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A met hod is suggested for computing t he p H of phosphate buffers from electromotive-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

The second ionization constant of malonic acid in aqueous solution was determined at 0° to 60° C from measurements of the electromotive force of galvanic cells without liquid junction. Solutions containing sodium acid malonate, sodium malonate, and sodium chloride were employed for these determinations.From the values of the ionization constants, the closest distance of approach, or the so-called diameters, of the ions in the solution was calculated for the different temperatures. These ionic diameters, the electromotive forc e, and the known molalities were then employed to calculate pH values of the solutions. The ionization constant at the different temperatures was determined from the experimental data by three different methods, using a least-square calculation in each. The values are different for the various temperatures, and may be computed for temperatures from 0° to 60° C, inclusive, by the equation 1053.08 log K 2= --T -+20.3223 log T-0.05838 T+0.0000236 T2-37.1402.Equations were formulated to express the variation of pH as a function of the ionic strength of the solution, to give the heat of ionization of the acid malonate ion, and to give the difference in the specific heats of the ions and the undissociated acid malonate ion at each temperature. The change in free energy and entropy for the ionization of the acid malonate ion were also calculated for each temperature. All of these quantities are of importance in arriving at explanations for the variation of hydrogen-ion activity with temperature.It has been found that solutions containing equal concentrations of sodium acid malonate, sodium malonate, and sodium chloride, each varying from 0.001 to 0.044 molal, have pH values ranging from 5.272 to 5.761 at 0° to 60° C. These solutions are suitable for use as pH standards.

For use in the calibration of electrometric pH assemblies, 17 standard buffer solutions have been investigated, and pH values at 20°, 25°. and 30° C have been assigned to them. The pH values of these solutions range from 2.27 to 11.68 and are considered accurate to ± 0.02 pH unit.Hydrogen-silver-chloride cells without liquid junctions were used for establishing the precise pH values of the buffer mixtures. The assumptions made to determine the activity coefficients of the ions in the mixt ures are discussed. The method of assigning an accurate pH value to a buffer mixture is outlined.Directions for preparing the mixtures from purified a nhydrous salts, standard solutions of acid and alkali, and pure water are given. Changes of temperature have a larger effect on the pH values of the buffers of pH greater than 7 than on those of the acid buffers. In all cases the effect of dilution is small; an error of 1 percent in the volume of solvent added results in a change of less t han 0.001 pH unit.The use of a pH meter of the glass-electrode-calomel-electrode type calibrated by means of a standard buffer may often in vol I·e greater uncertainties than those inherent in the pH value assigned to the buffer mixture. It should be recognized that errors arising from liquid junction, hysteresis, temperature, and salt effects may combine to give an uncertainty of 0.01 and 0.03 unit or more in practical pH t ests. CONTENTS