A PITZER MIXED ELECTROLYTE SOLUTION THEORY APPROACH TO ASSIGNMENT OF PH TO STANDARD BUFFER SOLUTIONS

The IUPAC recommendations for the pH scale for aqueous solutions are b ased on the Bates-Guggenheim (B-G) convention (1961)for the single ion activity coefficient of the chloride ion in the standard buffer(s). T his convention was adopted as a reasonable estimate based on the Debye -Huckel theory and is limited in its application to ionic strengths le ss than 0.1 mol-kg-1. This approach ignores the results of many worker s over the years on the properties of mixed electrolyte solutions and their prediction on the basis of the theories of Harned, Scatchard, Gu ggenheim and more recently of Pitzer. The literature data of EMF measu rements on appropriate weak acid systems have been reexamined to deter mine both the pK(a) values and values of appropriate Pitzer interactio n coefficients. The latter are used to calculate single chloride ion a ctivity coefficients for the chosen compositions of pH standard buffer s, and compared with the B-G convention values. Calculations were made to check the consistency of the pH values with determined pK(a) value s using the Pitzer treatment for all the required single ion activity coefficients. The overall aim was to remove the ionic strength restric tion of the B-G convention and rationalize the approach to pH standard ization for such diverse aqueous media as sea water, blood and acid-ra in water.