The degradation of isonicotinic acid hydrazide sodium methanesulfonate (IHMS) was examined in aqueous solution (pH 3.0-9.0). Isoniazid (INAH) and little unknown compound were found as degradation products and in addition to them, isonicotinic acid and 1, 2-diisonicotinoylhydrazine were found at pH 9.0. For the assay method, only IHMS was eluted from the sample solution by ion exchange resin, Amberlite CG-50 (H+), and the effluent submitted to colorimetry. This method gave satisfactory results. With this method, degradation of IHMS was examined and it was found that the main degradation in the reaction of IHMS is a catalytic reaction of OH- to IHMS anion, that equilibrium of IHMS concentration is established relatively rapidly, and that INAH and sodium hydroxymethanesulfonate (OMS) produce IHMS. The stabilizing effect of INAH and OMS, which are degradation products of IHMS, and NaHSO3, and formaldehyde, which are the degradation products of OMS, was investigated when they are added previously to the reaction system. OMS was found to be more effective.
