H70.0.01.6C-02 Operating Manual GMH 3531 page 7 of 18
5 Start Operation
Connect electrodes, turn device on via key.
After segment test the device displays some configuration:
if zero point or slope correction is active
(p.r.t chapter 7 „
Configuration“ and 9 „Input adjustment“)
Remove protective cap from electrode. (Attention: Cap should contain KCL 3 M or storage solution)
After that the device is ready for measuring.
6 Principles of the measurements
6.1 pH measurement
The pH value specifies the acid or alkaline behavior of aqueous solutions.
Solutions with a pH values below 7 are acid (the more below 7 the more acid), values higher than 7 mean
alkaline and pH = 7 means neutral.
The pH value is the negative common logarithm of the hydrogen ion activity (this is often approximately
equal to the concentration of dissolved hydronium ions):
log
H
∙
H
mol l
⁄
with H
: concentration of dissolved hydronium ions in mol/l
H
: activity coefficient (normally lower than 1)
The abbreviation “pH” stands for pondus Hydrogenii (Latin pondus: “weight“; Hydrogenium: “hydrogen“).
pH values should always be measured and saved together with the temperature of the solution:
i.e. pH 5.87; 22.8 °C.
Reason: The pH values of most liquids are depending on temperature.
The pH measurement is highly precise but also very sensitive. The measured signals are very weak (high
resistance), especially if measured in low-ion media. Therefore it is very important that:
- disturbances (electrostatic charge, etc.) are prevented.
- a stable value is reached by slow stirring.
- contact plugs are kept clean and dry.
- the electrode shaft is not submersed for a longer period (exception: special water-proof types).
- the electrode is calibrated often enough (see below). The needed calibration frequency depends on
the used electrode and application and varies between once every hour to once in several weeks.
- A suitable electrode is chosen. Please refer to chapter 6.4
6.2 ORP measurement
The ORP potential (also known as reduction potential or ORP) is a measure of the oxidizing or reducing
potential of the measured media compared to the standard hydrogen electrode.
This potential is often used in swimming pools to rate the disinfectant effect of chlorination. Also for aquarium
keepers the ORP value is an important parameter, because fishes need ORP values within specified
boundaries to live. Drinking water purification, water monitoring and industrial applications are some further
fields where the ORP value is of importance.
The measurement is done with a common silver chloride electrode (reference system with 3-molar
potassium chloride solution). The measured value can be directly displayed (mode mV) or converted to
“reference system: standard hydrogen electrode” and temperature compensated at mode mV
H
.
There is no calibration comparable with that of the pH measurement. However, the electrode’s capability can
be checked with ORP test solutions (for example GRP 100).
Suitable ORP electrodes: e.g. GE 105 BNC
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