The determination of sulphoxide in degraded phenothiazine formulations by difference spectrophotometry

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Abstract
A method is described for the assay of phenothiazine sulphoxides which may be formed in phenothiazine formulations during storage under unfavourable conditions. The assay is based upon the measurement of difference absorbance of the sulphoxide solution in 0·2 m hydrochloric acid relative to an equimolar solution reduced with zinc dust. A solvent extraction procedure avoids interference from colouring agents and coloured photolytic breakdown products of the phenothiazines. The assay is specific in the presence of intact drug, sulphone and co-formulated drugs for the formulations examined and is sensitive to 0·5% of the total phenothiazine present as sulphoxide. Many aqueous formulations stored in partially filled containers have been shown to contain up to 31·5% of the total phenothiazine as sulphoxide.