Determination of acidity constants of monoprotic and diprotic acids by capillary electrophoresis

Capillary electrophoresis as a tool to determine thermodynamic pK(a) values of monoprotic and diprotic acids has been extended to the pK(a) range between 1.5 and 3.4. Previously developed procedures for determinations of pK(a) values in this range have been difficult to implement using conventional fused-silica capillaries. The improved procedure uses a dynamic coating of a positively charged polymer to impart a positive charge to the capillary thus making the electroosmotic flow stable and less dependent on pH than is generally found with uncoated fused-silica capillaries. The dynamic coating reverses the electroosmotic flow allowing the acids to elute before neutral solutes. The procedure was evaluated using five monoprotic and three diprotic acids. For multiprotic solutes, a general model was presented. The pK(a) values in all cases were easily determined by nonlinear regression with the appropriate equation. It was found that for diprotic cases, where the two pK(a) values differ by approximately one pK(a) unit, the determination of the pK(a) from the migration data is less precise. This is an inherent limitation of this method that is also shared with any other method for determining the pK(a) values for multi-protic solutes.