HND-R106
HND-R106 K09/0422
page 11
9. Principles of the measurements
9.1 pH measurement
The pH value specifies the acid or alkaline behaviour 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):
with
c
(H
+
): concentration of dissolved hydronium ions in
mol/l
f
(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 9.4
9.2 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 mVH.
