Metrohm 746 VA Trace Analyzer User Manual
Page 545
7 Safety, Errors, Troubleshooting, Diagnosis, GLP
746 VA Trace Analyzer / 747 VA Stand
7-46
7.5.6
Complex formation
Substances determined polarographically can occur in various complexed forms,
depending on the composition of the analysis solution. As complexing is always
associated with a shift in the half-wave potential and the limiting current, difficulties
can arise in the peak evaluation. Such difficulties must be eliminated by appropriate
changes in the composition of the supporting electrolyte.
If it is not possible to remove the interfering complexing agents from the analysis
solutions or to mask them by suitable substances, it is often helpful to change the
pH of the supporting electrolyte. Another measure which is often used involves the
addition of a ligand of high complexing power (e.g. EDTA) to bring about 100%
change of the analyte to a definitive form. The latter possibility is also used in the
following example:
•
Copper determination in chloride-containing solutions
Copper can occur in chloride-containing solutions as both a CuCl
4
2–
and a
CuCl
2
–
complex. The two associated current peaks are near each other. In
unfavourable cases, the determination of copper is not possible. The diffi-
culties disappear after addition of the complexing agent EDTA as now all
copper is completely in the form of a Cu-EDTA complex. (Increasing the
chloride concentration [e.g. by addition of 1 mL of a 1.5 mol/L KCl solution
of the greatest possible purity per 10 mL analysis solution] would also give
a clearly defined current peak for CuCl
2
–
.)
Supporting electrolyte:
Supporting electrolyte:
without EDTA
with EDTA (0.001 mol/L)
Analysis solution:
25
µ
g/L Cu; 10
µ
L HCl 30%
Standard addition:
with 250 ng Cu
Electrode:
HMDE (enrichment 90 s at –600 mV)
CuEDTA
CuCl
4
2–
CuCl
2
–