Thermo Scientific Dionex ICS-5000, Operator'S Manual

Page: 93 / 482

2 • DC Description
Doc. 065342-06 12/11
77
the carbonate removal step during monosaccharide and disaccharide
chromatography) at high hydroxide concentrations do not affect the electrode
performance. If sustained highly alkaline eluent conditions are required, use a
disposable gold electrode on a PTFE substrate or a conventional gold
electrode. Refer to the disposable electrode manual (Document No. 065040)
for additional eluent compatibility information.
Because conventional working electrode blocks are made of Kel-F
®
and use a
gasket made of Ultem
®
, there is no restriction on the concentration of organic
solvents that can be used with them (providing the solvent is compatible with
PEEK tubing).
There is also no restriction on the use of organic solvents when disposable
gold electrodes on PTFE substrate and PTFE gaskets are used.
2.15.2 Combination pH
–
Ag/AgCl Reference Electrode
The pH-Ag/AgCl reference electrode is a standard combination pH electrode
containing a glass membrane pH half-cell and a Ag/AgCl half-cell. The
combination pH electrode monitors eluent pH.
The Ag/AgCl half-cell is typically used as the cell reference electrode. To
minimize changes in the baseline, the combination pH–Ag/AgCl electrode
can be used as the reference electrode during a pH gradient.
pH Dependence
The potentials at which many redox reactions take place on metallic
electrodes are pH-dependent, with the potential shifting -0.059 V per pH unit.
This is especially true for metal oxide formation, oxidation, and oxidative
desorption. Since the reference potential of the combination pH–Ag/AgCl
electrode also shifts by -0.059 V per pH unit, pH-dependent potential shifts at
the working electrode are canceled.
Correcting for pH Dependence
At an eluent pH of 7, the reference potential of the entire electrode is the same
as that of the Ag/AgCl half-cell. As the eluent pH is increased, the pH half-
cell potential decreases approximately 0.059 V per pH unit. For example, at
an eluent pH of 12, the reference potential of the pH half-cell would be -
0.295 V relative to the Ag/AgCl half-cell. Therefore, at pH 12, the potentials