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 precipita tion during far-from-equilibrium freezing of sodium phosphate buffers as a function of buffer composition, under experimental conditions relevant to p harmaceutical applications-sample volumes larger than a few microliters, ex periencing large degrees of undercooling and supersaturation. Methods. Buffer solutions were prepared by dissolving the monosodium and di sodium phosphate salts in the appropriate ratios to obtain initial buffer c oncentrations 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 confirme d by ion analysis and x-ray powder diffraction. Results. Precipitation of Na2HPO4 . 12H(2)O caused abrupt pH decreases afte r the onset of ice crystallization, at temperatures between -0.5 and -4.0 d egreesC. Decreasing the initial buffer concentration and/or initial pH resu lted in higher final pH values at -10 degreesC, farther removed from the eq uilibrium value of 3.6. At an initial pH of 7.4, the 50 and 100 mM buffer s olutions reached a pH of 4.2 +/- 0.1 at -10 degreesC, whereas the 8 mM solu tions reached a pH of 5.2 +/- 0.2. Solutions having an initial pH of 5.7 an d 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 concentra tions (activities) of precipitating ions, and are determined by both initia l pH and salt concentration. The ion activity product is a meaningful param eter when describing salt precipitation in solutions prepared by mixing sal ts containing precipitating and nonprecipitating ions.