Metrohm 797 VA Computrace - Manual (Page 281)

9.6 Voltammetric problems
797 VA Computrace – Software
269
After Purging (oxygen removed from solution):
Unsuitable bridging electrolyte in the reference electrode
When choosing the bridging electrolyte in the reference electrode,
possible complications with the substances present in the analysis
solution must be taken into account.
With regard to the bridging electrolyte solution KCl 3 mol/L used
in many cases, the following are examples of disturbances which
can appear:
• Precipitation of KClO
4
in the ceramic diaphragm
with supporting electrolytes containing HClO
4
With partial blockage, inexplicable side peaks can appear. To
avoid such precipitations, with analysis solutions containing
HClO
4
a bridging electrolyte solution free from potassium (e.g.
NaCl 3 mol/L) must be used.
• Introduction of chloride through KCl outflow from
the reference electrode
The outflow of bridging electrolyte from the ceramic dia-
phragm of the 6.1245.010 Electrolyte vessel (part of the refer-
ence electrode) is 2...5 μL/h. The chloride Cl
– flowing into the
analysis solution can interfere with the determination of vita-
min C or the determination of Cu (see also Complex formation,
section 9.6). The use of chloride Cl
– free bridging electrolyte so-
lutions (e.g. KNO
3
sat.) is recommended as a countermeasure.
Overloading of the working electrode
Under unfavorable conditions (high concentrations and/or long
deposition times), the deposition of species at polarized electrodes
leads to overloading phenomena such as non-linear standard addi-
tion curves or splitting into multiple peaks which are caused by
saturation and different deposition forms.
A shorter deposition time usually solves the problem. The fol-
lowing rule of thumb holds: In general, deposition 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 without