6 | Bases for measurement
PH20
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B-H86.0.11.DB201-1.0
6 Bases for measurement
6.1 pH measurement
6.1.1 Explanation
The pH value describes the acidic or alkaline behaviour of an aqueous solution. A pH
value below 7 is acidic, a value above 7 is alkaline. A pH value of 7 is neutral.
The pH measurement is very precise, but also sensitive. The measured signals are
very weak and high-ohmic. This is the case, in particularly in low-ion media.
NOTE
In order to detect the pH value of a solution, it should always be recorded together
with the measurement temperature, because most liquids change their pH value with
the temperature.
The following must be observed:
– avoid interference, electrostatic charges, etc.
– keep plug contacts clean and dry
– prevent electrodes which do not have any special waterproof versions from exten-
ded immersion above the shaft
– calibrate electrodes sufficiently often. The can range from every hour to several
weeks, depending on the electrode and the application
– Use a suitable electrode
6.1.2 pH electrode
NOTE
Normally, so-called pH single-rod measuring chains are used. They include all neces-
sary components that are integrated in an electrode.
6.1.3 Design
1. Coaxial cable
2. Reference electrode
3. Measuring electrode
4. Refill opening
5. Electrolyte
6. Internal buffer
7. Diaphragm
8. Glass membrane / source layer
The diaphragm, which establishes a connection between the electrolyte and the liquid
to be measured, can be designed in different ways. Clogging or soiling of the dia-
phragm is a frequent cause of a malfunctioning or sluggish electrode. Always handle
the glass membrane with extreme care. The so-called source layer forms there. This
is crucial for the measurement and must always be kept moist.
There are also electrodes with integrated temperature sensors.
