Metrohm 757 VA Computrace - Manual (Page 184)

8 Troubleshooting
757 VA Computrace – Software
174
Overcharging of the working electrode
Under unfavorable conditions (high concentrations and/or long
enrichment times), the enrichment of species at polarized elec-
trodes leads to overcharging phenomena such as non-linear
standard addition curves or splitting into multiple peaks which are
caused by saturation and different deposition forms.
A
shorter enrichment time usually solves the problem. The fol-
lowing rule of thumb holds: In general, enrichment should not be
carried out except in solutions with a mass concentration
ρ < 0.5
mg/L (= 0.5 ppm). In several cases work can be carried out with-
out enrichment even with concentrations
ρ > 100 µg/L (e.g. DP
voltammetry at the HMDE or also at the DME).
The effects of an enrichment time which is too long are shown by
the following two examples:
• Nickel and cobalt determination in the trace region by
cathodic adsorption stripping voltammetry (with di-
methylglyoxime complexes)
Prolongation of the enrichment time from 30 s to 120 s (keep-
ing all other measurement parameters constant) leads to non-
linear standard additions and in the case of nickel also to
shifts in the peak maximum:
Enrichment time: 30 s Enrichment time: 120 s
Analysis solution: 20 µg/L Ni; 20 µg/L Co
Supporting electrolyte: 0.1 mol/L NH
4
Cl/NH
3
; pH = 9.3
5⋅10 -4 mol/L dimethylglyoxime (DMG)
Standard addition: with 200 ng Ni / 200 ng Co
Electrode: HMDE
Co
Ni
Co
Ni