Voltammetric determination of stability constants and speciation of copper in salt-water lake (Bogoria) in Kenya
Kigunda, Japheth Irangu
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Differential Pulse anodic stripping voltammetry (DPASV) was used to measure cathodic shifts of copper oxidation peak potentials in the presence of carbonate, chloride, hydroxide, thiourea, fluoride or bicarbonate in aqueous potassium nitrate and in lake Bogoria water. Cyclic voltammetry was incorporated to determine reversibility of the peaks. Test solutions were held at a constant ionic strength of 2.34M. Peak potential shifts were uses to calculate stability constants of copper-Ligand complexes. These stability constants were then used to calculate species distributions of the identified complexes. Two copper concentration levels, 6.3 x10-5M and 6.3 x 10-8M, were used. Stripping voltammograms obtained at 6.3 x 10-5M showed Cu (I) and Cu (II) oxidation peaks at 0.302V and 0.020V versus Ag/AgCl respectively. Hence Cu (l) and Cu (ll) complexes were identified. Three carbonato Cu (ll) complexes were identified. Cu (CO3)4-3 was found to be most important at the carbonate level in the lake. Two Cu (l) carbonato complexes were established and Cu (CO3)3-2 was the major species. Four chloro (Cu (II) complexes were identified. CuCL2-4 was more abundant at the chloride level in the lake. Only one Cu (l) chloro complex was determined but its contribution to copper speciation in the lake was quite significant. Of two Cu (ll) hydroxo complexes identified, Cu (OH)+ was found to be the most abundant while Cu(OH)o contributed the most among the Cu(l) hydroxo complexes established. At the fluoride level in the lake, CuF+ and CuFo were found to be the most important fluoro species. Among the bicarbonato complexes identified, (Cu (HCO3)02 and Cu (HCO3)-2 were found to be the most important. Three Cu (l) and two Cu (ll) thiourea complexes were identified of which, Cu (CH4N2S)2+4 was found to be the most abundant species. Of the six ligands studied, hydroxo, fluoro and bicarbonato were of lesser importance in the speciation of copper than the other three ligands. The important physicochemical forms of copper are the chloro, carbonato and thiourea complexes. Apart from thiourea, no organic ligand was considered. Organic ligands such as urea, cysteine, acetate, phthalate, citrate and carbohydrates are expected to be present in the lake and they can form moderately to highly stable complexes which may affect the inorganic speciation of copper. Such effects would require a study of their own.