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

2–

and a

CuCl

–

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

–

Supporting electrolyte:

without EDTA

with EDTA (0.001 mol/L)

Analysis solution:

g/L Cu; 10

L HCl 30%

Standard addition:

with 250 ng Cu

Electrode:

HMDE (enrichment 90 s at –600 mV)

CuEDTA

CuCl

2–

CuCl

–