Yu. L. Lel’chuk, P. V. Kristalev, L. L. Skripova, L. B. Kristaleva

Extraction-Colorimetric Method for the Determination of Traces of Iron in High-Purity Tin

(Presented by the scientific seminar of the Faculty of Chemical Technology)

Using pure iron salts, mixtures of salts, and samples of high-purity tin, we studied the sensitivity, accuracy, and reproducibility of three photometric methods for determining microquantities of iron: the rhodanide, phenanthroline, and $\alpha,\alpha'$-dipyridyl methods, as well as the possibility of applying these methods to the determination of traces of iron in high-purity tin at iron contents of $4 \cdot 10^{-5}\%$ and below. Of all the methods studied, the best proved to be the extraction-colorimetric method with $\alpha,\alpha'$-dipyridyl and a mixture of benzene with cresol as the extractant for the iron complex.

$\alpha,\alpha'$-Dipyridyl began to be used for analytical purposes in the 1940s. The first works on its analytical application in the USSR were those of Alimarin [1] on the determination of iron in minerals and rocks, and the works of Poluektov and Nazarenko [2] on its microchemical application.

The principal advantages of this reagent over other reagents currently used for the photometric determination of microquantities of iron are: the high sensitivity of the reagent, its high degree of selectivity, the comparatively wide pH range of the solution in which it can be used as a reagent for Fe, the obedience of solutions of its complex with iron to the Lambert–Beer law, and the prolonged stability of solutions of the dipyridyl iron complex.

According to Sandell [3], the sensitivity of this reagent is estimated at $0.007\ \mu\mathrm{g}$ Fe/ml with a light filter having a transmission maximum at $552\ \mathrm{m}\mu$. It can be used for the determination of iron in solutions with pH from three to nine. An excess of reagent does not affect the optical density of solutions of the dipyridyl iron complex. The color of the complex solution is stable for up to 1 year.

$\alpha,\alpha'$-Dipyridyl is readily soluble in alcohol, ether, benzene, and chloroform. Its solubility in water is $0.5\ \mathrm{g}$ in $100\ \mathrm{ml}$ [4]. According to our data, its solubility in water increases significantly if from 0.01 to 0.1 gram-equivalent of hydrochloric acid per liter of solution is added to the solution. To increase the sensitivity of the method for determining traces of iron with this reagent, we used its 2% solution in 0.01 N aqueous hydrochloric acid.

With divalent iron, a colorless solution of $\alpha,\alpha'$-dipyridyl forms a colored divalent che-

chelate complex in which the ratio of iron to $\alpha,\alpha'$-dipyridyl is 1:3. The color of the solution depends on the concentration of Fe and, as it increases, changes from pale pink to reddish violet.

In recent years a number of works [5—8] have been published on the use of this reagent for determining microamounts of iron in natural waters, in gelatin, in phosphoric acid and its salts, in high-purity antimony, and in other objects.

Grá-Kabanak [7] investigated the question of the influence of other heavy metals on the photometric determination of traces of iron with $\alpha,\alpha'$-dipyridyl. According to his data, zinc interferes with the determination, weakening the color of the iron complex, if its content exceeds that of iron by 25 times; cobalt by 15 times; nickel by 22 times; and manganese by 88 times. Gasper [8] showed that up to 100 mg of $\mathrm{Al_2O_3}$ does not interfere with the determination.

In developing a method for determining traces of iron in high-purity tin with $\alpha,\alpha'$-dipyridyl, we experimentally tested a number of methods for bringing the weighed sample of the tin being analyzed into solution and for eliminating the interfering effect of tin on the course of the determination of traces of iron; studied the influence of the solution pH on the optical density of solutions of the dipyridyl iron complex; tested a large list of extractants for extracting the colored complex; investigated the action of a number of buffer mixtures for maintaining the required pH of the solution; studied the spectral characteristics of the iron complex and the conditions for its photometry on an FEK-N-57 photoelectric colorimeter-nephelometer; and investigated the influence of the principal impurities of high-purity tin on the accuracy of determining traces of iron with $\alpha,\alpha'$-dipyridyl.

