Electromotive force studies of electrolytic dissociation. Part 12.—Dissociation constants of some strongly ionising acids at zero ionic strength and 25°C

Abstract

A glass electrode-AgC1 cell, measured to kO.01 mV, has been used in evaluations of pK values at 25°C of four acids in the range pK 1-2.2. The practical work consisted of calibrating the cell with dilute HCl followed by additions of stock solutions containing the acid ligands. Three of the acids have been previously studied with Pt, H2-AgC1 cells so critical comparisons are possible.Attempts have been made to reduce the dependence of the pK values at zero ionic strength (when these are -c 2.5) upon the value of the ion-size parameter p used in the Debye-Huckel activity coefficient expression. The present calculations are based upon the use of a second parameter Q. Round values of this at different values of have been calculated from some HCI, salt data. A second but smaller feature which has been taken into account is the observations of Bates that standard electrode potentials (E") vary slightly with p if E" is derived by linear analysis. By using these two factors, (method C of table 3) the pK against p dependence is markedly diminished. Also, by using the p and Q parameters appropriate to HCl solutions, pK values close to the averages of those obtained by method C are obtained.For acids of pK values < 2.5, the values at zero ionic strength are markedly dependent upon the value assigned to p in eqn (2) when Q = 0. The dependence is much greater than is found with other methods such as conductivity 3* and the root

Thermodynamics of Dissolution of some Dicarboxylic Acids in Water/Ethane-1,2-Diol Mixtures

“…(1) and (3) Table 1, for each run the extrapolated K 1 and K 2 values at zero ionic strength were obtained from the linear least mean square plots of K 1 (or K 2 ) versus ionic strength [18,19]. The computed, pK 1 and pK 2 values at zero ionic strength for different solvent compositions and temperatures are shown in Table 2 from which it can be readily seen the influence of the composition of the medium on the first and second ionization constants.…”

“…The practical work has been simplified for K 1 and K 2 determinations by adopting a practice [18,19] which prevents movements of the electrode between its calibration, and usage by equilibrating the cell, and electrode in dilute HCl containing the weight percent of ethane-1,2-diol for the calculation of E • (cell) followed by addition of stock solutions containing the acid ligands of the required wt. % of ethane-1,2-diol.…”

Medium Effects on the Dissociation Reactions of Adipic Acid in Ethanol-Water Mixtures

“…K 2 was calculated from Eqns. (2) and (4) 23) , and was found to be valid and efficient under different experimental conditions. The computed pK| and pK 2 values at zero ionic strength for different solvent compositions and temperatures are shown in Table ( Hence, a decrease in D, by increasing the organic solvent composition in die medium, would increase the pK values as can be observed from Table (2).…”

“…respectively. The practical work has been simplified by adopting a practice" 2 ' 23 ' which prevents movements of the electrode between its calibration and usage. This is preferable to the technique (24,25) of transfering the glass electrode from an HC1 solution to an acid ligand solution followed by extrapolation of timed reading to compensate for any emf drifts.…”

“…This method' 12,23 ' has been proved to be the most precise and direct method for the evaluation of dissociation constants at zero ionic strength of the acid in water and organic solvent.…”