Acid dissociation constant (Ka)

The extent to which an organic chemical partitions between environmental compartments is determined by the properties of the chemical and the properties of the compartment it is discharged to. A major influence on the partitioning of ionisable organic compounds is attributed to the acid-base interaction between chemical and aqueous or soil/sediment compartment (Harris and Hayes, 1990). The acid dissociation (acidity) constant (Ka) indicates which chemical species will be present at a particular pH. The neutral, anionic or cationic species often have vastly different properties with regard to water solubility, volatility, UV adsorption, adsorption to soil/sediment, bioconcentration, and toxicity (Babic et al, 2007; ECHA, 2008a).

Water solubility measurements for regulatory purposes are usually made in distilled water at pH 6 to 9, whereas the pH of aquatic toxicity test media is usually 7 to 8 and pH values in the environment normally range from 5 to 8. Consequently, the solubility of an ionisable substance may differ significantly between these systems, especially if the pKa is between 5 and 9. This is because the extent of ionisation may vary according to pH or the level of counter ions in the test medium, and relatively small changes may significantly alter the equilibrium between dissociated and non-dissociated species. The dissociated and non-dissociated species may each have their own water solubility and partition coefficient, and therefore bioavailability and toxicity.

The Ka is also important in deciding which method or conditions should be used to determine the KOW (Section 2.2) and KOC (Section 2.3.1) (ECHA, 2008a).