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

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.

Catalytic and Kinetic Waves in Polarography

“…Kinetic currents limited by the rate of dehydration of the keto group were observed in the reduction of dehydroascorbic acid (A) [194,195]. The ilim value of the polarographic wave of reduction of this acid is smaller by about three orders of magnitude, as can be predicted by the Ilkovic equation, and shows a very high temperature coefficient.…”

Organic Polarography

“…Studies on ascorbic acid find that one oxidation wave occurs a t a pH less than 8 and two waves at a p H above this value. Dehydro-L-ascorbic acid, the product of the oxidation, gives, because of hydration (469) , a very small reduction wave.…”

Electrochemical oxidation of thiobenzanilide

“…Electrochemical techniques represent very useful tools for examination of electron-transfer reactions and can provide valuable information that helps to understand some of the biological redox reactions; for example, earlier works demonstrate good accordance between enzymatic oxidation and electrochemical oxidation of L-ascorbic acid [11,12] or uric acid [13]. The …”