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The pH Combination Electrode

This page gives info on the different types of pH combination electrodes, their uses, operation, care, maintainance,
reconditioning and glass membrane cleaning, as well as electrode and meter troubleshooting.

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Ion Selective Electrodes

Combination pH electrodes

Use electrode combinations that are compatible with the solutions to be measured.

General purpose combination electrode

Has silver chloride wire and KCl electrolyte. Refillable.
Used for aqueous solutions of compounds that do not react with silver.
Must not be used in solutions of :
1. Heavy metals
2. Proteins
3. Low ion solutions (DI water)
4. High sodium
5. Sulphides
6.Tris buffers
7. Organics.
If sample contains any of these contaminants, the pH electrode may work for a short time and will then fail to operate.
Reconditioning may restore it for a while, but the same short term usage may occur.

Non-refillable type

Permanently sealed with gel fill solution.
Rugged, but has a shorter life, easy to use and inexpensive.

Calomel combination electrode

Has KCl wire and mercuric chloride, with a KCl reference electrolyte. No silver wire. Refillable.
Fill solution is less reactive, does not react with sample, but is temperature sensitive.
Designed to work in solutions containing proteins, organics, low ion activity, tris buffers and heavy metals.

Double junction electrode

Two chambers. Upper chamber has silver chloride wire/KCl saturated with AgCl, and lower chamber has a KNO3 electrolyte solution.
Designed to work in same applications as for the calomel electrode, but at higher concentrations of problem materials.

Teflon junction electrode

Has AgCl wire and porous teflon liquid junction.
Designed so that solutions being measured do not clog the liquid junction.
Used for substances such as oils, foods, paints, gells, and pastes.

Solid-state electrode

Uses FET (Field effect transistor) sensitive to pH.
No glass bulb, non-glass sensor, zero maintainance, but proper electrode practice should be observed.

Making pH measurements

1. Remove fill hole cover when measuring. Fill hole must be open to avoid contamination of electrolyte from buffer solution.
2. Use ATC. pH is temperature sensitive.
3. Stir solutions using magnetic stir bar / stir plate.
4. Insulate stirrer base and sample container with piece of cardboard or styrofoam to minimize heat transfer.
5. Use fresh buffer solutions when calibrating meter.
6. Rinse electrode between measurements with distilled water, then with next sample to be measured.
7. Do not rub electrode glass bulb. Simply, touch-dry with tissue.
8. Never store pH electrode in distilled or deionized water because the filling solution will leak out into it.
9. Store pH electrode in pH 7.00 buffer with a drop or two of saturated KCl solution added.
For long term storage, fill electrode chamber with filling solution, cover the fill hole, then add a few drops of saturated KCl into the protective plastic cap and cap the electrode.
10. Do not touch or move the electrode cable while taking a measurement, because readings may become unstable due to the high impedence of the pH glass membrane.

Reconditioning and cleaning of pH electrodes

Poor response of pH electrodes. Prolonged use and natural aging, excessive alkaline immersion and high temperature operation will cause leaching of the membrane glass and a reduction in the Nerstian response, which will give rise to noisy and erratic or slow and sluggish electrode readings. There are twwo causes for this :
1. Contamination of the glass membrane.
2. Clogging of the liquid junction.
The following procedures will often provide renewed stability and pH sensitivity.

Cleaning the glass membrane

(a) Immerse electrode tip in 0.1M HCl for about 15 secs., rinse with distilled water, then immerse in 0.1M NaOH for another 15 sec.
Cycle the electrode through these solutions several times, then check for performance in pH buffer 4.00 and 7.00.
(b) If problem still exists, immerse electrode tip into a 20% ammonium bifluoride for 30 secs., or a 10% hydrofluoric acid solution for 15 secs.
Throughly rinse the electrode with distilled water, then immerse tip into conc. HCl for 30 secs. to remove residual fluorides, then rinse again with distilled water.
Soak electrode in pH 4.00 for 1 hour, then recheck electrode performance. If not restored, replace the electrode.

Other methods of reconditioning :

(a) Soak in 0.1M HCl or HNO3 for 1 hour.
Do a second cleaning : soak in 1 : 10 dilution bleach in a 0.1 - 0.05 % liquid detergent in hot water with vigorous stirring for 15 mins.
(b) soak in saturated KCl @ 50 °C for 1 hour. Allow KCl solution to cool down to room temperature, then rinse with deionized water.
This should clear all contaminants.

Removal of deposits

Protein : Soak in 1 % pepsin in 0.1M HCl for 15 mins.
Inorganic : Soak in 0.1M tetra sodium EDTA solution for 15 mins.
Grease and oil : Rinse electrode in mild detergent in methanol solution.

Unblocking the liquid junction

Inspect reference chamber for cryslallization. Remove crystals as follows :
1. Shake out filling solution from fill hole and flush and rinse reference cavity with distilled water until all crystals are dissolved.
2. Empty filling cavity and fill up with filling solution.
3. Pressurize the electrode and establish flow :
Insert the spout of the filling solution plastic dispensing bottle into the fill hole of the electrode to make an air tight seal, then squeeze bottle to exert a pressure into the electrode chamber. A bead of liquid should form at the liquid junction in about 30 secs. It may be necessary in some cases to maintain the pressure for several minutes. If still no flow, soak tip in conc ammonia solution for 5 - 10 mins. (hood), rinse with deionized water and apply pressure to filling hole.

For gel-filled electrodes:

Soak electrode in water @60 °C for 5 - 10 mins. or
Place electrode in warm saturated KCl @ 60 °C and allow electrode and solution to cool to room temperature.

Electrode test

1. Calibrate meter with pHbuffer 7.00 according to meter instructions.
Then measure mv for pH 7.00. It should read 0.0 mv ± 20mv. If reading is greater than this, the electrode should be reconditioned or replaced.
2. Perform a calibration with buffer pH4.00 or 10.00. Then measure mv reading of the buffer. Reading should be between 160 - 180 mv. If value is greater than ±12 mv of 177 mv, the electrode should be reconditioned or replaced.
Note :Actual mv values may change as electrode ages, but mv difference will remain between 160 and 180 mv.

Meter test

Perform pH 7.00 calibration according to meter instructions. If you cannot calibrate to pH 7.00, you may have a bad electrode.
Now, disconnect the electrode and short out the meter using shorting strap or pin. With the input shorted out, the mv reading should be 0.0mv, equivalent to pH 7.00.
If out of this range by ±2 mv, the meter is bad and should be electronically calibrated.
If you cannot adjust to pH 7.00 with the input shorted out, there is a problem with the meter.
If you can calibrate to pH 7.00 with the input shorted out, the electrode is bad and should be reconditioned or replaced.

Measuring electrode slope

New electrodes have a slope between 95 % and 102 %.
Determine the mv value of pH 4.00 and pH 7.00 buffers, and determine the net change in millivolts.
e.g. if pH 4.00 = 159.1 mv
and pH 7.00 = -10.0 mv
then net change = 169.1 mv.
Now, divide net change by 177.5, [177.5 mv is the ideal output at pH 4.00 (59.16 X 3 =177.5)] then multiply by 100 to determine the % of the electrode slope.
i.e. 169.1 / 177.5 x 100 = 95.3 %
If slope remains below 90.0 % or above 102 % after cleaning and /or reconditioning, replace the electrode.

References :

1. Ag / AgCl reference electrode manual. - Orion research
2. pH Combination electrode instructions. - Fisher Accumet

Signature: Dhanlal De Lloyd, Chem. Dept, The University of The West Indies, St. Augustine campus
The Republic of Trinidad and Tobago.
Copyright: delloyd2000© All rights reserved.