Polarographic Investigations of Vitamin C. I. On the Oxidation Waves of l-Ascorbic Acid and the Reduction Wave of Dehydro-l-ascorbic Acid

Abstract

Abstract l-Ascorbic acid (A. A.) shows an oxidation wave at pH less than 8 and two in a more alkaline range. After standing of A. A. in alkaline solution, the third oxidation wave has been observed. Dehydro-l-ascorbic acid (D. A. A.) has been also tested in alkaline solution. The mechanism of the electrode oxidation of A. A. and its decomposition in alkaline solution are discussed. The reduction wave of D. A. A. has been obtained at pH 2 to 5, in which the half-wave potential is about −0.3 to −0.5 V. (vs. N.C.E.). The wave height is about 1/1000 of its expected diffusion current at room temperature. The wave has all the typical characteristics of the kinetic current. The wave height is roughly proportional to the concentration and is not appreciably influenced by pH and the components of the buffer solution, but the shape of the wave is strongly deformed by adding small amounts of surface active substance such as gelatin. The reduction product of D. A. A. at the dropping mercury electrode has been polarographically proved to be A. A. Discussions are made on the configuration of the inactive form of D. A. A. and the reversibility of the A. A. and D. A. A. system.