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The extent to which Ca²⁺, Mg²⁺, Na⁺, K⁺ ions and pH independently mitigate acute copper toxicity for the cladoceran Daphnia magna was examined. Higher activities of Ca²⁺, Mg²⁺, and Na⁺ (but not K⁺) linearly increased the 48-h EC50 (as Cu²⁺ activity), supporting the concept of competitive binding of these ions and copper ions to toxic action or transport sites at the organism−water interface (e.g. fish gill, the biotic ligand). The increase of the EC50 (as Cu²⁺ activity) with increasing H⁺, however, seemed to suggest cotoxicity of CuOH⁺ rather than proton competition. Based on the biotic ligand model (BLM) concept, we developed a methodology to estimate stability constants for the binding of Cu²⁺, CuOH⁺, Ca²⁺, Mg²⁺, Na⁺, and H⁺ to the biotic ligand, solely based on toxicity data. Following values were obtained:  log K_CuBL = 8.02, log K_CuOHBL= 7.45, log K_CaBL = 3.47, log K_MgBL = 3.58, log K_NaBL = 3.19 …