Care and Maintenance of Electrodes for pH and Voltametric Measurements

1. Care and Maintenance of Electrodes for pH and Voltametric Measurements • Summary • The glass pH electrode • How it works • Calibration • Errors • Maintenance • Reference electrodes • Functioning • Types of reference electrodes • Problems • Maintenance

2. Glass pH Electrode • H+ ion selective electrode • Nernst Equation • Note that the electrode measures the activity of hydrogen ions and that pH is the negative log of the activity of hydrogen ions • The Nernst equation predicts that for every decade change in H+ ion activity (1 unit change in pH) the potential will change by about 60 mV

3. Glass pH Electrodes + - a sensing part of electrode, a bulb made from a specific glass sometimes the electrode contains a small amount of AgCl precipitate inside the glass electrode internal solution, usually 0.1MHCl for pH electrodes internal electrode, usually silver chloride electrode or calomel electrode body of electrode, made from non-conductive glass or plastics. reference electrode, usually the same type as 4 junction with studied solution, usually made from ceramics or capillary with asbestos or quartz fiber. Ag/AgCl reference electrode Glass pH electrode

4. The Gel Layer • The glass has a lithium silicate skeleton that forms a thin hydrated layer on both sides of the membrane. • Ions can penetrate this thin layer and alter the electrochemical potential. • Without the hydrated layer no pH measurements would be possible • The structure of the glass has been optimised so that virtually only H+ ions can enter the gel layer

5. Calibrating the Glass pH Electrode • In performing a pH measurement it is essential to: • Choose at least two standard buffers so that your measurement fits within the pH range of the buffers. E.g.: • sample pH 5, select pH 4 and pH 7 standard buffers • sample pH 3, select pH 2and pH 4 • Always determine the response of your glass electrode by recording the voltage or mV reading

6. Useful Buffers

7. Useful Buffers

8. Response of the Glass Electrode • Measure the voltage response of the glass electrode and not just the pH response when calibrating the electrode. • A new electrode should give a response of about 60 mV/pH under ambient conditions. • pH 7-4 the electrode potential should increase by +180 mV • pH 7-10 the electrode potential should decrease by -180 mV • As the electrode ages the slope decreases and the response time also becomes sluggish. The electrode should be discarded when the response is appreciably below 60 mV and the response time is slow.

9. Importance of knowing the response of the glass electrode ∆E = 59 mV/pH mV Inferior electrode, pH calibration forces a fit to the buffer pHs 4 7 10 pH

10. Electrode Response • As well as a decrease in slope or sensitivity, the response time of the glass electrode tends to get slower with age • This slow response can be attributed to contamination by organic and inorganic ions. The organic ions adsorb to the membrane whereas inorganic ions Na+ and K+ etc, may enter the gel layer.

11. Proprietary Metrohm Storage Solution 17 minutes 50 minutes pH measurement in (NaHCO3) = 0.05 mmol/L. Aquatrode electrode glass kept in the Storage solution shows a considerably shorter response time compared with the same type of glass electrode stored in KCl for the same length of time.

12. Errors in pH measurements • Improper calibration • Sodium (alkaline) error, occurs at high pH and is due to the electrode responding to Na+ ions. The pH reading is lower than the actual pH (actual pH is higher) • Acid error occurs at very low pH • Temperature effects

13. Sodium and Acid Errors 10 M acid 0.01-10 M NaOH

14. Temperature Effects • The normal temperature compensation on the meter only compensates for the mV output from the pH sensor (electrode) and does not correct for solution changes • To compensate for changes in solution pH with temperature, it is best to perform a pH/temp. calibration on your process solution and enter this into the program on your pH meter. Most modern research grade pH meters have this capability.

