Reassessment of pH reference values with improved methodology for the evaluation of ionic strength

Abstract

The conflict between pH as empirical number in routine control and the pH value regarded as conveying some information concerning the effective concentration or activity of hydrogen ions, a H, has caused much confusion. There are, however, reasons to conclude that the overwhelming amount of thermodynamic data is not sufficiently accurate—either due to ignorance of metrological concepts or due to insufficiently specified measurement processes of fundamental chemical quantities pH. The commonly used seven reference buffer solutions to which primary pH values have been conventional assigned, represent a selection out of a more extensive list, recommended by NBS (now NIST) in 1962. From then onwards conventions concerning the Debye–Hückel model of electrolyte solutions and ionic strength have been revised and the pH(S) values reassessed in conformity but only for these seven reference buffer solutions. The others have, so far remained unchanged, locking harmonisation of the conventionally assigned pH(S) values. In this work, ionic strength is calculated through complete equations derived from the acidity constants. Concentrations of the various species involved in the conventional assignment of pH and their corresponding activity coefficients are therefore, more rigorously known. The process proves particularly useful for poliprotic acids with overlapping acidity constants, where the ratio is less than 10 3. As a consequence, conventionally assigned pH values of reference buffer solutions are recalculated and corrections are introduced as appropriate.