The electrochemical determination of vitamin A. Part II. Further voltammetric determination of vitamin A and initial work on the determination of vitamin D in the presence of vitamin A

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Abstract
All vitamin A derivatives are found to give clean oxidation waves with a 0·05 M solution of lithium perchlorate in methanol-benzene as the supporting electrolyte, and a glassy carbon electrode as the working electrode. The determination of the vitamin in pharmaceutical preparations is carried out via direct extraction without prior saponification. The dependence on pH of the half-peak potentials of vitamins A and D, and of provitamins D, is examined with both carbon paste and glassy carbon electrodes. It is shown that the voltammetric technique does not distinguish between vitamins A1 and A2, nor between vitamins D2 and D3. A method for the determination of vitamin A in margarine is given. Some attempts have also been made to determine vitamin D in mixtures with vitamin A. Partition chromatography with Celite-polyethylene glycol gives a clean separation of vitamin D from vitamin A, which makes voltammetric determination of vitamin D possible without further purification.

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