The Potential of Inert Electrons in Sulfurous Acid Solution

Certain reducing substances, such as sulfurous acid, formic acid, or oxalic acid, show in their chemical behavior a much smaller reducing power than that calculated from the free-energy changes attending their conversion into their ordinary oxidation products, sulfuric acid or carbon dioxide and water. This fact is especially pronounced in the case of sulfurous acid, where a fairly definite electrode-potential (about -0.4 volts) results, which is in much better agreement with its known chemical behavior than is the potential (-0.14 volts) calculated for the conversion of SO2 (1 atm.) to SO4= (1 m.) The probable nature of the electrode process has been considered by Carter and James. In view of the apparent discrepancy with the calculated sulfite-sulfate potential, they considered the observed potential to be due to the tendency of the sulfur dioxide to be reduced rather than oxidized, an assumption which is also justified by its chemical behavior, which will be discussed later. Since experiments in which sulfur was added to the electrode vessel did not improve the constancy or reproducibility of the potential (which varied over 0.05 volts), they concluded that the sulfur was without effect, and that an intermediate reduction-product of sulfurous acid was concerned, which their experiments on the cathodic reduction in acid solution led them to believe was hydrosulfurous acid, H2S2O4. The present work is a further contribution to the interpretation of this sulfur dioxide potential. It is concerned chiefly with the potentials exhibited at a platinized platinum electrode in a half-cell containing sulfur dioxide in acid solution under various conditions of concentration, temperature, and agitation, and in the presence of certain other substances.