Discrimination of excess toxicity from baseline level for ionizable compounds: Effect of pH

The toxic effect can be affected by pH in water through affecting the degree of ionization of ionizable compounds. Wrong classification of mode of action can be made from the apparent toxicities. In this paper, the toxicity data of 61 compounds to Daphnia magna determined at three pH values were used to investigate the effect of pH on the discrimination of excess toxicity. The results show that the apparent toxicities are significantly less than the baseline level. Analysis on the effect of pH on bioconcentration factor (BCF) shows that the log BCF values are significantly over-estimated for the strongly ionizable compounds, leading to the apparent toxicities greatly less than the baseline toxicities and the toxic ratios greatly less than zero. A theoretical equation between the apparent toxicities and pH has been developed basing on the critical body residue (CBR). The apparent toxicities are non-linearly related to pH, but linearly to fraction of unionized form. The determined apparent toxicities are well fitted with the toxicities predicted by the equation. The toxicities in the unionized form calculated from the equation are close to, or greater than the baseline level for almost all the strongly ionizable compounds, which are very different from the apparent toxicities. The studied ionizable compounds can be either classified as baseline, less inert or reactive compounds in D. magna toxicity. Some ionizable compounds do not exhibit excess toxicity at a certain pH, due not to their poor reactivity with target molecules, but because of the ionization in water.