FAS622en_pH measurement

2 Measurement
The combination pH electrode consists of a pH glass electrode surrounded by the reference electrode.
The shaft can be made of glass or plastic. The important structural elements of this application
are the filling material of the reference electrode, the electrolyte, and in addition, an opening at
the bottom end of the reference electrode, the diaphragm, and a rounded glass tip at the bottom
end of the electrode – the glass membrane.
Electrolyte and diaphragm
The electrolyte and the diaphragm must be matched to one another according to the application.
For heavily contaminated liquids, such as waste water, suspensions or emulsions, diaphragms that
are insensitive to contamination are required, e.g. annular or ground. A stiffened electrolyte, gel or
polymerizate, reduces the electrolyte outflow and with it the maintenance costs.
For water that is optically relatively clear, such as swimming pool water and drinking water, electrolyte
solutions (possibly thickened slightly) are more suitable. Because of the lower viscosity, finepored
diaphragms, such as ceramic or glass fiber diaphragms, are necessary in this case.
For many applications, the electrolyte must contain no silver ions, or as few as possible. Solutions
containing sulfide, and even water with a low salt content, form silver compounds in the diaphragm
that are difficult to dissolve, and this can lead to degradation of the pH measurement.
Membran shapes
The membrane can be manufactured in different shapes, depending on the application. The spherical
and rounded tip membranes are particularly robust for operational use. These electrodes have
a low susceptibility to wear and are easy to clean.
Integrated temperature sensor
The temperature is a fundamental item of information for the temperature compensation function
of the transmitter, and it often has to be documented as additional information for the pH value as
well. Because of this, electrodes that already incorporate a temperature sensor are particularly
beneficial. Only one connecting cable and installation point is required for both sensors. However,
there were problems until now, as each manufacturer developed his own connection system,
which meant that measurement devices from different manufacturers were not compatible. Because
of the fully justified demands made by users, a team of experts at NAMUR (standards organization
for measurement and control technology in the chemical industry) tested the various connection
systems against defined criteria.
SMEK terminal head
Fig. 10:
SMEK terminal head
he result was a clear recommendation for the SMEK connection favored by JUMO. However, measurement
systems with the very robust VP (Variopol/Variopin) terminal heads are available on request.
Because pH electrodes involve parts subject to wear, careful consideration should be given
as to whether versions with integral temperature probes should be used: each time the electrode is
changed, the temperature probe is scrapped as well, and a replacement has to be paid for along
with the new pH electrode.
18 2 Measurement JUMO, FAS 622, Edition 04.07