1. Dr. Thomas Winkler ProMinent® Dosiertechnik Heidelberg
Background Information
pH-Electrodes
Dr. Thomas Winkler
ProMinent® Dosiertechnik Heidelberg

2. Dissoziation von Wasser
Definition pH
acid
Dissoziation von Wasser
(H20 H+ + OH- ),
pH = - log a H +
basic

3. Meaning of pH-value Change of pH-value by 1 step =
Change of H+ concentration by factor 10

4. Exampes for pH-values pH 3.6% HCl Coca Cola orange juice beer milk
blood
sea water
soap
4% NaOH
2.6
4.5
5.5
6,9
7.4 8 9 14

5. Constructions of pH-ElectrodesmV mV
1 glass membrane
2 internal electrolyte
3 internal electrode
4 reference electrolyte
5 external electrode
6 diaphragm
glass
electrode
reference
electrode
separate probe system
combination probe

6. pH-Measurement
Potentiometry: Measurement of the difference of potentials
Ag/AgCl/KCl/pHconst / /pHsample/KCl/AgCl/Ag
glass
membrane
saturated
KCl solution
The signal is a voltage [mV]
U = R * I
The measurement is performed
at high resistance (R)
at nearly zero current (I)
glass
membrane
sample

7. pH-Measurement Dependence of Electrode Signal on pH
the measured signal
slope
theoretical (Nernst)
57.2 mV/pH (20°C)
59.2 mV/pH (25°C)
61.2 mV/pH (30°C)
zero point (in practice)
+/- 0.5 pH
+/- 30 mV
failure at low pH
by experiment (practise)
theoretical (Nernst)
zero deviation
(Asymmetry-Potential)
failure at high pH

8. Dependence of Electrode Signal on Temperature
pH-Measurement
Dependence of Electrode Signal on Temperature
69,08 75
61,14 35
68,08 70
60,15 30
67,09 65
59,16 25
74,04
100
66,10 60
58,16 20
73,04 95
65,11 55
57,15 15
72,05 90
64,12 50
56,18 10
71,06 85
63,12 45
55,19 5
70,07 80
62,13 40
54,20
UN mV
Temp. [°C]

9. General Features of pH-Electrodes
glass membrane (H+- or pH-sensitive)
active gel (swelling) layer: exchange H+/Na+ ions
aging starts when electrode was manufactured
aging depends on temperature and extreme pH
for very low and very high temperatures: special glass required
glass should always be kept wet
very high electric resistance typical 100 M (25°C)
heavily temperature dependent
10-fold resistance when reducing temperature by 30 degree

10. Junction ceramic junction PTFE-ring junction open annular gap
The junction is the most critical part of the pH-probe beside the glass membrane!
ceramic junction
PTFE-ring junction
open annular gap
The junction should be a good electrical connection, but should let pass minimum electrolyte
The electrical resistance of a ceramic junction is approx. 3 kΩ

11. pH-Measurement diaphragm (junction medium - reference)
1 PHEP with ceramic diaphragm
2 PHER with PTFE-ring diaphragm
3 PHEX with open annular-gap diaphragm
4 PHED with double diaphragm
„double junction“ 1 2 3 4

12. pH-Measurement diaphragm (junction medium - reference)
porous ceramic diaphragm
constant quality is of utmost importance
defined porosity has to be complied with
typical electr. resistance 1 – 5 k
flow KCl typical 10 – 50 l/m WC per hour
porous PTFE-ring diaphragm (PHER)
perfect in sewage water – requires partial blockage,
since otherwise zero shift when „bleeding“
open annular gap (PHEX)
suspension, emulsion, soil content and high conductivity

13. Temperature dependence of pH-probes
temperature dependence of the slope
the slope increases with temperature
-57.2 mV/pH with 20°C, -61.2mV/pH with 30°C
the temperature dependence of the zero-point can be neglected
with values deviant from pH 7 and temperature variations:
automatic temperature compensation recommended
via measurement with Pt 100/1000
+ mV
50°C
20°C
pH 4
pH 7
pH 10
- mV

14. Cleaning of pH-probes 1. Glass membrane never mechanically!
lime, hydroxide (rust) with diluted hydrochloric acid, e.g. 0.5%
org. deposits (oil, grease) with alkaline cleaning agents, alcohol, acetone (acetone not with PHEK)
protein-containing media (milk, cheese, meat) with pepsin- hydrochloric acid solution (5 g/l – 0.5%)
2. Ceramic diaphragm also mechanically (carefully)
(e.g. with key file, nail file, sandpaper)
Sometimes works wonders!
sulfide (black) with thiourea + hydrochloric acid
inorg./org. deposits as above with acidic or alkaline
cleaning agents, alcohol

15. difference 160 mV/3pH = 53.3 mV/pH
Checking of pH-probes
Good probes reach the set value in buffers fast. In natural water also stable indication
Checking of the zero point and slope
switch the portable measuring instrument to Redox /ORP (mV indication)
pH-probe in buffer 7 e.g. –10 mV
pH-probe in buffer 10 e.g. –170 mV
difference 160 mV/3pH = 53.3 mV/pH
good zero point (pH 7): 0 mV +/- 30 mV
good slope: mV/pH (25°C)
slope of new probes: mV/pH (25°C)

16. Storage of pH-probes In a holder in KCl-solution 3-molar
in case of an emergency tap water, not DI-water!
drying out may destroy the probe (e.g. PHEX)
Storage at room ambient temperature
perfect: °C; avoid frost!
No long storage time
preferably max. ¼ year
Caution in the service station car!
avoid high temperatures in summer
avoid low temperatures in winter
Frost may destroy probes and may freeze buffers!