Dr. Thomas Winkler

ProMinent® Dosiertechnik Heidelberg

2

Dissoziation von Wasser

(H20 H+ + OH- ),

Definition pH

pH = - log a H +

pH H+ conc. commen log.

0 1 100

1 0,1 10-1

2 0,01 10-2

3 0,001 10-3

4 0,0001 10-4

5 0,00001 10-5

6 0,000001 10-6

7 0,0000001 10-7

8 0,00000001 10-8

9 0,000000001 10-9

10 0,0000000001 10-10

11 0,00000000001 10-11

12 0,000000000001 10-12

13 0,0000000000001 10-13

14 0,00000000000001 10-14

acid

basic

3

Change of pH-value by 1 step

=

Change of H+ concentration by factor 10

Meaning of pH-value

4

Exampes for pH-values

pH3.6% HCl

Coca Colaorange juicebeer

milkbloodsea water

soap

4% NaOH

0

2.64.55.5

6,97.4

8

9

14

5

Constructions of pH-Electrodes

1 glass membrane

2 internal electrolyte

3 internal electrode

4 reference electrolyte

5 external electrode

6 diaphragm

combination probeseparate probe system

referenceelectrode

glasselectrode

mVmV

6

pH-Measurement

saturatedKCl solution • The signal is a voltage [mV]

U = R * I

• The measurement is performed

• at high resistance (R)

• at nearly zero current (I)

Potentiometry: Measurement of the difference of potentials

glassmembrane

sample

Ag/AgCl/KCl/pHconst / /pHsample/KCl/AgCl/Agglass

membrane

7

pH-Measurement

zero deviation

(Asymmetry-Potential)

failure at low pH

by experiment (practise)

theoretical (Nernst)

failure at high pH

Dependence of Electrode Signal on pHthe 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

8

69,087561,1435

68,087060,1530

67,096559,1625

74,0410066,106058,1620

73,049565,115557,1515

72,059064,125056,1810

71,068563,124555,195

70,078062,134054,200

UN

mVTemp. [°C]

UN

mVTemp. [°C]

UN

mVTemp. [°C]

Dependence of Electrode Signal on Temperature

pH-Measurement

9

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

General Features of pH-Electrodes

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!

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

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 kflow 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

pH-Measurement

13

Temperature dependence of Temperature dependence of pH-probespH-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

pH 7 pH 10pH 4

20°C

50°C + mV

- mV

14

Cleaning of pH-probesCleaning 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

Checking of pH-probesChecking 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 (25°C)

slope of new probes: 55...59 mV/pH (25°C)

16

In a holder in KCl-solution 3-molarin case of an emergency tap water, not DI-water!drying out may destroy the probe (e.g. PHEX)

Storage at room ambient temperatureperfect: +10...30°C; avoid frost!

No long storage timepreferably max. ¼ year

Caution in the service station car!avoid high temperatures in summeravoid low temperatures in winter

Frost may destroy probes and may freeze buffers!

Storage of pH-probesStorage of pH-probes

17

Selection of pH - Probes

clear medium

ceramic PHEN (high accuracy, long lifetime)diaphragm temperature w/o pressure PHEN- 3D (low conduct. 50 μS/cm)

up to 60°Cmax. 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)

MEDIUM temperature w/o pressure PHEN- 3D (low conduct. 50 μS/cm)

above 60 °Cmax. 6 bar PHEP (galvanic, process)

max. 8 bar PHED (galvanic, process with species like Cr 6+, CN-)

medium with normal PTFE-ring PHER (municipal STP)solid material (suspension) diaphragm

extremly open circular PHEX (emulsion, suspension, sludge)(emulsion) diaphragm

medium withFluoride PHEF ( 7 bar, 50 °C /semiconductor industry, galvanic)