Solubility of pentachlorophenol in aqueous solutions: The pH effect

As indicated in most literature reports, the solubility of pentachlorophenol (PCP) in water ranges between 10 and 20 mg l−1. Since PCP is a weak acid (pKa 4.35), its solubility increases drastically with increasing pH. When PCP dissolves in water two forms are normally present: undissociated PCP (PCP°) and a dissociated anionic form, i.e. pentachlorophenolate (PCP−). Both forms differ in their physico-chemical properties and in their microbial response and toxicity. It is therefore important to know the solubility behaviour of PCP at specific pHs when working with site remediation and toxicity assessment. While anionic PCP is very soluble, experimental data have shown that undissociated PCP has a solubility limited to 10 μM (3 mg l−1). Theoretical calculations have confirmed this value to be independent of the solutionʹs pH. A non-empirical model was developed for estimating the “total” PCP aqueous solubility at different pHs using the standard solubility value and PCPʹs dissociation constant. The method may be extended to other chlorophenols and to most wastewaters.