Dr. Thomas Winkler
ProMinent® Dosiertechnik Heidelberg
Definition pH
pH
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
2
H+ conc.
commen log.
1
100
0,1
10-1
0,01
10-2
0,001
10-3
0,0001
10-4
0,00001
10-5
Dissoziation
0,000001 von Wasser
10-6
(H0,0000001
H+ + OH- ), 10-7
20
0,00000001
10-8
0,000000001
10-9
0,0000000001
10-10
0,00000000001
10-11
0,000000000001
10-12
0,0000000000001
10-13
0,00000000000001
10-14
acid
pH =
- log a H +
basic
Meaning of pH-value
Change of pH-value by 1 step
=
Change of H+ concentration by factor 10
3
Exampes for pH-values
pH
0
2.6
4.5
5.5
6,9
7.4
8
9
14
4
3.6% HCl
Coca Cola
orange juice
beer
milk
blood
sea water
soap
4% NaOH
Constructions of pH-Electrodes
mV
mV
glass
electrode
reference
electrode
separate probe system
5
1 glass membrane
2 internal electrolyte
3 internal electrode
4 reference electrolyte
5 external electrode
6 diaphragm
combination probe
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
6
sample
pH-Measurement
Dependence of Electrode Signal on pH
failure at low pH
by experiment (practise)
theoretical (Nernst)
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
zero deviation
(Asymmetry-Potential)
failure at high pH
7
pH-Measurement
Dependence of Electrode Signal on Temperature
8
Temp.
[°C]
UN
mV
Temp.
[°C]
UN
mV
Temp.
[°C]
UN
mV
0
54,20
40
62,13
80
70,07
5
55,19
45
63,12
85
71,06
10
56,18
50
64,12
90
72,05
15
57,15
55
65,11
95
73,04
20
58,16
60
66,10
100
74,04
25
59,16
65
67,09
30
60,15
70
68,08
35
61,14
75
69,08
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
9
Junction
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
10
pH-Measurement
diaphragm
(junction medium - reference)
1 PHEP with ceramic diaphragm
2 PHER with PTFE-ring diaphragm
3 PHEX with open annular-gap diaphragm
1
11
2
3
4
4 PHED with double diaphragm
„double junction“
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
12
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
13
pH 7
- mV
pH 10
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
14
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):
good slope:
slope of new probes:
15
0 mV +/- 30 mV
53...60 mV/pH (25°C)
55...59 mV/pH (25°C)
Storage of pH-probes




16
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: +10...30°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!
Selection of pH - Probes
clear medium
ceramic
diaphragm
MEDIUM
temperature
above 60 °C
medium with
solid material
medium with
Fluoride
17
temperature
up to 60°C
w/o pressure
PHEN (high accuracy, long lifetime)
PHEN- 3D (low conduct. 50 μS/cm)
max. 0.5 bar
PHE (swimmingpool, potable water)
max. 3 bar
PHES (swimmingpool, potable water)
max. 6 bar
PHEP (galvanic, process)
PHEN (high accuracy, long lifetime)
PHEN- 3D (low conduct. 50 μS/cm)
w/o pressure
max. 6 bar
PHEP (galvanic, process)
max. 8 bar
PHED (galvanic, process with species like Cr 6+, CN-)
normal
(suspension)
PTFE-ring
diaphragm
extremly
(emulsion)
open circular
diaphragm
PHER (municipal STP)
PHEX (emulsion, suspension, sludge)
PHEF ( 7 bar, 50 °C /semiconductor industry, galvanic)