Re-evaluation of the second stoichiometric dissociation constants of phosphoric acid at temperatures from (0 to 60) °C in aqueous buffer solutions with or without NaCl or KCL.: 2.: Tests and use of the resulting Huckel model equations

Previously (in Part 1), equations were determined for the calculation of the second stoichiometric (molality scale) dissociation constant (K-m2) of phosphoric acid in buffer solutions containing sodium, or potassium, dihydrogen phosphate, hydrogen phosphate, and chloride from the determined thermodynamic values of this dissociation constant (K-a2) and the molalities of the components in the solutions. These equations were based on single-ion activity coefficient equations of the Huckel type. In the present study (Part 2), these equations were further tested with all reliable thermodynamic data found in the literature from this dissociation reaction in NaCl and KCl solutions. In these data were included a considerable amount of literature results from measurements on cells without liquid junction using hydrogen and silver-silver chloride or mercury-mercury(l) chloride electrodes at temperatures from (0 to 95) degrees C. Also literature data from hydrogen and glass electrode cells containing liquid junctions were included in the present tests. The new model usually applies well to these data. The new model equations were then used to calculate pH values of the two phosphate buffer solutions recommended by IUPAC (in 1985 and 2002) at temperatures from (0 to 70) degrees C. Values Of p(m(H)), giving the molality of hydrogen ions directly, calculated from these equations are also tabulated for these buffers as well as for the buffer solutions with NaCl or KCl as major component and with KH2PO4 and Na2HPO4 at an equal molality of 0.0025 mol center dot kg(-1) as minor components.