Maintaining pH electrodes

Endress+Hauser Australia Pty Ltd
By John Immelman*
Wednesday, 11 February, 2009

Process instrumentation technicians who implement good pH electrode maintenance regimes get more than just greater accuracy in pH measurement. They also benefit from a longer interval between cleaning cycles and extended productivity during a longer life cycle, significantly reducing running costs.

Users often believe that a sluggish or non-responsive pH electrode indicates a failed probe. In most instances, the pH electrode can be revived just by applying the correct chemical cleaning procedure. The frequency with which pH electrodes require cleaning varies with the application and the process. Typically, in water and wastewater applications, electrodes may require cleaning every three months, while in harsher applications, such as the mining and chemical industries, it could be daily. An electrode must never be physically cleaned with a toothbrush or a scrubbing implement. Chemical cleaning with a solution of 4 or 5% hydrochloric acid followed by a rinse and wipe with a soft cloth delivers the best results.

The most common form of electrode failure, other than breakage, is a coated membrane or plugged reference junction. The reference junction consists of a porous diaphragm, usually ceramic or teflon and, like the membrane, must remain open, clean and in contact with the sample medium. When a coating builds up on the diaphragm or membrane, it becomes sluggish or non-responsive and is often wrongly interpreted as electrode failure.

To avoid plant disruption or process downtime, it is recommended that the electrode be swapped with a replacement before it becomes sluggish or ceases to function. Endress+Hauser offers inductively coupled, contact-free, pre-calibrated pH electrodes. These may be taken from the pH electrode store to the measuring point and plugged into the transmitter, where they will start functioning immediately. This is because with Memosens calibration is in the pH electrode itself, whereas in other electrodes, the calibration resides in the transmitter in the field. pH electrodes offered by other vendors must be calibrated at the measuring point in the field.

Not only is it inconvenient to perform the calibration at the measuring point, but there is a further downside. Apart from ending up with less accurate calibration doing it this way, it prolongs plant downtime since the calibration and cleaning procedure can take valuable time. There is also a chemical delay before the electrode functions properly after cleaning, and this also affects the calibration.

It is best to take the malfunctioning electrode back to the central calibration point where it can be cleaned, calibrated and stored in potassium chloride (KCl), ready to be swapped out again. Since all pH electrodes utilise a potentiometric reaction for their operation, they are considered consumable items and have a limited shelf life — typically around two years. It is not good practice to keep pH electrodes in the store for any length of time ‘just in case’. Using Memosens technology from Endress+Hauser facilitates the better management of electrodes, reducing the possibility of expired electrodes.

pH electrodes must remain wet at all times, whether on the shelf or in the process. In storage, the electrode is stored in a cap filled with 3 mL KCl solution. If it goes dry or dehydrates completely, the electrode will possibly not be salvageable.

It is important for technicians to adopt preventive maintenance measures and not wait for electrodes to fail before cleaning. Swapping electrodes regularly with pre-calibrated electrodes is the best way to maximise plant productivity and minimise process downtime.

*John Immelman is the Managing Director of Endress+Hauser Australia Pty Ltd.

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