| Abstract: | The toxicity of 13 anilines and 11 phenols to Daphnia magna has been assessed at pH values of 6.0, 7.8, and 9.0. Toxicity of anilines decreases with decreasing pH, whereas the toxicity of phenols decreases with increasing pH. These variations in toxicity, are clearly brought about by the effect of ionization, especially for the phenols. Toxicity decreased with an increasing proportion of ionized molecules. The relative contribution of the ionized and unionized molecules was assessed. It is believed that, at least for the phenols, the ionized form of the compounds does contribute to the toxicity of the molecules. However, the decreased overall toxicity suggests that a lower relative toxicity is attributable to the ionized form. Attempts were also made to develop quantitative structure‐activity relationship (QSAR) models for these toxicity data. Reasonable QSARs were produced for the anilines and phenols, acting as polar narcotics, considered together; these QSARs included a measure of hydrophobicity, pKa, and hydrogen bonding. When the phenols and anilines were considered separately, better, hydrophobicity‐based, QSARs were obtained. Most compounds, with the exception of 2,4‐dinitrophenol, resorcinol, and pentachlorophenol, were thought to be acting by a polar narcosis mechanism of toxic action. There was some evidence that 2,4‐dichlorophenol and 2,4,6‐trichlorophenol may be acting as weak acid respiratory uncouplers, despite earlier suggestions that they act as polar narcotics. © 2000 John Wiley & Sons, Inc. Environ Toxicol 15: 140–148, 2000 |
