Extraction-Amalgam-Polarographic Method for the Determination of Traces of Tin and Cadmium in High-Purity Iron
Abstract Generated abstract
This paper develops an extraction, amalgam, polarographic procedure for determining trace tin and cadmium in high-purity iron after separation from the iron matrix as iodide complexes extracted into diethyl ether. The authors examine the conductivity of ether, ethanol background mixtures and optimize potassium iodide concentration, sulfuric acid acidity, and extraction time to make the extract suitable for polarographic measurement while maintaining sensitivity. Under the selected conditions, tin and cadmium show little mutual interference over the studied concentration range, and the method is applied to grade 171913 ZhS-O iron, giving reported contents of approximately 5.28 · 10⁻⁷ for tin and 1.21 · 10⁻⁴ percent for cadmium with confidence estimates.
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EXTRACTION–AMALGAM–POLAROGRAPHIC METHOD FOR DETERMINING TRACES OF TIN AND CADMIUM IN HIGH-PURITY IRON
Yu. A. Karbanov, S. P. Tarzemyanova
(Presented by the scientific seminar of the Department of Analytical Chemistry)
For separating tin and cadmium from the matrix, extraction of them with diethyl ether in the form of iodide complexes was used [1]. The anhydrous extract obtained in this way, consisting of diethyl ether together with the extracted iodide complexes of cadmium or tin, is in itself unsuitable for carrying out polarographic measurements in it because of the high resistance of the solution (low electrical conductivity) [2].
One of the necessary conditions that background solutions in polarography must satisfy is their sufficiently high electrical conductivity, which, in order of magnitude, must be no lower than the electrical conductivity of a 0.1 N aqueous KCl solution. In this connection, the first question that arises is that of selecting an electrically conducting mixture, one of whose components is the extract being analyzed. We studied the electrical conductivity of the binary anhydrous mixture diethyl ether—96% \( \mathrm{C_2H_5OH} \), at different ratios of the indicated components in the mixture, but at a constant concentration of indifferent electrolyte in it, equal to \(0.2\,N\ \mathrm{NH_4NO_3}\).
The electrical-conductivity measurements were carried out by the Kohlrausch method in a vessel specially intended for solutions with high resistance. The apparatus included: an audio-frequency generator, an alternating-current bridge, and an electronic oscilloscope as the null instrument.
From analysis of the obtained isotherm of specific electrical conductivity it follows that, at a component ratio in the mixture equal to 1:1, the electrical conductivity is \(3.2 \cdot 10^{-3}\ \Omega^{-1}\cdot \mathrm{cm}^{-1}\), which, in order of magnitude, corresponds to the electrical conductivity of 0.1 N aqueous KCl. As the fraction of 96% \( \mathrm{C_2H_5OH} \) in the mixture is increased, its electrical conductivity continues to increase. On the basis of the data obtained, in the amalgam–polarographic determination of traces of tin and cadmium the anhydrous extract was mixed with 96% ethyl alcohol in a ratio of 1:2. We did not take ratios in which three or more volume parts of alcohol correspond to one volume part of extract, since with an increase in the proportion of alcohol in the background solution the concentration of the element being determined decreases and, consequently, the sensitivity of the method also decreases.
Further studies were carried out to determine the optimum concentration of ligand in the extraction of tin and cadmium in the form of iodide complexes from a sulfuric-acid medium. The data obtained are presented in Table 1.
As is evident from the data in the table, the optimum concentration in sulfuric-acid solution \((0.25\,N\ \mathrm{H_2SO_4})\) for cadmium is \(0.044\,N\), and for tin \(0.040\,N\).
Table 1
| \([\mathrm{KI}]\), g-mol/l | Cadmium: introduced, g-ion/l | Cadmium: found, g-ion/l | Cadmium: % extracted | Tin: introduced, g-ion/l | Tin: found, g-ion/l | Tin: % extracted |
|---|---|---|---|---|---|---|
| 0.01 | \(7.7\cdot10^{-7}\) | \(5.10\cdot10^{-7}\) | 66.0 | \(7.7\cdot10^{-7}\) | \(3.85\cdot10^{-7}\) | 50.0 |
| 0.02 | \(5.36\cdot10^{-7}\) | 69.7 | \(4.32\cdot10^{-7}\) | 56.1 | ||
| 0.03 | \(6.52\cdot10^{-7}\) | 84.7 | \(5.76\cdot10^{-7}\) | 74.7 | ||
| 0.04 | \(7.25\cdot10^{-7}\) | 94.2 | \(6.25\cdot10^{-7}\) | 81.2 | ||
| 0.05 | \(6.75\cdot10^{-7}\) | 88.0 | \(6.25\cdot10^{-7}\) | 81.2 | ||
| 0.06 | \(7.00\cdot10^{-7}\) | 91.0 | \(5.76\cdot10^{-7}\) | 76.0 | ||
| \([\mathrm{H_2SO_4}]\), g-mol/l | Cadmium: introduced, g-ion/l | Cadmium: found, g-ion/l | Cadmium: % extracted | Tin: introduced, g-ion/l | Tin: found, g-ion/l | Tin: % extracted |
| 0.25 | \(7.3\cdot10^{-7}\) | 95 | \(6.2\cdot10^{-7}\) | 80.5 | ||
| 0.10 | \(7.4\cdot10^{-7}\) | 96 | \(6.4\cdot10^{-7}\) | 83.0 | ||
| 0.05 | \(7.7\cdot10^{-7}\) | \(7.55\cdot10^{-7}\) | 98 | \(7.7\cdot10^{-7}\) | \(6.6\cdot10^{-7}\) | 86.4 |
| 0.04 | \(7.55\cdot10^{-7}\) | 98 | \(6.76\cdot10^{-7}\) | 88.0 | ||
| 0.01 | \(7.55\cdot10^{-7}\) | 98 | \(6.7\cdot10^{-7}\) | 87.3 |
Note: The ratio of the organic and aqueous phases was \(1:4\); the time of a single extraction was 3 min.
