Measurement of Association and Dissociation Rate Constants for Lead(II)/18-Crown-6 Using Square Wave Voltammetry at a Glassy Carbon Mercury Film Electrode

Understanding the rate parameters of metal ion-ligand complexes is necessary for sensing, separations, and responsive materials. The complexation between 18crown-6 and lead(ll) is of particular interest due to the potential use of this chemistry in sensors and separations. We have applied square wave voltammetry at a glassy carbon mercury film electrode to this problem. Lead(ll) in aqueous solution containing an excess of 18-crown-6, studied with different experimental time scales, yields stoichiometry, binding constants, and rate constants (25 °C). For pulse times longer than 10 ms, the glassy carbon mercury film electrode acts as a planar electrode. For shorter pulse times, a roughness correction factor must be used to calculate dimensionless current because of the increase in effective area due to the droplike nature of the adsorbed mercury. Lead(ll) forms a 1:1 complex with 18crown-6 in both nitrate and perchlorate media. Log K for the complex with the nitrate counterion is 4.13 +-0.09 (SEM); in the presence of perchlorate it is 4.35 ± 0.09 (SEM). The formation rate constants, k{, for the nitrate and perchlorate systems are (3.82 ± 0.89) *10^7 (5.92 ± 1.97) x 10^6M^-1S^-1 respectively. The dissociation rate constants, kd are (2.83 ± 0.66) x 10^3 s^-1 with nitrate as the counterion and (2.64 ± 0.88) x 10^2 s-1 with perchlorate as the counterion. The significant difference in rate constants for the two anions is probably caused by the ion pairing that occurs with lead(ll) nitrate.