A Study on the Determination of Microamounts of Arsenic by the PPN Method
Abstract Generated abstract
This study examines the determination of trace arsenic in high-purity materials by film polarography with accumulation using a gold electrode in hydrochloric and tartaric acid media. The authors characterize the arsenic anodic response, finding a maximum at a pre-electrolysis potential of -0.5 V, linear dependence on arsenic concentration up to 3.2 x 10^-5 g-ion/l, and an absolute sensitivity of 6 x 10^-7 g for a 1 minute electrolysis. Because cadmium interferes with the arsenic peak, a procedure is developed that combines vacuum distillation of cadmium, conversion and distillation of arsenic as volatile arsenic trichloride from hydrochloric-sulfuric acid solution, absorption in background electrolyte, and polarographic measurement. The method is applied to cadmium and adapted for nitric acid and water after preliminary concentration by evaporation.
Full Text
STUDIES ON THE DETERMINATION OF TRACE AMOUNTS OF ARSENIC BY THE PPN METHOD
A. A. Kaplin, N. A. Veits, A. G. Stromberg, L. F. Trushina
In work [1] the fundamental possibility of determining arsenic by the method of film polarography with accumulation (PPN) was shown. In work [2] a procedure was developed for determining arsenic by this method on a platinum electrode in hydrochloric acid.
The purpose of the present work is to study the possibility of determining trace amounts of arsenic in various objects by the PPN method. In view of the considerable increase in sensitivity and the satisfactory reproducibility (95%) of the anodic peaks of arsenic in the presence of gold ions, both on platinum [1] and on graphite electrodes, a gold electrode was used for analytical purposes. A saturated calomel electrode was used as the reference electrode. The background was 1 M hydrochloric and 1 M tartaric acids.
The dependence of the magnitude of the anodic current of arsenic on the pre-electrolysis potential passes through a maximum at \(\varphi = -0.5\ \mathrm{V}\). The dependence of the electrooxidation current of arsenic on the concentration of \(\mathrm{As}(3+)\) ions in solution is linear up to a concentration of \(3.2 \cdot 10^{-5}\ \mathrm{g\text{-}ion/l}\), which makes it possible under these conditions to determine the arsenic content by the method of additions. The absolute sensitivity of arsenic determination on a gold electrode with an electrolysis time of 1 min is \(6 \cdot 10^{-7}\ \mathrm{g}\).
Because of the interfering effect of cadmium on the anodic peak of arsenic (the arsenic peak disappears at a Cd concentration of \(1.8 \cdot 10^{-4}\ \mathrm{g\text{-}ion/l}\) in solution), the determination of arsenic in cadmium is carried out with separation of the matrix. In order to reduce the blank sample for separation of the matrix, the method of distilling cadmium in vacuum was used [3]. According to the data of [4], arsenic, interacting with cadmium with the formation of nonvolatile intermetallic compounds, is concentrated in the residue. The minimum residual amounts of cadmium containing arsenic, after distillation of the bulk (from a 10 g sample), are \(4 \cdot 10^{-4}\)—\(1 \cdot 10^{-3}\ \mathrm{g}\). Such amounts of cadmium interfere with the determination of arsenic by the PPN method. For quantitative separation of arsenic from the cadmium remaining after vacuum distillation and from most concentrated impurities of heavy metals, distillation of arsenic is proposed in the form of its readily volatile trichloride from hydrochloric–sulfuric acid solutions, followed by absorption of the \(\mathrm{AsCl}_3\) vapors by the background solution.
The distillation of arsenic in the form of \(\mathrm{AsCl}_3\) is hindered by \(\mathrm{NO}_3^{-}\) ions [15]. Heating a nitric-acid solution of the sample with a saturated solution of ammonium oxalate [6] does not give positive results, since the oxalate ion considerably reduces the sensitivity of arsenic determination. Therefore
Results of distillation of arsenic as \(\mathrm{As}^{3+}\) from hydrochloric–sulfuric acid solutions
Table 1
| Introduced into the evaporator, g | \(n\) | Found in the receiver, g \(x\) | \(r\alpha = 0{,}95\) | \(S_x\) | \(\pm E\) | \(S\) |
|---|---|---|---|---|---|---|
| \(1{,}2 \cdot 10^{-6}\) | 4 | \(1{,}134 \cdot 10^{-6}\) | \(1{,}028 \cdot 10^{-7}\) | 3,2 | \(1{,}014 \cdot 10^{-7}\) | \(3{,}33 \cdot 10^{-7}\) |
Results of the analysis of cadmium for arsenic content
Table 2
| \(n\) | Found As, %, mean | \(S_x\) | \(r\alpha = 0{,}95\) | \(S_x\) | \(\pm E\) | \(S\) |
|---|---|---|---|---|---|---|
| 4 | \(4 \cdot 10^{-6}\) | \(0{,}81 \cdot 10^{-6}\) | 3,2 | \(0{,}405 \cdot 10^{-6}\) | \(1{,}295 \cdot 10^{-6}\) | 0,32 |
dissolution of the residue after vacuum distillation was carried out either in conc. $H_2SO_4$ at 200–220°C, or in nitric acid (1:1), followed by removal of nitrogen oxides by evaporation with conc. $H_2SO_4$ to $SO_3$ fumes. As a reducing agent for arsenic to the polarographically active form As (3+) in a conc. $H_2SO_4$ medium, sulfuric-acid hydrazine is used for quantitative reduction. The results of distillation of $AsCl_3$ from hydrochloric–sulfuric acid solutions of synthetic samples containing 0.1 g of cadmium are presented in Table 1. On the basis of the investigations carried out, the following procedure is proposed for determining arsenic in high-purity cadmium.
10 g of cadmium is distilled off in vacuum at $10^{-1}$–$10^{-2}$ mm Hg and 400–450°C. The cadmium residue is dissolved in 0.5 ml of $HNO_3$ (1:1). $NO_3^-$ ions are removed by heating the nitric acid solution with 0.5 ml of $H_2SO_4$ (1:1) to $SO_3$ fumes. The evaporated solution is heated for 10–15 minutes with several crystals of hydrazine; after cooling, 0.5 ml of $H_2O$ is added, then 0.5 ml of 7–8 M HCl. $AsCl_3$ is distilled off at 140°C, while passing an inert gas, into a receiver with background solution, cooled with running water. After the distillation is complete, the solution is polarographed. The results of the cadmium analysis are given in Table 2.
The procedure for determining arsenic in nitric acid and water is based on preliminary concentration of the impurities by evaporating the sample, with subsequent operations analogous to the determination of arsenic in cadmium, but without separation of arsenic from accompanying impurities.
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