15. Care and Storage of the Glass Electrode • The best solution to store the glass electrode in is pure water. Storage in KCl and water which contains other salts will mean that cations can enter the gel layer and decrease electrode response. • Storing combination glass electrodes (where the glass and reference electrode are combined in one probe) in water can cause problems with AgCl precipitating and blocking the junction (porous membrane) of the reference electrode. • The best method, is to store the glass bulb in water and to leave the junction of the reference electrode exposed to air. • Metrohm have produced a proprietary storage solution which they claim works and the ICMT should consider giving this a try

16. Two Electrode System

17. Glass pH Electrodes a sensing part of electrode, a bulb made from a specific glass sometimes the electrode contains a small amount of AgCl precipitate inside the glass electrode internal solution, usually 0.1MHCl for pH electrodes internal electrode, usually silver chloride electrode or calomel electrode body of electrode, made from non-conductive glass or plastics. reference electrode, usually the same type as 4 junction with studied solution, usually made from ceramics or capillary with asbestos or quartz fiber. Ag/AgCl reference electrode Glass pH electrode

18. The Reference Electrode • Generally speaking, most problems in electrochemical measurements can be traced to a problem with the reference electrode. • These problems can be attributed to: • Blockage of the junction (the area where the electrode makes electrical contact with the solution) • Contamination of the internal filling solution • These problems can be largely overcome if you use reference electrodes in which the internal filling solution can be easily renewed and the junction can flushed (renewed) with fresh electrolyte. Such electrodes are available from Orion and Metrohm. • In the case of pH measurements, it is best to use separate single glass and separate reference electrodes. Combination pH electrodes have there place but for calibration it is best to have a meter setup in the lab. that uses a two electrode system.

19. Types of Reference Electrodes • Silver/silver chloride • -5o-100oC continuous use • -5o-130oC intermittent use. NB: the superior temperature performance of the Ag/AgCl electrode • Calomel Hg/Hg2Cl2 • -5o-80oC

20. Types of Reference Electrodes • Hg/HgSO4 electrode • Useful electrode to use where Cl- ion cannot be tolerated • Hg/HgO electrode • Suitable for work at high pH

21. Potential of Reference Electrodes vs the SHE @ 25oC

22. Problems with the Reference Electrode • Blockage of the junction causes an increase in the electrodes’ impedance, which inturn makes the measurement more susceptible to noise pick-up. High impedances should be avoided where possible in electrochemical measurements. • Blockage and/or contamination of the junction can result in a variable junction potential, which in-turn causes a variability in electrode response. The junction potential results from a separation of charge due to the different mobility of anions and cations and can be as large as 20 mV • Na+ K+ • Cl- Cl- - - - + + +

23. Ag/AgCl Reference Electrode Care and Maintenance • Problems with this electrode can often be traced to the presence of soluble AgCl. • AgCl is sparingly soluble in highly concentrated chloride solutions due to formation of the silver chloride complex • If the reference electrode is stored in water and solutions of low chloride activity, AgCl solid will form and lead to the junction blocking. • The AgCl can decompose to Ag2O which is a black/purple colored deposit. • For this reason it is best to store the electrode in 3M KCl solution. • The dilemma is that this solution is not suitable for storing the glass electrode.

24. Ag/AgCl Reference Electrode Care and Maintenance • Blockage of the junction can also take place due to precipitation of KCl. • This problem can occur at low temperature and if the electrode is stored in air. • For this reason the use of saturated KCl as the internal filling solution should be avoided. Better to use 3M KCl.

25. Types of Reference Electrodes (Ag/AgCl) Electrodes with renewable liquid junctions Metrohm reference electrodes Orion double junction reference electrode Single junction reference electrode

26. Copper/Copper Sulphate Reference Electrode

27. pH Meters • There are various types of pH meters, hand-held, portable and research grade pH meters. • A pH meters is an electrometer, the feature which distinguishes a pH meter from an ordinary volt meter is the input impedance. Research grade pH meters have an input impedance of the order of 1013 ohms. • The input impedance enables the meter to measure the potential of glass electrodes which have an impedance of 50-500 M (106)ohms • A reference electrode has an impedance of the order of 1-3 kilo ohms (103) • The high impedance means that only a small current is taken from the system during a measurement, thus preventing a change in the equilibrium voltage. This is particularly important when recording an equilibrium voltage with a low exchange current density (10-8 amps cm-2). • E.g. for a measurement of 1 V, if the impedance = 1013 the current = 10-13 amps.

29. THE END