Aiming to determine the polyprotic acid dissociation constants, we have modified a potentiometric titration technique and the spectrophotometric method. The 2-hydroxyisophthalic acid (1) and its 5-nitro derivative (2) are relatively strong acids that have been chosen as model triprotic acids with well-separated pKas. Direct potentiometric titration gave only the values of pKa2 (4.72 and 4.23). For pKa3, an indirect method for the titration data treatment was used. The pH of the solution, after 3 equivalents of base per one equivalent of acid were added, was measured and subtracted from the imaginary pH computed, assuming that the acid being analyzed was diprotic. This difference is attributed to the reaction HA2− + OH− → A3− + H2O, so that [A3−] = Δ[OH−]. At equilibrium, [HA2−] = [H3A]total − [A3−]. These values and [H+] at the final point were used to compute the pKa3 (11.11 and 10.69). The pKa1 of (1) could not be determined potentiometrically due to its low solubility in water; for (2), an indirect method was used. Assuming that the equilibrium [H3A]eq = 0, the imaginary pH at the first midway titration point was computed. The pH at this point was measured from the actual titration curve. The difference Δ[H+] was computed, giving Δ[H+] = [H3A]eq, and [H2A−] = [H3A]total − [H3A]eq. These values were used for computing the pKa1 = 0.72. The pKas for both (1) and (2) were also determined by a modified spectrophotometric method as the spectra of both acids are pH sensitive and the extinction coefficients are high. General strategy was that the spectrum of each pure H3A, H2A−, HA2− and A3− had to be identified and used as a reference to compare with spectra at several pHs where the concentration of the next protonation form was progressively increasing. The differential curves were obtained at several fixed pHs with variable concentration of the analyte. A set of values of the absorbance differences at the selected analytical wavelength and at fixed pH, plotted against the analyte’s concentration, produced a straight line. Similar graphs were obtained at several fixed pHs. The slopes were used along with [H+] or [OH−] to calculate the pKa1 (1.92 and 0.87), pKa2 (4.57 and 4.05) and pKa3 = 11.12 (for 2).

Determination of the dissociation constants of triprotic acids with broadly ranging pKas

253rd National Meeting (2017)

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