Metrohm 746 VA Trace Analyzer - Manual (Page 542)

7.5 Vagaries of the chemistry
746 VA Trace Analyzer / 747 VA Stand
7-43
7.5.3 Unsuitable bridging electrolyte solution in the reference
electrode
When choosing the bridging electrolyte in the reference electrode, possible compli-
cations with the substances present in the analysis solution must be taken into ac-
count.
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, with complete
blockage one of the following error messages appears:
VA STAND - ERROR 2: Working electrode faulty
VA STAND - ERROR 4: Reference electrode faulty
To avoid such precipitations, with analysis solutions containing HClO
4
a
bridging electrolyte solution free from alkalis or alkaline earths (e.g. acetate
buffer) must be used.
• Introduction of Cl – through KCl outflow from
the reference electrode
The outflow of bridging electrolyte from the ceramic diaphragm of the
6.1245.010 Electrolyte vessel (part of the reference electrode) is 2...5 µ L/h.
The Cl – flowing into the analysis solution can interfere with the determination
of vitamin C or the determination of Cu (see also section 7.5.6).
The use of Cl
– free bridging electrolyte solutions (e.g. KNO
3
sat.) is rec-
ommended as a countermeasure.
7.5.4 Overcharging of the working electrode
Under unfavorable conditions (high concentrations and/or long enrichment times),
the enrichment of species at polarized electrodes leads to overcharging phenom-
ena 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 following 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 without enrichment even with concentrations ρ > 100 µ g/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 dimethylglyoxime com-
plexes)
Prolongation of the enrichment time from 30 s to 120 s (keeping all other
measurement parameters constant) leads to non-linear standard additions
and in the case of nickel also to shifts in the peak maximum: