ph electrodes: what users should know

Transcription

1 Electrochemistry Application Knowledge Base ph electrodes: what users should know - parts of a ph electrode - different types of junction - filling level - contamination - ph glass membrane - ph probe acid / sodium error November 2010 Dr. Axel Bier Hach-Lange GmbH Düsseldorf 1

2 General differences in ph electrodes Use model - laboratory liquid filled (need to be refilled) glass and plastic body variety of liquid junctions - field / portable gel filled (no refill) plastic body open, ceramic pin, PTFE ring Application - contaminated waste water brackish water surface water seawater plating bath - not contaminated drinking water pool & spa boiler/cooling water deionised water pharmaceutical water 2

3 ph electrode design and types of liquid junction A B C D ceramic pin ceramic pin ceramic pin A C open ring PTFE or glass frit sleeve B D 3

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5 Correct use of ph electrodes 5

6 What happens if the ph probe is not filled correctly? Part I Refill hole fill level Reference element Electrolyte Diaphragm Indicating element ph glass bulb normal filling sufficient filling insufficient filling, reference element can be damaged filling much too low, reference element is damaged 6

7 What happens if the ph probe is not filled correctly? Part II There must be a minimum volume of inside the probe. Otherwise the is contaminated with sample and in a worst case the reference element can be damaged. fill level hydrostatic pressure of the on the inner end of the diaphragm is high enough - normal. hydrostatic pressure is not sufficient, sample can move through the diaphragm and can contaminate the hydrostatic pressure is much too low, sample contaminates the hydrostatic pressure is much too low, sample contaminates the. Reference is damaged. 7

8 What happens if the ph probe is not filled correctly? Part III sufficient insufficient outflow of KCl contaminated with sample Insufficient inside the probe gives not enough hydrostatic pressure on the inner junction. The result is not KCl flowing out, but sample getting through the junction and contaminates / poisons the reference electrode. 8

9 What is the problem with having sample inside the? The usual inner solution is KCl in specific concentrations e.g. 3 molar, 3.5 molar or saturated. Based on this constant composition the potential of the reference electrode (mainly Ag/AgCl) is constant (only depending on temperature). During a calibration of the probe ph buffers or standards are measured against the reference electrode potential and give a SLOPE and OFFSET for the complete probe. If the is contaminated with sample, the ph and ion concentration of the will change and therefore the potential of the reference electrode is no more constant. In that case the calibration parameter are not longer valid and the measurement is wrong. In addition the sample inside the can of course react with the reference element and destroy the AgCl layer or even attack the Ag wire. 3 molar KCl??? molar KCl 9

10 What about the lower ph glass bulb? Part I Does an air bubble inside the inner tube interfere? The air bubble at the top of the indicating filling is necessary to have space for the liquid to expand at higher temperatures reference element Electrolyte inside the inner tube ending in the glass bulb is KCl, e.g. 3 M or saturated, maintaining an inner ph of 7.0. indicating element The indicating element is like the reference element Ag/AgCl. As long as it is in KCl solution, the indicating element gives a stable potential. If an air bubble is inside the glass bulb, the indicating element shows a different potential and the complete chain of reference Ag/AgCl // KCl (ph 7) <-> indicating Ag/AgCl // KCl (ph 7) is different to it s normal function. Therefore the probe does not measure correctly. Shaking the probe like a fever thermometer removes the air bubble from the glass bulb. 10

11 What about the lower ph glass bulb? Part II As long as the inner potential E i is constant, the outer potential E o results from the outer ph only (temperature is the other variable). ph buffer solution ph 7.0 E O E i KCl * ph 7 E = E O -E i ph glass membrane E i = potential on inner side of the glass E o = potential on outer side of the glass Theoretically E is zero, if the conditions of inner and outer material and solution are equal. However, inhomogeneity of the glass composition and variances during production can lead to E <> 0. This E is called offset potential. With a ph buffer calibration the specific parameters of the ph glass element and the reference element are measured and stored. * The inner indicating also contains a ph buffer (phoshpate) 11

12 What about the lower ph glass bulb? Part III How does the ph glass membrane work? Glass usually consists of SiO 2 (sand) and some additions, like Na 2 CO 3, and depending on the use model other elements like B boron or Al aluminum. There is a main net of SiO 2 tetraeder which are randomly connected, building SiO 4 2- modules. In the caves, to neutralize negative SiO 4 2-, positive cations like Na +, H + or Ca 2+ are included. While bigger cations like Ca 2+ are fixed in the net, smaller cations like Na + and H + can replace each other. If glass gets in contact with water an equilibrium develops between ions in solution and ions in the glass. Na+ Na+ Na+ Ca++ +H +H Na+ Ca++ Na+ Ca++ Na+ Ca++ Silicium oxygen +H +H 12

13 What about the lower ph glass bulb? Part IV How does the ph glass membrane work? For instance, starting from sodium calcium silica glass: Na 2 CaSiO 4 + H 2 O NaCaHSiO 4 + Na + + OH - Contact with water results in an exchange of sodium ions from the glass with hydrogen ions from the water until equilibrium. If more H + ions are present, the glass membrane can take up more H + than Na +, what results at very low ph (< 1) in an acid error of the ph electrode: the electrode is overloaded with H + ions. The measured ph is incorrect. In high alkaline solutions (ph >12), like in NaOH with a majority of Na + ions, the equilibrium exchange in the glass net between H + and Na + ions is forced to more Na + uptake. That leads to a potential shift due to an overload with Na +, called alkaline error or sodium error. The measured ph is incorrect. Samples containing fluoride ions hardly attack the glass membrane and can destroy it within a short period of time. The following reaction takes place: SiO HF SiF H 2 O SiF HF H 2 SiF 6 First a layer of SiF 4 disables the membrane to interact with H + ions, then the SiF 4 2- ion is soluble and the membrane looses material. 13