Process instrumentation technicians who implement good pH electrode maintenance regimes get more than just more accurate pH measurement. They also benefit from a longer interval between cleaning cycles and extended productivity during a longer lifecycle, 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/wastewater applications, electrodes may require cleaning every three months, while in harsher applications such as mining and chemical industries it could be daily. An electrode should never be physically cleaned with a toothbrush or a scrubbing implement. Chemical cleaning with a solution of 4% or 5% hydrochloric acid (HCl) followed by a rinse and wipe with a soft cloth delivers good 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 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 offer inductively coupled, contact free, pre-calibrated pH electrodes. These may be taken from the pH electrode ‘store’ to the measuring point and plugged in to 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.
It is best to take the malfunctioning electrode back to the central calibration point where it can be cleaned, calibrated and stored in 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. It is not good practise to keep pH electrodes in the store for any length of time. Using Memosens technology, from Endress + Hauser, facilitates 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 3ml potassium chloride (KCl) solution. If it goes dry or dehydrates completely, the electrode will not be salvageable.
It is important for technicians to adopt preventative maintenance measures and not wait for electrodes to fail before cleaning. Swapping electrodes regularly with pre-calibrated electrodes is the suitable way to maximise plant productivity and minimise process downtime.