Lead and Cadmium in fresh tobacco and its products and the potentiometric study of their complexes with nicotinic acid
Muhavini, Cleophas Wawire
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Tobacco and its products contain heavy metals, which if accumulated in the body, could cause biological and psychological disorders. The availability of the heavy metals both to animals and plants depend very much on their transport to body organs via water as a solvent. Therefore this work reports the results of a study done to determine the proportions of Pb and Cd compounds in both tobacco leaves and the processed tobacco products that are soluble in selected polar and nonpolar solvents. Extractions were done in solvents of varying polarities and the levels of these metals in both the residues and the extracts were determined by Atomic Absorption Spectrophotometry (AAS), and Total Reflection X-ray Fluorescence (TXRF). Polarographic technique was used to determine the labile forms of cadmium and lead in the water extracts. A study on the thermodynamics of the interaction of Pb and Cd with nicotine acid ligand was done potentiometrically. The stability constants obtained of the modelled complexes were used to model a likely speciation of Cd and Pb with the ligand. The results indicate that larger portions of the these metals form complexes which are insoluble in both water and non-polar solvents. Using water for instance, the residues contained as much as 98% of the total metal concentration, with only 2% being extracted using water. With hexane, the extract had as much as 36.7% and the residue 63.3% of the total metal. Levels of the extractable metal increased with decreased solvent polarity. The fraction of labile metals as determined by polarography was negligible. Nicotinic acid, a typical ligand with O donor atoms was found to form very weak complexes with Pb and Cd. The stability constants determined, compared well with literature values. From these results it appears that lead and cadmium in tobacoo and its products are in forms that are not labile. However, smoking could convert them into forms that are easily absorbed.