User manual

H70.0.21.6C-01 Operating Manual GMH 3511 page 8 of 14
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6.3 pH electrode
6.3.1 Design
In most cases so-called combination electrodes are used. That means that
all needed elements are integrated in a single electrode (including
reference electrode).
Sometimes even a temperature sensor is integrated.
The picture on the right shows an electrode without temperature sensor.
There are several design types for the diaphragm, but generally said it is
the connection between electrolyte and the measured solution. A blockade
or soiling of the diaphragm is often the reason for the electrodes idleness
and erratic behavior.
The glass membrane has to be treated with care. The hydrated gel layer
forms on the surface of the glass membrane, which is of highest importance
for the measurement. The electrode has to be kept wet to preserve the
hydrated gel layer (see below).
6.3.2 Further Information
pH-electrodes are wear parts which need to be replaced, if the values
required can no longer be kept even after thorough cleaning and recovery
or the electrode signal gets to slow. The actual lifetime of an electrode
depends highly on the chemical or mechanical stress it is subjected to.
Please take into account that there are several materials that are in
aqueous solutions aggressive to glass; other chemicals may react with the
KCl-solution in the electrode thus causing blockades in the diaphragm.
Examples:
- with solutions containing protein, like they are used on the medical and biological sector, KCI may result in
the denaturation of the protein.
- coagulated varnish
- solutions with a relatively high concentration of silver ions
Any material depositing on the measuring membrane or the diaphragm will influence the measurements and
have to be removed at regular intervals. This can be done by means of automatic cleaning equipment.
Electrodes have to be stored in a way that they are kept wet. An adequate solution is to
store them with suitable protective cap filled with KCl 3 M. Please consider also the
instructions in the electrodes manual!
6.3.3 pH electrode suggestions
Different applications require different electrodes
1. Measurements in low-ion media (rain water, aquarium water, VE-waters)
GE 104 BNC (as of 20 µS/cm)
2. Sea water aquariums
Standard pH electrodes with 3mol KCl (GE 100 BNC, GE 114, GE 117)
3. Swimming pools
Standard pH electrodes with 3mol KCl (GE 100 BNC,GE 114, GE 117)
4. Soil checks
Glass electrodes with several diaphragms (GE 101 BNC); use insertion mandrel!
5. Electroplating, some paints and lacquers
Glass electrode GE 151 BNC
6. Cheese, fruit, meat
Insertion electrode (GE 101 BNC or GE 120 BNC).
When taking measurements in cheese, milk and other high-protein products use special cleaning agent to
clean electrode. (pepsin solution - GRL 100).
Standard cleaning: apply 0.1 molar HCl-solution for at least 5 minutes or protein cleaning agent.
The average service life of an electrode is 8 to 10 months but may be increased to 2 years if electrode is well
maintained and treated carefully. We regret not being able to give a more detailed information as this is
highly dependent on the individual case of application.