All reagents were of analytical grade and ultra pure water (18 MΩ) was used. 0.9216 g L−1 bismuth (III) standard stock solution was prepared by dissolving an appropriate amount of Bi(NO3)3·5H2O (99.999% Sigma-Aldrich) in 5 mL of 20% (v/v) nitric acid and taken up to 25.0 mL with water. 0.0922 g L−1 bismuth (III) working solution was prepared by a suitable dilution of the standard stock solution. 2000 mg L−1 lead stock solution was prepared by dissolving an appropriate amount of Pb(NO3)2 (Merck) in 20% (v/v) nitric acid and taken up with water. 20 mg L−1 lead working solution was prepared by a proper dilution of the standard stock solution. An acetate buffer solution (0.1 mol L−1, pH 4.5) was employed as supporting electrolyte. Nafion, a perfluorinated ion-exchange resin, (5%, w/v solution in a mixture of water and lower alcohols) was purchased fromAldrich. A 0.5% (v/v) ethanolic Nafion solution (NA) was prepared by diluting 100 μL of Nafion with ethanol (Merck p.a.). Multi-wallet carbon nanotubes (MWCNT) were purchased from Aldrich (inner diameter 110–170 nm and 5–9 μm of length). N-N-dimethylformamide (DMF) and potassium hexacyanoferrate(II) were provided from Aldrich.