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

Abstract

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

Uranium, Mining and Hydrogeology

“…With pKa2 = 8.56 ± 0.05, determined by potentiometric titration and comparable to those in the literature (Bates et al 1943, Zollinger et al 1953), the hydroxyl group should not be deprotonated at the investigated pH 2.5. Thus, the sulfonic acid group can be regarded as completely dissociated.…”

Dissociation constant of 4-aminopyridinium ion in water from 0 to 50 C and related thermodynamic quantities

“…It has been shown [ 14 ] that the hydroxyl ion concentration can be obtained with the accuracy needed to correct the buffer ratio by where K w is the ion product constant for water [ 9 ]. This approximation is quite satisfactory if the base is not too strong (i.e., when m OH is not too large).…”

pH values of acid-salt mixtures of some aromatic sulfonic acids at various temperatures and a criterion of completeness of dissociation

“…Differential titration with carbonate-free sodium hydroxide [7] indicated a purity of 100.1 percent. Both methods of assay have an estimated uncertainty of 0.1 percent.…”

“…MATERIALS Sodium p-phenolsulfonate obtained from a commercial source was recrystallized four times from distilled water and dried to constant weight at 105° C. The purity of the material Wl,l.8 found to be 98 percent by bromometric assay [7,8]. The original sample contained traces of sulfate and phenol, which were absent in the purified material.…”

“…Use of the mass law (eq 2) and emf data by Bates, Siegel, and Acree [7], including the correspondin~ values of the activity coefficients of the ions, gave a good second IOnization constant for phenolsulfonic acid. Practically the same value was obtained by Sager, Schooley, and Acree by spectrophotometric methods [8].…”