A colorimetric method for the determination of phenacetin and paracetamol. Part II. A manual procedure for the determination of phenacetin or paracetamol in formulations

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Abstract
A manual colorimetric method for the determination of phenacetin or paracetamol in pharmaceutical preparations has been developed from the automated method described in Part I of this series. The drug (about 140 µg of paracetamol or about 200 µg of phenacetin) is added to a hydrochloric acid-sodium hypochlorite mixture at pH 3·4 and the excess of hypochlorite is removed with sodium arsenite. A solution of phenol is then added, followed by borate buffer solution (pH 9·9), and the optical density of the blue indophenol dye solution is measured at 625 nm. The method is used to determine phenacetin in Aspirin, Phenacetin and Codeine Tablets B.P. and paracetamol in Paracetamol Tablets B.P. and paracetamol elixirs. Common tablet excipients, and drugs frequently compounded with paracetamol and phenacetin, do not significantly interfere with the reaction; the method should therefore be widely applicable to phenacetin or paracetamol formulations.

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