Provisional pH values for certain standard buffer solutions

Abstract

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

The second dissociation constant of p-phenolsulfonic acid and pH values of phenolsulfonate-chloride buffers from 0 degrees to 60 degrees C

“…The calculation of the second dissociation constant, K 2 , of a weak acid and the pH values of solutions of the acid mixed with its salts from emf measurements of hydrogen-silver-chloride cells has been outlined in earlier publications from this Bureau [2,6,7] . Mention should be made, however, of the assumptions upon which these calculations are based.…”

“…= (E-EO)/k+ log (mRPSmc,/mps) +2A-l""p'/( 1 + Bar/j;) (7) It is usually possible to choose a value of at that will make pK; a linear fun ction of J.I. The slope of the plot of pK; with r espect to J. l .…”

“…Few acids are known whose dissociation constants fall in the region 10-: 7 to 10-9 • Consequently, few buffer standards with pH values between 7.0 and 9.0 are available for the calibration of pH equipment and for the control of acidIty. Regulation of pH in this slightly alkaline region is, however, of great importance in studies of biological processes and of the body fluids, in the treatment of water, and in the processing of foods and medicines.…”

Dissociation constants and pH-titration curves at constant ionic strength from electrometric titrations in cells without liquid junction: titrations of formic acid and acetic acid

“…PtjH2jH+, Cl-jAgCljAg, (1) where H+ and Cl-represent the components of the cell electrolyte to which the cell is reversible. The emf of the cell at a centigrade temperature, t, is related to its standard potential, EO, and to the activities, a , of these two ions by the equation…”

“…from the emf measurements of such a cell often involve considerable uncertainty, and in unfavorable cases they may be in error by more than 0.05 pH unit [1,2]. 1 The principal reason for this difficulty is neglect of the contribution of the potential at the liquid junction or in the application of improper or inadequate corrections for it. Such corrections are laborious and unsatisfactory and require a knowledge, often unavailable, of the mobilities and activities of the ions of which the solutions are composed.…”