Influence of pH on the toxicity of substituted phenols to fish

The 96-hr LC50 values of six chloro-, bromo-, and nitro- substituted phenols to the guppy (Poecilia reticulata Peters) were determined by a semistatic method in the pH range of 5 to 8. The pH did not affect appreciably the toxicity of 4-chlorophenol, which is primarily nonionized over the whole range, but the toxicity of more acidic phenols decreased as the pH increased. The changes in toxicity were substantially smaller than they would be, if only the nonionized phenol form were toxic. The results could be explained by assuming that the phenate ion also contributes to the toxicity, but its molar toxicity decreases with rising pH.