Measurement of surface pH of pharmaceutical solids: a critical evaluation of indicator dye-sorption method and its comparison with slurry pH method.

Two methods for the measurement of surface pH of pharmaceutical solids, namely, the dye‐sorption method and the slurry pH method, were compared. High purity drug substances, instead of excipients, were used as model solids, because acidic or basic impurities present in excipients could influence slurry pH. Solid test samples were prepared by sorption of methanol–water solutions of several indicator dyes, and their diffuse reflectance UV‐visible spectra were measured. The solid surface pH values were estimated by comparing base‐to‐acid peak ratios of the diffuse reflectance UV‐visible spectra of solid samples to the calibration plots of dye solutions in aqueous standard buffers of known pH. In the slurry pH method, pH values of concentrated slurries of the compounds in water were considered to represent solid surface pH. The agreement between the two methods was mixed and depended on the compound or the indicator used. It was concluded that in many cases calibration plots of indicator dye spectra in aqueous buffers were not applicable to the solid state, and, as a result, the reliability of the method was low. The slurry method provided a simple and reliable measurement of surface pH indicating that concentrated slurry may closely represent solid surface pH.