Chemical Speciation, Stability and Toxicity of Sulphate and Phosphate Salts of Copper

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Chemical modelling involves use of published stability constant data to predict equilibrium distribution of metal species under various conditions of water quality together with known concentrations of various ions and suspended solids in the water and to complete the equilibrium concentrations (or activities) of the various species. An extensive thermodynamic data consisting of bank of all conceivable stability constants for a system is needed.To model the speciation of Cu with the common environmental ligands SO42- and PO43-method. Cu was analysed by atomic absorption spectrophotometer. Turbidimetric method using a Uv-visible spectrophotometer, Spectroscan 30 calibrated was used to study SO42-at 420 nm providing a light path of 2.5 to 100 nm. Phosphate was determined with a spectrophotometercalibrated at 690 nm using ammonium molybdate, concentrated sulphuric acid, stannous chloride reagent and standard phosphate solution. Chemical modelling was performed using the specific ion interaction theory (SIT). SIT uses an extended Debye-Hückel expression and goes beyond the more simple Davies equation. It does this by adding medium specific properties and by including ion-pairing between the medium ions and the species involved in the equilibrium reaction.Results:The distribution of CuSO4 (aq) was 19.37 % with the free sulphate distribution at 80.28%.CuSO4(aq) is stable while at pH 8.0 and [Cu2+]T =1.078533 mmo/L, the maximum concentration of CuHPO4 (aq) occurs at pH 8.0; it reaches 31.69 % of [CuII]T when [PO43-]T = 0.44892 mmol-1and a maximum of 62.32 % when [CuII]T = 0.09 mmol/L. CuHPO4(aq) is unstable.