Similarity of salt influences on the pH of buffers, polyelectrolytes, and proteins

Changes in pH induced by the addition of electrolytes to buffers, polyelectrolytes (a polycarboxy polymethylene and a polyethyleneimine), and proteins (casein, whey, and lysozyme) solutions are explored systematically. The two buffer systems are triethanolamine/triethanolammonium chloride and citric acid/sodium citrate. These are chosen because of the similarity of their acid-base equilibria with those of amino acids predominant in most proteins, that is, amino acids that include carboxylate or ammonium groups in their structures. The pH of triethanolamine and of citrate buffers respectively increases and decreases when salt is added. At low electrolyte concentrations (< 0.15 mol/kg), the phenomenon is well accounted for by standard electrostatic theories. pH values at higher salt concentrations are not reliable when measured with a commercial glass electrode without cross-checking by a standard hydrogen electrode. The changes of the pH values of polyelectrolyte and protein solutions with added salts turn out to be remarkably similar to the salt induced pH changes in the buffer solutions. It is even possible to qualitatively predict these changes in protein solutions simply from the primary protein structure. At least in the systems considered here, the specific ion effects on pH seem to correlate with the bulk activity coefficients of the added electrolytes, at least at moderate salt concentrations.