different electrodeposition potentials and then the metals are
stripped off the mercury electrode by oxidation. By increasing
the concentration of HCl, separation between the peak potentials
(Ep) of Cd2+ and Pb2+ increases gradually up to 0.1 M
HCl (pH = 1) which provides the best peak resolution. This
medium is found to be most suitable for the determination
of Cd2+ and Pb2+ with good peak separation. The effect of
chloride ion concentration on the peak separation was studied
as a function of NaCl concentration. It is clear that by increasing
the chloride ion concentration, the Ep of Cd2+ shifts to
more positive values. The resolution of the peak of Cd2+ is
found to increase with increasing the NaCl concentration up
to 2 M, where the best peak resolution is obtained. This means
that the optimal supporting electrolyte for the determination
of Cd2+ and Pb2+ by DPASV consists of 0.1 M HCl
(pH= 1) and 2M NaCl. The NaCl added has no effect on
the peak resolution of the other elements under investigation.
In this highly acidic medium, a major problem is that the
oxidation peak of Cd2+ is masked by the hydrogen evolution.
Accordingly, Cd2+ was determined by the same technique
(DPASV) after raising the pH of the medium to 2 by adding
ammonia/ammonium chloride (as a basic solution).
3.2. Optimization of the instrumentation conditions
The most reasonable scan rates for the determination of the
two elements are found to be 5–10 mV/s, depending on the element
determined. The electrode position potentials (Ed) of the
two elements were studied separately, and plotted against the
peak current (ip) values to make it easy to select the optimal
deposition potential for each metal ion Fig. 1. The optimal
deposition potentials for metal ions under investigation in
the specified solution and instrumental conditions were determined.
The relation between the peak current (ip) and deposition
time (td) is studied separately for the two metal ions. The
linearity is valid for the investigated metal ions through long
periods within the concentration levels normally found in fish
samples.