Application of Graphite Paste Electrode Modified with CuHSA
The electro-oxidation of the graphite paste electrode modified with CuHSA for the electro-oxidation of nacetylcysteine and l-cysteine in 1.0 mol [L.sup.-1] KCl is illustrated in Figures 7 and 8, respectively.
The intensity of the anodic peak current of CuHSA increases proportionately with the analyte concentration.
This increased anodic current intensity occurs due to electrooxidation of the analytes (n-acetylcysteine and l-cysteine) by the mediator CuHSA. Fe(In) produced during the anodic scanning chemically oxidizes n-acetyl cysteine (l-cysteine), while this is reduced to Fe(II), which in turn is electrochemically oxidized to Fe(III).
Therefore, in view of this l-cysteine reaction, the oxidation of n-acetylcysteine on the electrode surface of the graphite paste electrode modified with CuHSA can be described by the following reactions:
The electrochemical behavior of the oxidation of nacetylcysteine in the electrode modified with CuHSA differs from the electrode modified with Prussian blue film, in which there is as a current increase in both oxidation potentials (0.79 and 0.95 V) when in the presence of n-acetylcysteine (1.0 mol [L.sup.-1], KCl, pH 3.5 versus Ag/AgCl) .
However, the Prussian blue film  is degraded by the hydrolysis reaction in solutions with higher pH values than five, which does not occur with CuHSA.