The chemical degradation of phenylmercuric nitrate by disodium edetate during heat sterilization at pH values commonly encountered in ophthalmic products.

The stability following heat sterilization (121 degrees C for 15 min) of phenylmercuric (PM) nitrate in the presence of disodium edetate at pH values 5-8 has been investigated by both high-performance liquid chromatography (HPLC) and atomic absorption spectroscopy (AAS). A stability-indicating HPLC method involving formation of the diethylamine-dithiocarbamate complexes of phenylmercuric and mercuric ions was found to suffer a pH-dependent interference from disodium edetate. A second method was therefore also employed involving selective extraction into diethylether of the phenylmercuric ion followed by HPLC of the piperidinedithiocarbamate complex with concomitant analysis of the unextracted mercuric ion by AAS using the cold-vapour technique. An HPLC method was also developed for benzene in the degraded mixtures. The application of these methods to autoclaved solutions containing PM nitrate and disodium edetate demonstrates that under the conditions of heat sterilization the phenylmercuric ion is degraded to mercuric ion and benzene to the extent of 15% at pH 8, 80% at pH 7 and completely degraded at pH 5 and 6.