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Dr. Thomas Winkler ProMinent ® Dosiertechnik Heidelberg.

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


2 Dr. Thomas Winkler ProMinent ® Dosiertechnik Heidelberg

3 2 Dissoziation von Wasser (H 2 0 H + + OH - ), Definition pH pH = - log a H + acid basic

4 3 Change of pH-value by 1 step = Change of H+ concentration by factor 10 Meaning of pH-value

5 4 Exampes for pH-values pH 3.6% HCl Coca Cola orange juice beer milk blood sea water soap 4% NaOH ,

6 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 reference electrode glass electrode mV

7 6 pH-Measurement 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) Potentiometry: Measurement of the difference of potentials glass membrane sample Ag/AgCl/KCl/pH const / /pH sample /KCl/AgCl/Ag glass membrane

8 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 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

9 8 69,087561, ,087060, ,096559, , ,106058, ,049565,115557, ,059064,125056, ,068563,124555,195 70,078062,134054,200 U N mV Temp. [°C] U N mV Temp. [°C] U N mV Temp. [°C] Dependence of Electrode Signal on Temperature pH-Measurement

10 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

11 10 Junction ceramic junctionPTFE-ring junctionopen 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

12 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“ 1234

13 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 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 pH-Measurement

14 13 Temperature dependence of pH-probes  temperature dependence of the slope the slope increases with temperature 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 7pH 10pH 4 20°C 50°C + mV - mV

15 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

16 15 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 7e.g. –10 mV pH-probe in buffer 10e.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)

17 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: °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! Storage of pH-probes

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