Abstract
The effect of the adsorption of tetraethylammonium (TEA) cations, which present both ionic and organic characteristics, on the reduction of Cd(II) ions have been studied from dc and ac measurements at the dropping mercury electrode. The resistance to the charge transfer (Rct) and Warburg coefficient (σ) parameters have been determined through impedance measurements. Thus, the global velocity constant has been obtained. The reduction process of Cd(II) in perchloric media is reversible and is affected by the adsorption of TEA cations, especially at high TEA concentrations. Values of E1/2, half wave potential, and DO, diffusion coefficient, obtained from both dc and ac measurements agree. The velocity constants show a decrease as TEA concentration increases, with values ranging from 0.6 to 0.01 cm·s−1. The inhibitory effect of TEA adsorption on the electrode process and the relationship between electrode coverage, θ, and velocity constants, K, using several isotherm equations, have been discussed. The best fit was obtained with the equation K = 0K(1 − θ)a with an a value close to three, indicating a blocking effect and electrostatic repulsion due to TEA.
Highlights
The reduction of Cd(II) ions at the dropping mercury electrode (DME) has been widely used to test models for electrode reactions
The catalytic effect of thiourea has been studied [5,6], and it was concluded that thiourea adsorption changes the mechanism of Cd(II) reduction; the inhibition effect of sucrose [8] only influence the values of the velocity constants but the CEE mechanism remains
The working electrode (WE) was a DME; the counter electrode (CE) was a pool of mercury; and the reference electrode (RE) was a sodium-saturated calomel electrode (SSCE or NaSCE) connected to the cell via a salt bridge filled with the supporting electrolyte
Summary
The reduction of Cd(II) ions at the dropping mercury electrode (DME) has been widely used to test models for electrode reactions. The catalytic effect or inhibition by organic substances could introduce changes in the mechanism or only influence values of the parameters and velocity constants. The catalytic effect of thiourea has been studied [5,6], and it was concluded that thiourea adsorption changes the mechanism of Cd(II) reduction; the inhibition effect of sucrose [8] only influence the values of the velocity constants but the CEE mechanism remains. Tetraethylammonium (TEA) ions have been selected because of their double character of ionic and organic, and because their adsorption has been studied previously [10,11]. The impedance data were analyzed by fitting them to the expressions valid in the case of Randles behavior [21,22], giving the values of the Warburg coefficient, σ, the charge transfer resistance, Rct, the irreversibility quotient, p = Rct/σ, and the capacitance of the double layer, Cdl
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