solutions containing 1 and 0.1 μg of phosphorus in 1 ml are prepared. Dilution is carried out with bidistillate.

Into a series of quartz dishes used for the decomposition of tin samples, volumes of the standard solution corresponding to phosphorus amounts of 0.00; 0.01; 0.02; 0.04; 0.05; 0.075; 0.10; 0.125; 0.15 μg are introduced.

The contents of each dish are taken through all stages of treatment of the tin sample (evaporation with hydrochloric acid, hydrobromic acid and bromine, obtaining phosphomolybdenum blue, re-extraction, formation of molybdenum rhodanide, and its extraction with isoamyl alcohol). The same reagents, and in the same quantities, are used as in the work with tin samples. In this way a standard scale is obtained, the color intensity of each point of which corresponds to a definite amount of phosphorus.

Obtaining such a standard scale is laborious. Moreover, the color of the rhodanide–molybdenum complex extracted with isoamyl alcohol is stable for only about three hours. Therefore, the color intensity of each point of the scale was imitated by us with a mixture of solutions of potassium dichromate and cobalt chloride.

To imitate the color intensities of molybdenum rhodanide extracted with isoamyl alcohol, obtained from phosphomolybdenum blue with a phosphorus amount from 0.00 to 0.125 μg, a 0.5% potassium dichromate solution and a 5% cobalt chloride solution are used; with a phosphorus amount of 0.125 μg and more, a 2% potassium dichromate solution and a 15% cobalt chloride solution are used.

The standard scale prepared in this way is stable for more than a month. It need be checked only when a new batch of reagents is used in the analysis.

The accuracy of the method was checked on standard phosphorus solutions carried through all stages of the analysis (Table 1), and by the method of additions (Table 2).

The results of parallel determinations of phosphorus in high-purity tin are given in Table 3.

Conclusions

A visual extraction-colorimetric method has been developed for the indirect determination of phosphorus in high-purity tin by the rhodanide …

molybdenum. The sensitivity of the method is \(1 \cdot 10^{-6}\%\). The relative error of determination is \(\pm 10\text{–}20\%\). The time required for five parallel analyses is about 10 hours.

REFERENCES

1. A. K. Babko, A. T. Pilipenko. Colorimetric Analysis. Moscow, Goskhimizdat, 247, 1951.

2. R. I. Alekseev. Quantitative separation and determination of anions of phosphoric, arsenic, and silicic acids by selective extraction. Zavodskaya laboratoriya, 11, 122, 2–3, 1945.

3. Colorimetric (Photocolorimetric) Methods for the Determination of Nonmetals. Collection edited by E. Busev, A. I., Moscow, Izd. inostr. lit., 12, 1963.

4. I. P. Onufrienok, Z. N. Tereshkova. Method for determining arsenic in high-purity tin. Report under economic contract No. 68/61, Tomsk, 1962.

5. S. Yu. Fainberg, N. A. Filippova. Analysis of Nonferrous-Metal Ores. State Scientific-Technical Publishing House for Ferrous and Nonferrous Metallurgy Literature, Moscow, 386, 1963.

6. N. A. Tananaev, A. P. Lokhvitskaya. On the colorimetric determination of molybdenum in the form of the rhodanide complex. Zavodskaya laboratoriya, 11, 6, 1, 1945.

7. B. E. Reznik, G. M. Ganzburg, V. V. Sachko. An accelerated variant of the rhodanide method for determining molybdenum. Zavodskaya laboratoriya, 28, 277, 3, 1962.