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

Abstract

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.

Definitions of pH Scales.

“…-/ 2.303RT. £ = Er--=log aci (10) From equations 8a and 10 we have paH --log }"m, -^-- (11) and the equivalent expression, £o' + £,• = £ -fc (paH) (11a)…”

“…Use of equations 11 and 12 to determine the paH of so-called unknowns entails the use of previously determined values of £0' + £,• or a redetermination of £0' + £,• at the time of measurement by calibration of the cell with a reference solution of known or assigned paH (equation 9). These two procedures will receive further attention in a later section.…”

“…A concentrated solution of ammonium nitrate is not as effective as saturated potassium chloride for this purpose (15), but mixed chlorides and nitrates appear to possess certain advantages (106). The bridge solution of potassium chloride is often regarded as reducing the residual liquid-junction potential to a few tenths of a millivolt, when the pH of each solution forming the boundary is greater than 3 and less than 11 (88,123).…”

“…For this purpose, the values of log/? given in table 10 for the chloride, bromide, and iodide series were fitted to equations of the type: The parameters a and ß°, obtained by the method of least squares, are listed in table 11. The average difference, , between the "observed" values of log fr and those calculated by equation 48 is given in the last column.…”

“…Primary standards such as these can be used to redetermine the value of E0> + Ej in equation 11 and, with the aid of cells of type II or IV, the paH values of the many convenient secondary standards of somewhat higher concentrations and varying types. The computation of hydrogen-ion activity at high or even moderate ionic strengths is quite uncertain and probably will remain so.…”