Metrohm 797 VA Computrace - Manual (Page 279)

9.6 Voltammetric problems
797 VA Computrace – Software
267
3. If a second element is present at the same potential: add this
element to the sample and carry out the analysis again. If the
second peak has become higher then the second element is
present. Might it be possible to selectively mask this second
element with a
complexing agent?
4. For Cu: work without chlorides in the electrolyte or increase the
chloride concentration massively.
5. Has any substance formed a precipitate in the measuring vessel
(e.g. lead perchlorate standard with KCl as electrolyte)?
6. Try out eluents with different compositions (addition of comp-
lexing agents).
7. Check method parameters.
8.
Try another measurement mode like AC1. If one substance is
reversible and the second one irreversible, only the reversible
substance is detected by AC1.
With MME:
1.
Check MME. If necessary, change capillary and replace sealing
needle (see Hardware Manual).
With RDE/SSE:
1. Check RDE and polish if necessary (see Hardware Manual).
Standard addition peaks displaced
With all types of electrodes:
1. Standard solutions have been made too acidic.
2. Buffering capacity of the electrolyte is not sufficient: increase
electrolyte volume.
3. Electrolyte solution too old: make up a new one. Its working
life with organic additives may be as short as 1 day or less.
With HMDE:
1. If HMDE is used potential displacements of more than 20…30
mV are often normal and have to be accepted;
particularly in
adsorptive stripping voltammetry.
With RDE/SSE:
1.
Electrode surface overloaded : reduce sample volume.
No addition
With all types of electrodes:
1.
Has the correct standard solution been used or is the concen-
tration of the solution too low: increase the volume of the
standard addition solution or use a higher concentration or re-
duce the sample amount accordingly.