Metrohm 757 VA Computrace - Manual (Page 183)

8.6 Voltammetric problems
757 VA Computrace – Software
173
Oxygen interference
Oxygen can be electrochemically reduced and produces two
waves in the voltammogram, one of which is characterized by the
appearance of a pronounced maximum. The oxygen reduction
can interfere for two reasons:
• The signals of the analytes are masked by the oxygen waves.
This becomes noticeable primarily in trace analysis as the
oxygen is present in a relatively high concentration in solu-
tions saturated with air (ca. 8 mg/L at room temperature).
• The hydrogen peroxide formed in the first step of the oxygen
reduction can react further with certain substances.
For these reasons, oxygen must be removed from the analysis
solution before the polarographic analysis by saturation with inert
gas (usually nitrogen). With the inert gas flow rate of ca. 20 L/h set
on the 757 VA Computrace Stand in the factory, a purging time of
3...5 min usually suffices.
Unsuitable bridging electrolyte in the reference electrode
When choosing the bridging electrolyte in the reference electrode,
possible complications with the substances present in the analy-
sis 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 alkalis or alka-
line earths (e.g. acetate buffer) 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 forma-
tion). The use of chloride Cl
– free bridging electrolyte solutions
(e.g. KNO
3
sat.) is recommended as a countermeasure.