Determination of Methimazole and Carbimazole Using Polarography and Voltammetry

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Abstract
Anodic waves of methimazole (I) (1-methylimidazole-2-thiol) and carbimazole (II) (1-ethoxycarbonyl-3-methyl-2-thio-4-imidazoline) on mercury electrodes correspond to mercury salt formation. Both compounds form in the thiono form a soluble complex at pH < 6, compound (I) at higher pH-values a slightly soluble salt of the thiol form. Electrode processes involving the thiol form are complicated by adsorption. Oxidation at solid electrodes occurs only at potentials more than 0.5 V more positive. For compound (I) spectrophotometry indicated pKa=12.0 ± 0.2. By d.c. polarography in 0.1 M H2SO4 containing 10% ethanol the determination of both compounds is possible between 4 × 10− and 1 × 10−3 M, by differential pulse polarography between 1 × 10− and 1 × 10−4 M, by differential pulse voltammetry at HMDE between 5 × l0−7 and 6 × 10− M.

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