Dr. Thomas Winkler

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Background Information
pH-Electrodes

• Dr. Thomas Winkler
• ProMinent Dosiertechnik Heidelberg

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Definition pH
acid
pH - log a H
basic
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Meaning of pH-value
Change of pH-value by 1 step Change of H
concentration by factor 10
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Exampes for pH-values
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Constructions of pH-Electrodes
mV
mV

• 1 glass membrane
• 2 internal electrolyte
• 3 internal electrode
• 4 reference electrolyte
• 5 external electrode
• 6 diaphragm

reference electrode
glass electrode
combination probe
separate probe system
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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
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pH-Measurement
Dependence of Electrode Signal on pH
the measured signal slope theoretical
(Nernst) 57.2 mV/pH (20C) 59.2 mV/pH
(25C) 61.2 mV/pH (30C) 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
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Dependence of Electrode Signal on Temperature
pH-Measurement
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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?
  (25C)
• heavily temperature dependent
• 10-fold resistance when reducing temperature by
  30 degree

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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 kO
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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
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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

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Temperature dependence of pH-probes

• temperature dependence of the slope
• the slope increases with temperature
• -57.2 mV/pH with 20C, -61.2mV/pH with 30C
• 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
50C
20C
pH 7
pH 10
pH 4
- mV
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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

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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 53...60 mV/pH (25C)
• slope of new probes 55...59 mV/pH (25C)

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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 10...30C 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!

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