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 mM and pH values between 5.7 and 7.4 at 25 degrees C. Temperature and pH were monitored in situ during cooling to -10 degrees C (at a rate of 0.3 to 0.5 degrees C/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, 12H2O caused abrupt pH decreases after the onset of ice crystallization, at temperatures between -0.5 and -4.0 degrees C. Decreasing the initial buffer concentration and/or initial pH resulted in higher final pH values at -10 degrees C, 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 degrees C, 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 degrees C, 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.