Letters to the Editor: The Assay of Mersalyl

The Assay of Mersalyl SIR,-Methods of assay of mersalyl [sodium salt of salicyl-(r-hydroxymercuri-j3-methoxypropy1)amide-0-acetic acid], phenylmercuric nitrate, and unguentum hydrargyri nitratis dilutum as proposed by Waterhouse' and modified by Pierce' have become official in the British Pharmacopceia, 1948.All involve reduction to metallic mercury under reflux, solution of the precipitated mercury in nitric acid, and titration with ammonium thiocyanate.It was found that erratic results were sometimes obtained in the assay of mersalyl, and also when this assay process was applied to injection of mersalyl.An investigation was therefore undertaken into the cause.Under the conditions of assay, mercury being to some extent volatile, it was found that vigorous boiling caused condensation of the metal on the cold part of the redux condenser, mercury in such a form often being difficult to wash off.It is suggested, therefore, that glass jointed apparatus be used, and that after the reduction, the condenser should be washed with water and the precipitate transferred to the filter paper as directed.Nitric acid (20 ml.) and water (10 ml.) are then placed in the flask and refluxed in the apparatus for 10 minutes.The condenser is finally washed with 10 ml. of water, and the acid, after cooling, used to dissolve the zinc amalgam in the usual way, the assay then being completed in accordance with the directions given.