AMALGAM-POLAROGRAPHIC DETERMINATION OF TRACE AMOUNTS OF TIN IN IRON

Yu. I. Ivanov, L. F. Zaichko, A. A. Kaplin

(Presented by the Scientific-Methodological Seminar of the Department of Physical Chemistry)

A limited number of works are known on the amalgam-polarographic determination of microimpurities of tin in iron and alloys based on it [1]. The authors eliminated the interfering influence of $\mathrm{FeCl_3}$ by adding a concentrated solution of ascorbic acid. As noted in the work, the reproducibility and sensitivity of the method are somewhat lower than usual because of the significant magnitude of the residual current. Thus, at a tin content of $\leq 1 \cdot 10^{-7}$ M/liter, analysis is impossible. In the presence of large amounts of copper in the sample, the anodic peak of tin decreases owing to the formation of the intermetallic compound $\mathrm{Sn—Cu}$.

In the presence of lead in the sample being analyzed, the quantitative determination of tin is difficult, since the electrodissolution of these elements occurs practically together on acid backgrounds [2]. The combination of extraction separation of microamounts of tin from the matrix with an amalgam-polarographic finish makes it possible to develop a highly sensitive procedure, but it is more difficult to implement because of the cumbersome analytical part.

We propose an amalgam-polarographic procedure for determining microimpurities of tin in iron, free from the shortcomings listed.

In this work, distillation of $\mathrm{SnCl_4}$ from boiling hydrochloric acid solutions of $\mathrm{FeCl_3}$ in an argon atmosphere was used in a sublimation apparatus developed for this purpose (Fig. 1). All determinations were carried out on an OH-101/1 polarograph. An electrolyzer with insert cups was used; the stationary electrode was a mercury film on a silver substrate [3]. A standard solution of $\mathrm{SnCl_4}$ ($10^4$ μg) was prepared by dissolving a weighed portion of metallic tin in concentrated hydrochloric acid in the presence of hydrogen peroxide. Solutions of lower concentrations were prepared by successive dilution in 4 N HCl.

Studies were carried out to determine the optimum conditions for distilling $\mathrm{SnCl_4}$ on synthetic mixtures and iron samples. From [4] it is known that $T_{\text{boil}}\,\mathrm{SnCl_4}=115^\circ\mathrm{C}$, and $T_{\text{boil}}\,\mathrm{FeCl_3}=320^\circ\mathrm{C}$, but practically complete separation of traces of $\mathrm{SnCl_4}$ from the matrix—$\mathrm{FeCl_3}$—takes place, according to our data, at a higher temperature. Figure 2 presents experimental results on the quantitative separation of $\mathrm{SnCl_4}$ (calculated as metal) as a function of temperature from 4 N HCl solutions (curve 1) and in the presence of 0.04 g of iron in the form of $\mathrm{FeCl_3}$ (curve 2). From the results given it follows that the optimum temperature

The distillation temperature of \( \mathrm{SnCl_4} \) from \( \mathrm{FeCl_3} \) solutions is \(250^\circ\mathrm{C}\). When the iron content in a boiling hydrochloric-acid solution exceeds 0.04 g, considerable amounts of \( \mathrm{FeCl_3} \) volatilize together with the microcomponent being determined, which hampers the amalgam-polarographic determination of traces of tin owing to an increase in the residual current. This is explained by the fact that iron(III) chloride begins to sublime already at \(100^\circ\mathrm{C}\) [5].

Fig. 1. Sublimation apparatus. Material—quartz.
1—argon fitting, 2—evaporation vessel, 3—adapter, 4—opening for argon discharge into the atmosphere, 5—capillary, 6—receiver, 7—cooling jacket.

Fig. 2. Dependence of the completeness of extraction of Sn (calculated as metal) from hydrochloric-acid solutions on temperature.
1—from 4 M HCl solutions; 2—from 4 N HCl solutions in the presence of 0.04 g of iron as \( \mathrm{FeCl_3} \).

The completeness of the determination of tin under analytical conditions was studied. Experimental conditions: iron sample—0.04 g, distillation temperature—\(250^\circ\mathrm{C}\), electrolysis potential—\(-1.0\) V, electrolysis time—3 min, polarograph sensitivity—\(3 \cdot 10^{-8}\) A/mm, receiver—3 ml of 2 M HCl. The results are given in Table 1.

On the basis of the investigations carried out, the following procedure was developed for determining traces of tin in iron.

Reagents: hydrochloric acid, twice distilled in quartz apparatus;
water, twice distilled in quartz apparatus;
hydrogen peroxide, not subjected to additional purification.

Procedure. A weighed portion of iron, 0.2 g, is dissolved in 10 ml of 8 N HCl in a 25-ml volumetric flask while heating to 70–80°C.

Table 1

After cooling, 2 ml of hydrogen peroxide is added dropwise to the resulting solution to oxidize Fe²⁺ to Fe³⁺ and Sn²⁺ to Sn⁴⁺, and the flask is brought to the mark with 2 N HCl. Five milliliters of the resulting FeCl₃ solution are placed in the sublimation apparatus (Figs. 1, 2), and SnCl₄ is distilled off at \(T = 250^\circ\mathrm{C}\). Beforehand, 3 ml of 2 N HCl is poured into receiver 6 (Fig. 1). Argon heated to the same temperature is used as the carrier. Distillation is carried out until the hydrochloric acid has completely evaporated, with ignition of the residue for 30 min at the specified temperature.

The polarographic determination of trace tin is carried out directly in the receiver. The receiver is inserted into the electrolyzer, gaseous argon containing not more than 0.001% oxygen is passed through the solution under investigation, and electrolytic accumulation of Sn is carried out for 3 min at a potential of −1.0 V. All potentials in this work are given relative to the saturated calomel electrode. The tin content is determined by the method of standard additions. The distillation time is 1.5 h; the time for the amalgam-polarographic completion is 15 min.

Five parallel samples of PZhS-2 iron were analyzed. Tin found was \((8.2 \pm 1.2)\cdot 10^{-5}\)% by weight. The maximum sensitivity of the determination is \(3\cdot 10^{-6}\)% by weight at an anodic peak height of 0.5 cm.

Similar studies were carried out for the determination of Sb in iron. Separation of SbCl₅ from FeCl₃ does not occur, probably because of the formation of the complex \(\mathrm{SbCl_5\cdot FeCl_3\cdot 8H_2O}\) [6].

Conclusions. A procedure has been proposed for the amalgam-polarographic determination of tin in iron and iron-based alloys with preliminary separation of Sn from the matrix—Fe—by distillation. The maximum sensitivity of the determination is \(3\cdot 10^{-6}\)% by weight. The analysis time for one sample is 2 h. The determination error is ±15%.

REFERENCES

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3. V. A. Igolinskii. Methods for the Analysis of Chemical Reagents and Preparations. Proceedings of IREA, issue 5–6, 1963.
4. Chemist’s Handbook, vol. 2, Moscow–Leningrad, Goskhimizdat, 1964.
5. P. P. Korostelev. Preparation of Solutions for Chemical-Analytical Work. Moscow, Nauka Publishing House, 1964.
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