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4115314 
Journal Article 
Redetermination of the second dissociation constant of phosphoric acid and calculation of the pH values of the pH standards based on solutions of dihydrogen and hydrogen phosphate ions at 298.15 K 
Partanen, JI; Minkkinen, PO 
1996 
Yes 
Acta Chemica Scandinavica
ISSN: 0904-213X
EISSN: 1902-3103 
50 
12 
1081-1086 
WOS:A1996VY70900019 
Thermodynamic properties of aqueous mixtures of sodium or potassium hydrogen phosphate, dihydrogen phosphate and chloride at 298.15 K were studied by means of a recently developed method where a simple equation is used for ionic activity coefficients. The experimental data for these studies were taken from the literature. It is shown that almost all data used in the tests up to an ionic strength of 0.6 mol kg(-1) can be interpreted within experimental error by the model used. The results of this method were compared to those of a model obtained recently on the basis of the Pitzer approach to mixed electrolyte solutions. A value of 6.360x10(-8) was determined here for the thermodynamic value of the second dissociation constant of phosphoric acid. The pH values of the two standard reference solutions based on the mixtures of KH2PO4 and Na2HPO4, ie the values of 6.865 and 7.413, can accurately be predicted by the equations determined in this study. The present results lead to the possibility of preparing a phosphate buffer whose pH can have any value between about 6.5 and 7.2; this value is probably known within +/-0.005 pH units.