Study of the Conditions for the Polarographic Determination of Arsenic(III) on a Platinum Electrode
Abstract Generated abstract
This study examines conditions for determining microamounts of arsenic(III) by polarography at a platinum electrode, using preliminary cathodic concentration of arsenic as an elemental film followed by measurement of the anodic oxidation current. In 0.1 N hydrochloric acid, the anodic peak current was found to be directly proportional to arsenic(III) concentration over the tested range and to increase linearly with electrolysis time from 1 to 10 minutes under unstirred conditions. The effect of accumulation potential was also evaluated, with diminished current at more negative potentials attributed to hydrogen evolution and possible arsine formation, leading the authors to select 0.8 V versus SCE as the working potential for further measurements.
Full Text
Study of the Conditions for the Polarographic Determination of Arsenic(III) at a Platinum Electrode
L. F. Trushina, T. P. Barsukova
(Presented by the seminar of the Department of Inorganic Chemistry, KhTF)
There are comparatively few works in the literature on the polarographic determination of arsenic(III), mainly at amalgam and mercury dropping electrodes [1–5].
The aim of the present work was to study the possibility of determining microamounts of arsenic from the electrooxidation current of arsenic(III), preliminarily concentrated in the form of a film on the surface of a platinum electrode [6].
As shown in [6], the reduction of arsenic(III) against the background of a 0.1 N hydrochloric acid solution proceeds with the formation on the electrode of a film of elemental arsenic.
We investigated the dependence of the magnitude of the anodic current of arsenic(III) on its concentration in solution for the interval \(10^{-3} - 5 \cdot 10^{-5}\) mol/L, with an electrolysis time of 10 min and a volume of the solution studied of 5 ml, Table I. The value \(I_{\text{p.an.}}^{\mathrm{As^{3+}}}\), as is evident from Table I, is directly proportional to the concentration of arsenic(III) ions in solution (the ratio
\[ \frac{I_{\text{p.an.}}^{\mathrm{As^{3+}}}}{C} \simeq const \]
).
Table I
| \(C\) of arsenic, mol/L | \(I_{\text{p.an.}}^{\mathrm{As^{3+}}}\), μA | \(\dfrac{I_{\text{p.an.}}^{\mathrm{As^{3+}}}}{C}\) |
|---|---|---|
| \(1 \cdot 10^{-3}\) | 42.0 | 42 |
| \(5 \cdot 10^{-4}\) | 22.5 | 45 |
| \(2 \cdot 10^{-4}\) | 8.6 | 43 |
| \(1 \cdot 10^{-4}\) | 4.1 | 41 |
| \(5 \cdot 10^{-6}\) | 2.1 | 42 |
The dependence of the magnitude of the anodic current of arsenic on the electrolysis time under conditions in which the solution was not stirred was studied in the range of 1–10 min. As the results of the studies showed, this dependence is linear.
The effect of the electrolysis potential on the magnitude of the anodic current of arsenic(III) was studied from \(+0.1\) to \(-1.8\) volts (vs. SCE).
It was found that at potentials of 1.5–1.7 V (vs. SCE) the anodic current of arsenic decreases somewhat (20–30%). The decrease in the magnitude of \(I_{p.\,an.}\) of arsenic at an accumulation potential of 1.5–1.7 volts can apparently be explained by two causes:
1) discharge of hydrogen ions at the electrode with the formation of gaseous hydrogen, which screens the platinum surface;
2) formation of arsine, as a result of which the concentration of arsenic(III) ions in the solution decreases. The latter is in agreement with the data of [4]; however, the evolution of arsine occurs at a cathode potential of \(-1.4\)–\(-1.5\) V, whereas in [4] this potential is \(-0.55\)–\(-0.6\) V. As the optimum potential at which our further studies were carried out, a potential of 0.8 V was chosen.
Conclusions
The conditions (concentration of arsenic(III) in solution, electrolysis time, electrolysis potential) for the polarographic determination of arsenic(III) at a platinum electrode have been studied.
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