The investigations were continued in order to determine the optimum acidity of the initial medium at a constant concentration of potassium iodide in it, equal to \(0.044\,N\) (Table 1). It follows from Table 1 that, as the acidity of the solution is varied in the range \(0.25 \div 0.01\,N\) with respect to \(\mathrm{H_2SO_4}\), the percentage extraction of cadmium remains practically constant. For tin, however, the optimum acidity is \(0.04\,N\) with respect to \(\mathrm{H_2SO_4}\).
Using cadmium as an example, the optimum time for a single extraction was investigated. The concentrations of potassium iodide and sulfuric acid were maintained at their optimum values. The extraction time was varied within the limits from 30 sec to 5 min. As a result of these investigations it was found that the optimum time for establishing extraction equilibrium between the aqueous and organic phases, at their ratio of \(4:1\), is 2.4 min.
The mutual influence of tin and cadmium during their simultaneous separation by extraction with diethyl ether from a sulfuric-acid medium in the form of iodide complexes was also studied. The concentration of tin and cadmium in the initial solution varied within the limits \(1\cdot10^{-6}\)–\(5\cdot10^{-7}\) g-ion/l.
The initial solution was prepared as follows. A weighed portion of especially pure iron of 0.1 g was dissolved, with gentle heating, in 5 ml of \(4N\ \mathrm{HCl}\). After dissolution, the contents were evaporated to a moist residue, cooled, and dissolved in 5 ml of \(\mathrm{H_2O}\). An addition of standard tin and cadmium solution was introduced. To the mixture obtained were added
Table 2
| Number of parallel determinations | Standard deviation of a single measurement | Standard error of the arithmetic mean | Values of the parameters $\alpha$ and $t$ | Absolute deviation of the mean from the true value | Result of the analysis, % |
|---|---|---|---|---|---|
| Tin content | Tin content | Tin content | Tin content | Tin content | Tin content |
| 9 | $S_x=\sqrt{\dfrac{\sum (x_i-\bar{x})^2}{n-1}}=0.09\cdot10^{-3}$ | $S_{\bar{x}}=\pm\dfrac{S_x}{\sqrt{n}}=\pm0.03\cdot10^{-3}$ | $\alpha=0.95$ $t=2.31$ |
$\varepsilon=\pm t\cdot S_{\bar{x}}=\pm0.07\cdot10^{-3}$ | $(5.28\pm0.07)\cdot10^{-7}$ |
| Cadmium content | Cadmium content | Cadmium content | Cadmium content | Cadmium content | Cadmium content |
| 7 | $S_x=\sqrt{\dfrac{\sum (x_i-\bar{x})^2}{n-1}}=0.069\cdot10^{-4}$ | $S_{\bar{x}}=\pm\dfrac{S_x}{\sqrt{n}}=\pm0.026\cdot10^{-4}$ | $\alpha=0.95$ $t=2.45$ |
$\varepsilon=\pm t\cdot S_{\bar{x}}=\pm0.064\cdot10^{-4}$ | $(1.21\pm0.06)\cdot10^{-4}$ |
0.15 ml of 1.5 N KI in 1.5 N H₂SO₄. The extraction conditions were kept optimal. Experience shows that when the cadmium concentration in the initial solution is changed, and at a constant concentration of tin in it, the cadmium content in the extract changes in proportion to the cadmium concentration in the initial solution, and vice versa.
Thus, within the indicated limits \((1 \cdot 10^{-6} \div 5 \cdot 10^{-7}\ \text{g-ion/l})\), the mutual influence of tin and cadmium in their simultaneous determination is practically not manifested.
Procedure for the extraction–amalgam–polarographic determination of traces of tin and cadmium in high-purity iron
A 0.1 g weighed sample of iron is dissolved in a quartz beaker with gentle heating in 5 ml of 4 N HCl. After dissolution, the contents are evaporated to a moist residue. With gentle heating, the residue is dissolved in 5 ml of tridistilled water. It is transferred to a 25 ml separatory funnel, mixed with 0.15 ml in 1.5 N H₂SO₄, and, after addition of 1.5 ml of diethyl ether, extracted once for 3 min. The aqueous phase is discarded, and the ether extract is washed with 5 ml of H₂O (re-extraction of iron and removal of H₂SO₄). The washed extract is transferred to a polarographic cell, mixed with 3.5 ml of 0.2 N NH₄NO₃ in C₂H₅OH (96%), and after the solution has been purged with nitrogen for 5 min, it is polarographed.
Analysis conditions: electrolysis potential \(\varphi = -1.2\ \text{V}\); electrolysis time \(\tau_\text{e} = 5\ \text{min}\); instrument sensitivity \(1 \cdot 10^{-9}\ \text{A/mm}\).
The results of the analysis of samples of iron, grade 171913 ZhS-O, for tin and cadmium are presented in Table 2.
REFERENCES
- J. Morrison, G. Freiser. Extraction in Analytical Chemistry. Leningrad, State Scientific-Technical Publishing House of Chemical Literature, 1960.
- Yu. A. Karbainov, A. G. Stromberg. In the collection Modern Methods of Chemical and Spectral Analysis. Moscow, Metallurgizdat, 1967, p. 230.