KROHNE SMARTSENS PH 8320 EN - Manual (Page 28)

6
TECHNICAL DATA
28
SMARTSENS PH 8320
www.krohne.com 11/2013 - 4002543902 - MA SMARTSENS PH 8320 R02 en
Technical data
6.1 Measuring principle
6.1.1 pH measurement
The measuring principle of a pH sensor is based on a pH sensitive glass (membrane glass).
When the pH sensitive glass gets into contact with a liquid, a thin layer of hydrated gel develops
on the surface, enabling an ion exchange between the glass surface and the liquid. The so-called
Nernst potential builds up on the glass surface. If both sides of the glass are in contact with
liquids, a voltage may be detected between the two surface potentials. The voltage correlates to
the difference in H
+ ion concentration and thus to the difference of pH values in both liquids.
The pH sensor contains an internal buffer solution with a known pH value. If the pH value of the
measuring medium on the outside of the sensor is equal to the pH value of the inner buffer, the
resulting voltage is 0 V.
If the pH value of the medium differs from the internal pH value, a voltage between the internal
and the external layer can be measured. From the resulting voltage, the pH difference of the two
liquids can be calculated.
The voltage is measured using a measuring electrode and a reference electrode; both are built
into the sensor. The measuring electrode is in contact with the known buffer solution in the pH
sensitive glass bulb. The reference electrode is immersed into a saturated solution of potassium
chloride (KCl). The KCl solution itself is in electrical contact with the measuring medium by
means of a diaphragm. The diaphragm prevents the measuring medium from penetrating into
the reference system but still allows electrical contact with the measuring medium.
Figure 6-1: Measuring principle for pH measurement
1 Reference electrode
2 Measuring electrode
3 Diaphragm in contact with KCl solution and measuring medium
4 Inner pH 7 buffer solution
5 Surface potential on the inside (contact with buffer solution)
6 pH sensitive glass (membrane glass)
7 Surface potential on the outside (contact with measuring medium)
8 Measuring medium
.book Page 28 Monday, November 18, 2013 10:42 AM