Role of the activity coefficient in the dissemination of pH: comparison of primary (Harned cell) and secondary (glass electrode) measurements on phosphate buffer considering activity and concentration scales

Despite recent efforts devoted to assessing both the theoretical rationale and the experimental strategy for assignment of primary pH values, these have not yet been accomplished satisfactorily. Traceability and comparability of pH values are achieved only within the constraints of internationally accepted conventions and predefined conditions that cannot account for all possible situations when pH is measured. Critical parameters to be defined are, in particular, the activity coefficients (γ _i) of the ionic species involved in the equilibrium with the hydrogen ions in the solution, which are usually estimated with the approximation typical of the Debye–Hückel theoretical model. For this paper, primary (Harned cell) measurements (traceable to the SI system) of the pH of a phosphate buffer have been considered and the results have been compared with secondary (glass electrode) measurements obtained by considering either the activity (paH) or concentration (pcH) scale of the hydrogen ions. With conventional approaches based on measurements related to activity or concentration scale, discrepancies emerge which have been assigned to incomplete inferences of γ _i arising from chemical features of the solution. It is shown that fitting and comparable paH and pcH results are attainable if evaluation of γ _i is performed using better estimates of the ionic strength, according to an enhanced application of the Debye–Hückel theory.