Presentation is loading. Please wait.

Presentation is loading. Please wait.

Dr. Thomas Winkler ProMinent® Dosiertechnik Heidelberg

Similar presentations

Presentation on theme: "Dr. Thomas Winkler ProMinent® Dosiertechnik Heidelberg"— Presentation transcript:

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-Electrodes
mV 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!


Download ppt "Dr. Thomas Winkler ProMinent® Dosiertechnik Heidelberg"

Similar presentations

Ads by Google