Thallium (III) sulfate in dilute sulfuric acid quantitatively oxidizes isoniazid to nitrogen and 4-pyrldine carboxylic acid. The test material is reacted with a measured excess of the reagent, the residual amount being back-titrated with ascorbic acid using o-dianisidine or p-ethoxychrysoidine as indicator. 4-Aminosalicylic acid, thiacetazone, thiamine hydrochloride, lactose and a number of other excipients do not Interfere with the analysis.
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Verma et al. (1985) studied this question.
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