Acute toxicity of benzoic acids to the crustacean Daphnia magna

Abstract

The acute immobilization toxicity of benzoic acids substituted with hydroxyl and/or methoxyl groups on the aromatic ring was determined for the freshwater crustacean Daphnia magna under neutralized condition (initial pH: 7.45 ± 0.05). Toxicity, expressed as EC 50 value, varied depending largely on the number and position of phenolic hydroxyl groups. Especially, benzoic acids with ortho-substituted hydroxyl groups were more toxic than benzoic acids with meta- and/or para-substituted hydroxyl groups. Whereas the limited data indicated that methoxyl substitution had relatively small and variable effects on the toxicity. Of the tested compounds, 2,4,6-trihydroxybenzoic acid showed the highest toxicity with the 48 h EC 50 of 10 μmol l −1. This was 700 times as toxic as the parent benzoic acid (48 h EC 50 = 7.0 mmol l −1) and about two orders of magnitude higher than those previously reported for monohalogenated benzoic acid derivatives in Daphnia. Within the subgroups based on the number of hydroxyl groups ( N OH), the toxicity variations due to the position of hydroxyl groups appeared to be correlated with the logarithms of n-octanol/water partition coefficients (log P ow). The toxicity of benzoic acids existing almost entirely as their ionized forms could be expressed as simple structure–toxicity relationships using these two descriptors ( N OH and log P ow).