Polarographic studies of copper speciation in lake Elmenteita
Theuri, Gikandi Johnstone
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The stability constants of copper complexes with the ligands, chloride (CL-); Fluoride (F-): carbonate (CO2-3); bicarbonate (HCO-3) and hydroxide (OH-) were determined using differential pulse polarography (DPP) and differential pulse anodic stripping voltametry (DPASV). For DPP method the copper complexes were studied at a metal concentration of 6.3X10-4M whereas for DPASV method, the complexes were studied at both 6.3 x 10-4M. and 6.3 x 10-7M copper concentrations. The experimental work covered both the artificial lake water (aqueous KNO3) and the actual lake water media at the same ionic strength (0.270). The lake water was sampled from lake Elmenteita and the artificial lake water was maintained to the ionic strength of the lake water using 1 molar analytical reagent potassium nitrate solution. In determining the complexes of copper with the CO2-3 and F- ligands, it was found necessary to add 0.005% gelatin solution to suppress the maxima and minima in the polarographic peaks of copper. This was found to improve on both the symmetry and the reversibility of the peaks. The methods of Lingane38 and DeFord-Hume54 were used to determine the copper complexes existing and their respective stability constants in both the lake water and the KNO3 media. The distribution of these complexes in the lake water was calculated using the method given by Incenzy51. It was noted that while it was possible to obtain the peaks for the reduction of Cu2+(aq) ion at -0.010V in pure Cu2+(aq) solutions versus saturated calomel electrode (SCE), it was not possible to obtain the complexes of Cu2+(aq) ion with the addition of ionic ligands. This was due to the reduction of Cu2+(aq) to Cu+(aq) which in turn formed copper (l) complexes. The peak for the reduction of Cu+(aq) ion on the mercury electrode occurred at 0.380V against SCE. The values of stability constants of copper (l) complexes with the ligands under study together with their species distributions were determined. From the values of stability constants and the species distribution, this study revealed that copper (l) in Lake Elmenteita would predominantly exist in the carbonato form while the bicarbonato complexes contributed least to the speciation of copper (l) in the lake. The concentration of copper (ll) seemed to have an effect on the number of peaks produced. At high levels of copper (ll) (10-4M), two peaks appeared at -0.010V and +0.380V. The peak at -0.010V was due to the reduction of Cu2+(aq) ion while the one at +0.380V was due to the reduction of Cu+(aq) ion. However, at low concentrations of copper (ii) (10-6M 10-7M) only the peak due to the reduction of Cu+(aq) appeared while at 10-8M, no peak appeared. The DPASV method was found to be more sensitive than DPP method because with the latter method, it was only possible to study complexes at [Cu+] while with the DPASV method; it was possible to study complexes at [Cu+] as low as 10-7M. The following complexes of copper (l) which have not been reported before were observed: - CuF-2, CuCO-3, Cu(CO3)3-2, Cu(CO3)5-3, CuHCO3 and CU(HCO3)-2.