The toxicity of pentachlorophenol (PCP) and 2,3,6-trichlorophenol (TCP) to the crayfish (Astacus fluviatilis L.) was measured at two pH levels. The eight-day LC50 estimates obtaine were 53 ppm for PCP and 19 ppm for TCP, at pH7·5 (13°C). Lowering the pH to 6·5 increased the toxicity of PCP by a factor of 5·9 and that of TCP by 3·5. Both factors are significantly smaller than could be expected if only the concentrations of the non-ionised chlorophenol molecules were essential. On a weight basis, TCP was more toxic than PCP also at pH 6·5. The LD50 estimates obtained by injection were, however, lower for PCP than for TCP, suggesting that the latter may penetrate more rapidly into the crayfish. This is in agreement with the smaller degree of ionisation of TCP due to its higher pKavalue. © 1977.
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