Influence of pH measurement inaccuracy on the values of acidity constant determined on the basis of electrophoretic and thermophoretic data

The accuracy of the acid dissociation constant (pKa) determined by experimental techniques depends on the potential errors in the pH measurement. This dependence is obvious, but, according to common practice, this effect is either ignored or treated in a greatly simplified manner when discussing the credibility of obtained pKa values. This paper investigates the influence of incorrect pH measurement on the pKa values obtained by capillary electrophoresis (CE) and microscale thermophoresis (MST). A simple self-control procedure has been proposed to control and reliably predict the corresponding total uncertainty of the acidity constant. The significance of the pH measurement error was also investigated in relation to the thermodynamic analysis, which requires determining the thermal dependency of pKa values. The obtained results clearly indicate that the investigated effect should not be ignored, and the actual accuracy of methods using electrophoretic separation may be worse than commonly assumed. It also points at the need to develop alternative methods that do not require measuring pH, such as a known internal standard-based approach. Besides pH-related effects, other sources of inaccuracy of pKa constant should also be considered.