The thermodynamic dissociation constant of the phenol group of p-phenolsulfonic acid was calculated from electromotive-force measurements of hydrogensilver-chloride cells without liquid junction. Thirty-nine buffer mixtures of potassium p-phenolsulfonate, sodium hydroxide, and sodium chloride were studied in five series of experiment s from 0° to 60° C at intervals of 5 degrees. In three series, t he molal ratio of phenolsulfonate ion to phenolate-sulfonate ion was unity, and in two series the buffer ratio was 2 :3. For two series of experiments, one at each buffer ratio, the molality of sodium chloride was m aintained constant near 0.05 for all dilutions of t he buffer. In the other experiments, the molality of each component of t he solut ion varied bet ween 0.0037 and 0.1.The values of pK2' the negative of the common logarithm of the second dissociat ion constant, between 0° and 60° C are given by the equation012139T, where T is in degrees Kelvin.Equations were formulated to give the changes of free energy, heat content, entropy, and heat capacity that accompany t he dissociation, at infinite dilution, of a mole of phenolsulfonate ion at any temperature between 0° and 60° C. For the dissociation of the phenol group at 25° C, ~F o is 12,351 cal, ~Ho is 4,036 cal, ~So is -27.9 cal deg-1, and ~c o p is -33 cal deg-1•The pH value of each buffer-chloride mixture was calculated from the experimental data and the activity coefficients that were found to characterize each series of solutions. The pH values of other phenolsulfonate buffers which have m olal ratios, mdm2, of phenolsulfonate ion to bivalent phenolate-sulfonate ion between 2/3 and 1 can be computed for temperatures between 0° and 60° C with an accuracy of ± 0.002 unit from their compositions and from the dissociation constants and ionic parameters given in this paper. The equation used is pH =pK2 -log (mt/m2) -3A.JJ;f(1 +8B-V JL), where JL is the ionic strength . Buffer solutions of this type are suitable pH standards in the range 8.6 to 9.0. CONTENTS

An improved method for obtaining the titration curves of monobasic acids is outlined. The sample, 0.005 mole of the sodium salt of the weak acid, is dissolver! in 100 ml of a 0.05-m solution of sodium chloride and titrated electrometrically with an acid-salt mixture in a hydrogen-silver-chloride cell without liquid junction.The acid-salt mixture has the composition: nitric acid, 0.1 m; pot assium nitrate, 0.05 m; sodium chloride, 0.05 m. The titration therefor e is performed in a. medium of constant chloride concentration and of practically unchanging ionicstr ength (1'=0.1) . The calculations of pH values and of dissociation constants from the emf values are outlined . The tit ration curves and dissociation constants of form ic acid and of acetic acid at 25 0 C were obtained by this method. The pK values (negative logarit hms of the d issociation constants) were found to be 3.742 and 4. 754, respectively.