ph of aqueous mixtures of potassium dihydrogen phosphate and disodium hydrogen phosphate at 0 degrees to 60 degrees C

Abstract

A met hod is suggested for computing t he p H of phosphate buffers from electro-motive-force measurements of cells wi thout liquid junction. Each of the 33 buffer solutions studied was prepared from equal molal quantities of potassium dihydrogen phosphate and disodium hydrogen phosphate. The solutions were divided int o fi ve series wit h respect t o t he a m ount of sodium chloride added. The ratios of the molalit y of ea ch buffer salt to that of sodium chloride in the five series were about 1, 2, 3, 8, and 10. The pH values were computed from measurements of cells with hydrogen elec trodes and silver-silver-chloride electrodes by a procedure that involves extra polation of a function of t he emf to zero concentration of sodium chloride. The values of the second dissociation constant of phosphoric acid given in a n earlier paper (RP1524) were confirmed. The mean values of pK, t he negat ive of the common logarithm of t he second dissociat ion constant, are given as a function of absolute t emperature, T, by t he equation pK= 2073.0/ T-5.9884 + 0.020912 T between T = 273.16 and T = 323.16 (0° to 50° C). Equations are given t o express the change of pH wit h molality of sodium chloride. The pH values from 0° to 60° C of eight phosphate buffers without chloride are listed. The densities of the buffers were determined. At 25° C, the pH of buffers con t aining equal molal quantities of pot assium dihydrogen phosphate and of disodium hydrogen phosphate is given by t he equations and pH=7.162 + 2.18 m-2.237{m pH = 7.169 + 2.39 c-2. 324y'G, where m and c, the molality and the molar concentration of ea ch buffer salt, lie between 0.005 and 0.1. CONTENTS Page