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Short-term toxicity to aquatic invertebrates

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Potassium dichromate is recommended as a reference substance in the acute toxicity to Daphnia test (Method C.2; EEC, 1992). A ring test involving 129 EC50 determinations from 46 laboratories determined the mean 24h-EC50 value as 1.5 mg K2Cr2O7/l (EEC, 1992). This is equivalent to an EC50 of 0.53 mg Cr/l, expressed on a concentration of chromium basis. The toxicity of chromium (VI) to invertebrates in short-term tests appears to depend on water properties such as hardness, pH and temperature. Persoone et al. (1989) noted decreasing EC50 values for Daphnia magna with decreasing hardness and with increasing temperature. Although the conditions included some which were outside those recommended, checks were carried out to make sure that they did not cause mortality or stress in the controls.

For some invertebrates, toxicity data is available for more than one of the chromium (VI) compounds included in this assessment. The limited available information indicates that, when expressed on a total chromium concentration, there are no significant differences between the toxicity of sodium chromate, sodium dichromate and potassium dichromate (allowing for differences in water properties). This is as would be expected if the equilibria between the chromate and dichromate anions are established in the test medium. Little information is available for ammonium dichromate and chromic acid, but it would be expected that their toxicity would be similar to that of the other chromates/dichromates, when expressed on a total chromium concentration basis.