Effect of initial buffer composition on pH changes during far-from-equilibrium freezing of sodium phosphate buffer solutions

Purpose. This study aims to assess the pH changes induced by salt precipitation during far-from-equilibrium freezing of sodium phosphate buffers as a function of buffer composition, under experimental conditions relevant to pharmaceutical applications-sample volumes larger than a few microliters, experiencing large degrees of undercooling and supersaturation. Methods. Buffer solutions were prepared by dissolving the monosodium and disodium phosphate salts in the appropriate ratios to obtain initial buffer concentrations in the range of 8-100 nM and pH values between 5.7 and 7.4 at 25 degreesC. Temperature and pH were monitored in situ during cooling to -10 degreesC (at a rate of 0.3 to 0.5 degreesC/min) and for 10-20 min after the sample reached the final temperature. Salt crystallization was confirmed by ion analysis and x-ray powder diffraction. Results. Precipitation of Na2HPO4 . 12H(2)O caused abrupt pH decreases after the onset of ice crystallization, at temperatures between -0.5 and -4.0 degreesC. Decreasing the initial buffer concentration and/or initial pH resulted in higher final pH values at -10 degreesC, farther removed from the equilibrium value of 3.6. At an initial pH of 7.4, the 50 and 100 mM buffer solutions reached a pH of 4.2 +/- 0.1 at -10 degreesC, whereas the 8 mM solutions reached a pH of 5.2 +/- 0.2. Solutions having an initial pH of 5.7 and initial buffer concentrations of 8 and 100 mM experienced less pH shifts upon freezing to -10 degreesC, with final pH values of 5.1 +/- 0.1 and 4.7 +/- 0.1, respectively. Conclusions. Precipitation-induced pH shifts are dependent on the concentrations (activities) of precipitating ions, and are determined by both initial pH and salt concentration. The ion activity product is a meaningful parameter when describing salt precipitation in solutions prepared by mixing salts containing precipitating and nonprecipitating ions.