Pseudopolarography of trace metals. Part II. The comparison of the reversible, quasireversible and irreversible electrode reactions

The theoretical and experimental pseudopolarographic curves of reversible, quasireversible and irreversible electrochemical reactions were compared and evaluated. The measurements were performed on a stationary mercury drop electrode (SMDE, PAR 303A), using differential pulse anodic stripping voltammetry (DPASV). A good agreement between the theoretical and the experimental shift of the half-wave potential with an increasing accumulation time was obtained for the reversible pseudopolarograms of 10−7 mol dm−3 Cd(II) (in 0.1 mol dm−3 NaClO4, pH ∼2). As compared with the curve of the logarithmic analysis of the polarogram, the corresponding curve of the pseudopolarogram is steeper in the region of the half-wave potential. It has been shown that even though the pseudopolarograms are quasireversible or irreversible, there is a range at the foot of the curves with a reversible slope (usually below 10% of the total/limiting current). It has been verified that the range of this reversible slope can be extended by increasing the accumulation time, lowering the mercury drop size and diminishing the thickness of the diffusion layer. The estimated value for its approximative evaluation is about 1% of the total/limiting current. This is essential for the determination of the corresponding electrochemical parameters, such as: the formal potential (E°′), transfer coefficient (α) and rate constant (ks). From the experimentally obtained reversible slope of the (pseudo)polarographic curves of Zn(II) (in 1 mol dm−3 NaClO4, pH 4.7 ± 0.1), the parameters for the quasireversible electrochemical reactions were estimated as follows: E°′=−0.964 ± 0.002 V, α=0.24 ± 0.02 and ks∼2–3×10−3 cm s−1. It is shown that an accurate transfer coefficient can be calculated from the curves of the logarithmic analysis of the quasireversible pseudopolarograms, which is not the case for the polarographic curves. The irreversible system, tested on the electrochemical reaction of the CdNTA complex (in 0.1 mol dm−3 NaClO4, pH 7.9 ± 0.1), shows relatively good agreement between the experimental and the theoretical dependences. The (pseudo)polarographic measurements enabled approximate estimation of the electrochemical parameters (E°′=−0.835 ± 0.010 V, α=0.55 ± 0.02 and ks=1.0 ± 0.4 × 10−4 cm s−1) which are in fairly good agreement with the literature data.