3.3. Recommended analysis scheme
The concentrations of the two elements Cd2+ and Pb2+ dissolved
in fish samples can be determined by the method of
standard addition under the optimal conditions described
above, according to the following analysis scheme:
1- After filtration and acidification of the fish samples to
0.1 M HCl (pH =1), NaCl was added up to 2M, the
investigated solution was deoxygenated for 10 min by
purified nitrogen while stirring. Since the metal ions of
Pb2+ dissolve in mercury by forming amalgams, they
can be determined by DPASV. Preconcentration of this
metal is carried out at a potential of 0.35 V (vs. Ag/
AgCl) at a suitable deposition time, according to the following
reaction:
Pb2þ þ 2e þ Hg ! PbðHgÞðPreconcentration stepÞ
At the end of the deposition period, the stirring is
stopped and after a small rest period (30 s) the potential
is scanned anodically to-0.1 V with a scan rate of 10 mV/
s for Pb2+and a pulse height of 25 mV, the dissolved
amalgam is then reoxidized back to the following
equation:
PbðHgÞ ! Pb2þ þ 2e þ HgðDPASV stepÞ
1- The pH of the solution is raised up to 2.0 by adding
ammonia/ammonium chloride solution (as a basic solution)
and then the concentration of Cd2+ can be determined
by DPASV after preconcentration at Ed of
0.68 V. After a rest period of 30 s, the voltammogram
is then recorded anodically within the potential range
0.75 to 0.45 V with a scan rate of 10 mV/s and a
pulse height of 25 mV:
Cd2þ þ 2e þ Hg ! CdðHgÞðPreconcentration stepÞ
CdðHgÞ ! Cd2þ þ 2e þ HgðDPASV stepÞ
3.4. Test of linearity of calibration plots
The linearity between ip and concentration C was tested for the
investigated elements by the standard addition method under
the optimal conditions Fig. 2. Satisfactory linearity was obtained
over the concentration range generally found in the fish
samples for all investigated elements. The slope values of the
calibration curves of the investigated elements are reported
in Table 2.