About the author: Frederick J. Kohlmann is Midwest product marketing manager for analytical products for Endress+Hauser. Kohlmann can be reached at [email protected] or 262.820.1167.
Frederick J. Kohlmann
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The first steps of cleaning a pH sensor are to wipe away debris and rinse the sensor in warm tap water.=
For a pH sensor to maintain an accurate reading, the sensor must remain clean. Specifically, the glass measuring electrode cannot become coated, and the reference electrode assembly must not become coated, plugged or otherwise contaminated by the process solution. Here is how to clean pH sensors to ensure correct operation.
General Cleaning Procedure
- Remove the bulk of contaminant by carefully blotting/wiping away debris using a clean damp cloth or towel. Be careful not to rub too vigorously, as this may cause static charge.
- Rinse the sensor in warm tap water or distilled water.
- Prepare a cleaning solution containing a soap and water mixture. Use dishwashing detergent and warm water. Use only soaps that do not contain abrasives or lanolin.
- Soak the sensor in this solution for five minutes and then use a soft bristle brush to gently scrub the bulb and reference area of the sensor.
- Rinse the pH sensor in warm tap water and then check/standardize the sensor in buffer solutions. If the readings in buffers are still out of tolerance and the contaminants are acidic, soak the sensor in a weak caustic (less than 4% NaOH). If the contaminants are alkaline or scale, soak the sensor in a 5% to 10% HCI acid solution for less than five minutes. Caution: do not follow this procedure if the sensor has been used in a solution containing cyanide, as this may produce poisonous cyanide gas.
- Rinse the sensor in warm tap water and then place the sensor into dishwashing detergent and warm water for two minutes to neutralize any remaining acid, and to let the sensor come to equilibrium.
- Rinse in warm tap water and check/standardize the sensor in buffer solutions.
If the above procedures yield results within operational tolerances, the pH sensor is once again suitable for use. However, if the above results do not bring the readings of the pH sensor within tolerance, it is time to replace the sensor.
Cleaning Tips
If the pH sensor has a slight coating or scaling, these usually can be removed using a water jet from a faucet or spray bottle. More entrenched coatings may require the use of a gentle acid brush or toothbrush to carefully remove the coating.
Depending on the nature of the scale or coating, it may be necessary to dip the sensor in a hot water solution containing dishwashing detergent and then lightly scrub the electrode for a few seconds to facilitate cleaning.
For a more aggressive coating of the sensor where the detergent cleaning procedure outlined above does not suffice, dip the brush in a 2% HCI acid solution and then lightly scrub the electrode for a few seconds or so to facilitate cleaning. You may have to allow the sensor electrodes to soak in a similar solution for a few minutes to really work at attacking the contaminant. Immediately after cleaning, rinse the sensor in water, then soak the pH sensor in tap water or a 7-pH buffer solution for a few minutes to allow the pH sensor to stabilize.
Knife blades or wire brushes shouldn’t be used to clean deposits left on the glass bulb, as they can scratch the glass or break off the bulb at the neck. A soft-bristle brush should be used instead.
