Stability of Vancomycin Hydrochloride Solutions at Various PH Values as Determined by High-Performance Liquid Chromatography

A stability-indicating HPLC assay method for the quantitation of vancomycin HCl has been developed. The developed method was used to study the effect of pH, phosphate buffer concentration and ionic strength on the stability of vancomycin HCl. The pH-rate profile curve showed 3 regions, acid-catalyzed, general base-catalyzed and the pH-independent region. The region of maximum stability was between pH 3.0 to about 5.7. On increasing the buffer concentration, phosphate ions hastened the decomposition of vancomycin HCl, the effect being more pronounced at pH 6.85 than at pH 4.43. In the pH region of 1-3, the decomposition was catalyzed by H⁺ and between pH 3-7 by HPO₄^2-, H₂PO₄^- and OH^-. An increase in the ionic strength decreased the rate of decomposition.