Acidity has long been recognised as an important property, and methods for measurement of pKa are well established. The principal procedures involve experimental determination of the pH and the concentrations of the dissociated and undissociated forms of the chemical substance (Eq. 7). In principle, there are two ways to determine Ka: (i) titrating a known amount of substance with a standard acid or base, or (ii) determining the relative concentrations of the ionised and neutral forms and their pH dependence. Traditionally, titration, spectrophotometry and conductometry have been the most useful techniques due to their accuracy and reproducibility. The details are given in the OECD guideline 112 (OECD, 1981a) and the EPA OPPTS 830.7370 (US EPA, 1996). The three reference methods are summarised in Appendix A.
Acid dissociation constants in the range of pKa 3 to 11 can generally be measured with a high degree of accuracy. However, the value of Ka is affected by several parameters, including: Molecular structure of the organic acid, increasing ionic strength of the aqueous medium (increasing ionic strength favours the ionic form of the conjugate acid/base pair), and temperature of the medium. However, the latter is of unimportant in the environmentally relevant temperature range.
The influence of the solution’s ionic strength and the temperature is small compared to the influence of the molecular structure of the acid, i.e. Ka values generally change by less than one order of magnitude between 5 and 60°C (typically < 10%) (Harris and Hayes, 1990). If a significant temperature dependency is suspected, the pKa determination should be conducted at different temperatures with intervals of 10°C (±0.1°C).
The strategy for measuring pKa is determined by the solubility of a compound. The compound should be soluble and stable to ensure equilibrium is maintained.
Substances need to be submitted for pKa measurement as free bases or inorganic acid salts in pure form, and must be of accurately known composition. In general, no reliable measurements can be made on organic acid salts. A suitable analytical method with sufficient sensitivity for the different species should be available. The analytical method used to determine the amounts of dissociated and non-dissociated forms present in solution should not affect the equilibrium, and should be capable of distinguishing between the chemical species involved.