As a result of the investigation it was established that the rate of decomposition of the weighed sample is affected by the thickness of the sample of the tin being analyzed. Dissolution and decomposition of the sample proceed most rapidly if, before taking the weighed portion, the tin to be analyzed is converted into thin sheets 0.1–0.2 mm thick and weighing close to 1 g. Conditions were developed for obtaining such sheets without any accidental contamination.

Of the many reagents tested by us for decomposing the weighed sample of the tin being analyzed, the best results were given by a 1:4 mixture of hydrochloric and nitric acids. Decomposition lasts no more than 5 minutes. For decomposing 1 g of the tin being analyzed, 10–15 ml of the mixture is consumed. The mixture is prepared from hydrochloric and nitric acids purified by distillation in quartz distillation apparatus.

A study of the influence of the solution pH on the optical density of aqueous solutions of the complex of divalent iron with $\alpha,\alpha'$-dipyridyl showed that the optimum pH is in the range 3–4. However, the tin salts formed during decomposition of the weighed sample at this pH produce an abundant precipitate of hydrolysis products, making photometry of the iron complex impossible. Of the various variants for eliminating the interfering influence of tin, the best results are given by binding tin into a stable tartaric-acid complex or by complete removal of tin through decomposition of the sample with hydrochloric acid, hydrobromic acid, and bromine, followed by threefold evaporation of the solution with hydrochloric acid at a temperature of 80–90°C.

Our data on the spectral characteristics of the iron complex with $\alpha,\alpha'$-dipyridyl differ little from the latest literature data. We obtained the best results in photometry of the complex on an FEK-N-57 photoelectric colorimeter-nephelometer using a green light filter with a transmission maximum at 508 m$\mu$. As extractants for the dipyridyl iron complex we tested: benzene, nitrobenzene, chloroform, carbon tetrachloride, cre-

benzene and benzene–cresol mixtures. The best proved to be a mixture of benzene with meta-cresol. It was decided to use this mixture in visual determinations of traces of iron in the range of \(10^{-5}\)—\(10^{-6}\%\) by the standard-series method.

Independently of us, Lipshits, Smirnova, and Kulikov [6] arrived at a similar solution to a number of questions concerning the procedure for determining microquantities of iron with \(\alpha,\alpha'\)-dipyridyl in another object. The authors proposed using, for binding antimony into a stable complex, reducing trivalent iron to divalent iron, and maintaining the pH of the solution, a mixture—purified of traces of iron—of tartaric acid, sodium sulfite, sodium chloride, and sodium hydroxide hydrate. Our experiments showed that this mixture can also be used successfully in the determination of iron in high-purity tin. In our procedure we call it the complexing reducing buffer mixture.

To prepare 500 ml of the mixture, 7.5 g of sodium chloride, 30 g of tartaric acid, 15 g of sodium sulfite, and 10–11 g of solid sodium hydroxide hydrate are used. The mixture is purified from traces of iron by treating it twice with a 2% solution of \(\alpha,\alpha'\)-dipyridyl, followed by removal of the iron complex formed by extraction with a mixture of benzene and cresol (1:1).

As a result of the investigations carried out, two variants of a highly sensitive procedure for determining traces of iron in high-purity tin were developed. According to the first variant, a weighed portion of tin in a quartz beaker is dissolved in a mixture of hydrochloric and nitric acids; the solution is evaporated under an infrared drying lamp on a table covered with Teflon in an organic-glass box at a temperature of 80–90°C; the residue is treated with the complexing reducing buffer mixture, by which the pH of the solution is brought to 3.5; 2 ml of a 2% solution of \(\alpha,\alpha'\)-dipyridyl is added; the volume of the solution is brought with bidistillate in a volumetric flask to 25 or to 50 ml; and, after standing for one hour, the optical density of the solution is measured on an FEK-N-57 photoelectric colorimeter–nephelometer in 50-mm cuvettes with a green light filter having a transmission maximum at 508 \(m\mu\). Bidistillate serves as the comparison solution.

A blank is prepared analogously, to which all the same reagents are added and in the same sequence, only without the weighed portion of tin. The true optical density is found by subtracting from the optical density of the high-purity tin sample the optical density of the blank. To construct the calibration graph, a standard iron solution prepared from pure iron wire dissolved in hydrochloric acid is used.

