Voltammetric investigation of stability constants of lead in salt-water lake, Bogoria in Kenya

Abstract

The purpose of this research work was to determine the shifts in peak potentials (Ep) of the voltammograms. These were then to be used to obtain stability constants and hence calculate the distribution of the complexes with simple ligands. The ligands used were: hydroxide (OH-), Bicarbonate, (HCO3-), carbonate, (CO3-2), chloride, (Cl-), fluoride (F-), and thiourea, (CH4N2S) in salt-water lake. The lake water was obtained from lake Bogoria in the Rift Valley Province of Kenya. The highly saline lake is situated in the Baringo District.

The determinations were carried out in lake water and also in aqueous solutions at an ionic strength of I= 1.31. The Lake water ionic strength had been determined as at I = 1.78. Potassium nitrate (KNO3), was used as the supporting electrolyte for maintaining the ionic strength of the aqueous media. Two voltammetric methods were used i.e. Differential Pulse Polarography, (DPP) and Anodic Stripping Voltammetry, (ASV). These were used for measuring the shifts in peak potentials, (Ep). Two levels of lead concentrations were used, 1.45 x 10-4M at which both methods were applied and 4.83 x 10-7M at which only ASV was used.

The observed Ep shifts were then used in calculating the stability constants using the DeFord and Hume68 method. Lingane's24 method was only used as a supplement to DeFord and Hume's68. The obtained stability constants were then used to calculate the species distribution using Inczedy10 method. The results showed that four hydroxo complexes exist in both lakewater and aqueous potassium nitrate. The distribution of these differed in the two media. In the KNO3 (aq) media, Pb (OH)20 dominates while in the lakewater its contribution is minimal. Three bicarbonato complexes were found for the potassium nitrate aqueous media with Pb (HCO3)20 dominating while four were detected for the lakewater media with PbHCO3+ and Pb (HCO3)4-2 being the most abundant. Two carbonato complexes were detected with the Pb(CO3)2-2 being the highest contributor in both cases. Two chloro complexes were identified for the aqueous media in which PbCl20 was the highest contributor while three complexes were observed for the lakewater with PbCl+ being the highest contributor. Three fluoro complexes were noted in both media. PbF20 was the highest contributor in the aqueous media while PbF+ was the highest contributor in the lakewater media. Three complexes of thiourea were identified in both media. In the aqueous media, Pb(CH4N2S)2+2 and Pb (CH4N2S)3+2 contribute to the species distribution almost equally. In the lake media, it is Pb (CH4N2S)2+2, which is the most abundant complex.

Of the six ligands studies, fluoro, chloro, and thiourea complexes are of minor importance in the speciation of lead. At the naturally high pH of the lake, the bulk of lead present would be as the hydroxo, bicarbonato, and carbonato complexes. The low level of the non-alkali metals was attributed to the high level of carbonate in the lake. These would be precipitated as the metallic carbonates. The most abundant lead complexes were;

Pb(OH)3- logK3 = 9.88± 0.02

PbOH+ logK1 = 8.40± 0.09

Pb(OH)4-2 logK4 = 10.23± 0.07

PbHCO3+ logK1 = 3.60± 0.18

Pb(HCO3)4-2 logK4 = 5.74± 0.56

Pb(CO3)2-2 logK2 = 7.63± 0.47

PbCO3o logK1 = 6.40± 0.19

In this study, two complexes namely Pb (HCO3)4-2 and Pb (OH)4-2 were observed which had not been reported before in the literature. The complex Pb (HCO3)4-2 was detected in the lakewater media alone. The complex Pb (OH)4-2 was detected in both KNO3 (aq) and the lakewater media.