Polarography of 5-nitro-orotic acid

Abstract

Polarography of 5-nitro-orotic acid in aqueous medium has been carried out in the pH range 1–10, using different buffer systems. There are two well-defined steps up to pH 9·0; the first step is purely diffusion-controlled (6 e) reduction at all pH values, and the second step (4 e) is purely diffusion-controlled in the acidic range and shows adsorption characteristics in the alkaline. Above pH 9, the compound is reduced in three steps: the first is purely diffusion-controlled (4 e); the second (4 e) and third (2 e) steps have adsorption character. At pH > 11, the reduction involves three steps, but the wave heights are not reproducible. The number of electrons involved in the diffusion-controlled steps was determined by comparing the wave heights with that of the first step of nitrobenzene reduction under identical conditions, which was confirmed by finding the diffusion coefficient D with the help of the McBain-Dowson cell. Kinetic parameters (α n a and −log k°) have been computed for the diffusion-controlled steps, using Koutecky's method. The probable reduction mechanism is discussed.