When the iron content in the tin being analyzed is within tenths of a microgram (lying at the sensitivity threshold of the method in the photometric finish), a blank to which 10 \(\mu\)g of iron is added is used as the comparison solution. From the optical density measured in this way, the iron content in the analyzed tin sample is found on a calibration graph constructed by an analogous procedure.

When the iron content is in the range \(2\cdot10^{-5}\)—\(1\cdot10^{-6}\%\), the photocolorimetric finish is replaced by a visual finish by the standard-series method after extracting the dipyridyl iron complex with 1 ml of a mixture of benzene with cresol.

The second variant of the procedure differs from the first in that the bulk of the tin is removed in the form of tin chloride. According to this variant, the weighed ...

weighed sample of tin in a quartz beaker is treated with 2.5 ml of hydrochloric acid, 2.5 ml of hydrobromic acid, and, while cooling, 1.5 ml of bromine is added dropwise. The solution is evaporated to a slightly moist residue; 0.5 ml of hydrochloric acid is added twice, and in both cases the solution is evaporated to dryness. The dry residue is treated with 0.5 ml of hydrochloric acid, 10 ml of a complexing reducing buffer mixture is added, the solution is heated and boiled for 3–4 minutes, 2 ml of a 2% solution of α,α′-dipyridyl is added, mixed, and allowed to stand—

Table 1

Check of the accuracy of iron determination by the first variant of the procedure

Table 2

Results of parallel determinations of iron in high-purity tin

Table 3

Check of the accuracy of iron determination by the second variant of the procedure

Table 4

Verification of the accuracy of the second variant of the procedure by the method of additions

is maintained for 1 hour. The solution is then made up to 25 ml in a volumetric flask and photometered, or 1 ml of the benzene–cresol mixture is extracted and compared with the standard scale in the same way as in the first variant of the procedure.

In the second variant of the procedure, the consumption of the complexing, reducing, buffer mixture is standardized and considerably reduced. For each determination of iron only 10 ml of the mixture is consumed. In the first variant, the consumption of the mixture for bringing the pH of the solution to 3.5 varies greatly and in individual determinations reaches 50 ml or more.

Tables 1 and 2 give part of the data obtained by us in checking the accuracy and reproducibility of the determination of traces of iron by the first procedure; Tables 3 and 4 give the corresponding data for the second procedure.

Conclusions

A highly sensitive method has been developed for determining traces of iron in high-purity tin. The sensitivity of the method with photocolorimetric completion is \(2 \cdot 10^{-5}\%\); with completion by the method of standard series after extraction of the complex with a benzene–cresol mixture it is \(1 \cdot 10^{-6}\%\). The duration of the determination is 3–4 hours. The error does not exceed 10–12% relative.

Literature

1. I. P. Alimarin. Determination of ferric oxide in minerals and rocks. Zavodskaya laboratoriya, 9, 545, 1940.

2. N. S. Poluektov, V. A. Nazarenko. On the use of the ferrodipyridyl complex in microchemical analysis. Zh. prikl. kh., 10, 2105, 1937.

3. Sendel. Colorimetric Determination of Traces of Metals. Goskhimizdat, 212, 1949.

4. V. I. Kuznetsov et al. Chemical Reagents and Preparations. Moscow, Goskhimizdat, 308, 1953.

5. Grat—Cabonac M. Photometric determination of iron in phosphoric acid and its salts, Analyt. Chim. Acta, No. 6, 588, 1957.

6. B. M. Lipshits, G. K. Smirnova, F. S. Kulikov. Determination of iron in high-purity antimony. Zavodskaya laboratoriya, 10, 1199, 1961.

7. Grat—Cabonac M. Photometric determination of iron using \(\alpha,\alpha'\)-dipyridyl. RZh khimiya, No. 1, abstract 952, 1959.

8. Gasner. Photometric determination of iron in orthophosphates using \(\alpha,\alpha'\)-dipyridyl. RZh khimiya, No. 6, 299, abstract 19586, 1957.