On the Concentration of Trace Amounts of Variable-Valence Elements in Accumulation Polarography
Abstract Generated abstract
This paper examines the concentration of trace variable-valence metals for stripping polarography, focusing on cases where such metals cannot be reduced to zero valence on stationary electrodes under ordinary conditions. Using molybdenum as a model element, the authors study electrodeposition at controlled current density on a graphite electrode in alkaline solution, with the deposited amount assessed by neutron-activation analysis. The experiments identify an approximate optimal current-density range of 20 to 100 mA/cm² and show that current yield decreases sharply as molybdenum concentration falls, although the deposited amount may still be sufficient for subsequent anodic dissolution and polarographic determination. The work demonstrates the feasibility of concentrating microamounts of variable-valence metals by controlled-current electrodeposition for use in stripping polarography.
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On the Concentration of Trace Amounts of Variable-Valence Elements in Stripping Polarography
V. A. Bramin, A. A. Kaplin
(Presented by the scientific-methodological seminar of the Department of Physical Chemistry)
Methods of stripping polarography (APN, PPN) make it possible to determine microimpurities under the condition that the latter are electrodeposited on an electrode in the form of atoms of zero valence. However, many metals of variable valence, for example Nb, Ta, W, Mo, V, Ti, etc., cannot be reduced to zero valence on stationary electrodes under the usual conditions of the stripping polarography method. To determine trace amounts of variable-valence metals by stripping polarography methods, it is necessary at the electrodeposition stage to select conditions for depositing these metals on stationary electrodes in a stable form.
The literature examines in considerable detail the processes of electrodeposition of variable-valence metals for obtaining galvanic coatings [1—3]. Electrodeposition is carried out from electrolytes in which the concentration of the element being deposited is 1—100 g/l, which is 5—7 orders of magnitude higher than the concentrations of interest for the analytical chemistry of trace elements.
The present communication is devoted to a discussion of the possibility of electrodeposition of trace amounts of metals at a controlled current density and to the selection of optimal conditions for the maximum deposition of trace amounts of molybdenum. Preliminary studies are described in [4].
To investigate the possibility of quantitative electrodeposition of trace amounts of variable-valence metals at a controlled current density, molybdenum was selected. The amount of metal deposited on the electrode was monitored by the neutron-activation method.
Experimental Procedure
A weighed portion of molybdenum oxide (1—10 mg) is irradiated in pure aluminum foil with thermal neutrons for 4—10 hours. Then the irradiated molybdenum oxide is dissolved in 10 ml of a 5% NaOH solution.
To obtain initial solutions with concentrations of \(10^{-3}\)—\(10^{-1}\) mg/ml, successive dilution with a 5% NaOH solution is carried out.
Electrodeposition is carried out on a PO-312 potentiostat in a cell with replaceable beakers. A graphite electrode is used as the indicator electrode, and a saturated calomel electrode as the auxiliary electrode. Deposition is carried out while stirring the solution with a stream of nitrogen.
Using the proposed procedure, the dependence of the deposition rate of microamounts of molybdenum on current density was studied. It was shown that the optimum current density is 20–100 mA/cm² (Table 1), which agrees quite well with the literature data [3].
Table 1
| $i$, mA/cm²—electrode activity (cpm) | 6 | 12 | 20 | 40 | 60 | 100 |
|---|---|---|---|---|---|---|
| 0.5 mg/ml, Mo | 120 | 228 | 225 | 260 | 228 | 205 |
| 0.1 mg/ml, Mo | 4115 | 5090 | 6970 | 6850 | 7080 | 8300 |
From the data in Table 2 it follows that, as the concentration of molybdenum in solution decreases, its current yield decreases significantly and, consequently, so does the absolute amount of metal deposited on the electrode1. For example, at a molybdenum concentration in solution of $10^{-5}$ g/ml, about $10^{-6}$ mg is deposited in 10 min. A decrease in the degree of depletion of the solution upon going to microamounts of the element being determined does not, however, preclude the possibility of polarographic determination of molybdenum after anodic dissolution of the deposit.
Table 2
| Concentration, mg/ml | 3.0 | 0.5 | 0.3 | 0.1 | 0.01 |
|---|---|---|---|---|---|
| $i$, mA/cm² | 80 | 40 | 60 | 20 | 40 |
| $\tau$, min | 10 | 10 | 10 | 10 | 10 |
| Current yield, % | 24.5 | 14.4 | $3 \cdot 10^{-2}$ | $1.67 \cdot 10^{-2}$ | $10^{-4}$ |
Electrode activity is a quantity proportional to the amount of metal deposited on the electrode. To determine the conditions for complete electrodeposition of microamounts of molybdenum from a solution containing the minimum concentration of the element, a series of experiments was carried out in which the concentration of molybdenum in the solution was gradually decreased. The results are given in Table 2.
Conclusions
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Conditions have been found for the electrodeposition of microamounts of molybdenum at a controlled current density. Neutron-activation analysis was used to estimate the current yield.
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The fundamental possibility of applying this method for concentrating microamounts of metals of variable valence in stripping polarography has been demonstrated.
Literature
- L. F. Yntema, “J. Am. Chem. Soc.,” 54, 3775 (1932).
- P. M. Vyacheslavov. Electroplating of Noble and Rare Metals. Leningrad, “Mashinostroenie,” 1970.
- V. I. Lainer. Modern Electroplating. Moscow, “Metallurgiya,” 1967.
- V. A. Bramin, S. V. Obrazov, A. A. Kaplin. Abstracts of Reports of the III All-Union Conference on Activation Analysis. Tashkent, 1972.
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In the proposed concentration of Mo on the electrode in the form of oxides of variable valence, a relative increase in the fraction of soluble oxides with decreasing Mo concentration is not excluded. ↩