Double Polarographic Wave of Cobaltous Ion Near 0" C. in Thiocyanate Medium SIR: The polarographic reduction of divalent cobalt at ordinary temperatures and in a thiocyanate medium produces only one wave. This wave is due to a single reduction step involving two electrons (1). At temperatures close t o 0" C. two well defined waves of approximately equal height are formed. This behavior is best exhibited when cobalt concentrations of less than about 5 X lo-' M are used and potassium thiocyanate concentrations are between 0.1 and 0.2M. The double wave is believed to be due to the stability of monovalent cobalt complexes at the relatively low temperatures at which electrolysis takes place, and to the reducibility of these complexes at a more negative potential than that of divalent complexes. EXPERIMENTAL
The usual polarographic technique and apparatus mere used. Polarograms were obtained with a Sargent Model XV polarograph. Both the saturated calomel and a mercury pool were used as
anodes, the latter being more convenient because the agar bridge connecting the calomel tended to crack at low temperatures. A dropping mercury electrode with a flow of 1.5 to 1.7 mg. per second and drop times of 4 to 5 seconds was used. Cobalt solutions were made from reagent grade cobaltous chloride in distilled water, with the desired concentrations of potassium thiocyanate as complexing agent and supporting electrolyte. Oxygen was removed by bubbling nitrogen through the solution. The cell was immersed in a thermostated bath and was brought to the desired temperature, while nitrogen was still bubbling through the solution. In this neutral solution (pH about 6.5) it is unlikely that any oxidation of cobaltous ion could take place. DISCUSSION
The double wave of cobaltous ion, which is obtained when the electrolysis takes place at temperatures near 0' C. in a thiocyanate medium, is probably the result of two electroreduction steps, which can be visualized to occur as follows :
It appears that a t low temperatures monovalent cobalt complexes can exist, and they require a more cathodic potential for reduction than the divalent complexes. As the temperature rises, the stability of monovalent complexes is decreased; a t akout 18" C. there are practically no such complexes in the solution, and only one wave is produced involving two electrons. I n Figure 1 are shown polarograms obtained with the S.C.E. as an anode. I n Figure 2 is shown the effect of temperature on the polarographic waves; these polarograms Mere obtained against a mercury pool anode. I n order t o lower the temperature below zero, solutions containing about 5% methanol were used. The ratio of the wave heights does not change by further lowering of the temperature below zero. Were this not so, the explanation for the
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