PROVISIONAL pH VALUES FOR CERTAIN STANDARD BUFFER SOLUTIONS

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. Hydrogensilver-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 anhydrous 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.