pH and Dissolved Oxygen Electrode Care and Troubleshooting
This article is focused on pHionics STs Series™ electrodes and sensors but every cleaning procedure is relevant to other manufacturer products unless otherwise specified.
Water quality sensors should always work. I wish they did.
Unfortunately, we run into many problems such as slow stabilization, drifting, or various connection issues. These problems can sometimes be solved by simple cleaning and other times require complete replacement of the electrode or device. The pHionics STs Series™ is designed to mitigate these issues to ensure that water quality measurement is easy but it is impossible to eliminate all of them. This guide to pH and dissolved oxygen electrode care and maintenance provides both general maintenance instructions and diagnosis steps for troubleshooting
Please reach out to us if any problems are not solved after following these steps.
Precautions
It is extremely important to know that pH and dissolved oxygen measurements rely on the surface of the electrode being clean. The electrode surfaces are sensitive and do not read properly if oil residue from fingers is left on the tips. Unlike conductivity and ORP, the sensing elements should not be cleaned with a brush or any other mechanical technique as the electrodes will be damaged. The sensing element does not include the junction on the pH electrode.
Diagnosis Steps
Is the signal drifting or inaccurate?
1. Accumulated debris on the sensing element (tip of the electrode) may be the cause. Perform a visual inspection on the tip and follow the steps provided in the table below for cleaning depending on what type of debris may be building up on the electrode. Even if there are no visible signs of build-up, a general cleaning in 0.1M HCl is recommended to be done consistently. Recalibrate the electrode and pay attention to how long the output takes to stabilize (Stabilization is defined as less than 0.01mA change per minute). Slow stabilization may mean that the electrode reference solution is diluted.
2. Dilution occurs whenever the solution being measured differs from the reference solution and some solutions cause faster dilution. Dilution will result in drift at an increasing rate over the life of the electrode. To check this, calculate the slope error and offset of the electrode after cleaning and calibrating. If unsure how to calculate these, please reference our Sensor Calibration article. Values outside of accepted ranges indicate that the reference electrolyte is diluted or dirty. The further out of range these values are, the greater the drift rate will be.
Some electrodes are refillable and will allow the reference to be replaced by the user, while others are replaceable and will require purchase of a new electrode. pHionics currently has replaceable electrodes because they provide the lowest maintenance time and fewer complications.
3. Slight and extreme inaccuracies can result from nearby sources of interference such as pumps, boats, or radio frequencies. The pHionics STs Series provides excellent noise reduction to prevent interference but sensors and transmitters from other manufacturers may not ever function properly in environments with high levels of background noise.
For a more in-depth guide, check out our new pH Drift Troubleshooting Guide.
Is the signal erratic (jumping up and down) or extremely low (≤ 4 mA)?
1. Check the inner connectors, electrodes, and cables for signs of wear or damage. This may require disassembly to check the connectors for oxidation so please use the resources found on our products page to ensure proper handling of pHionics products. It is a simple process but there are a few precautions to consider during disassembly that can have a serious impact on the performance of your product. All parts of pHionics STs Series™ devices are fully replaceable for pHionics so please contact us to order replacements if damage does occur.
2. Check the electrode tip for air bubbles. Any air present in the bulb or under the dissolved oxygen membrane will affect the measurement capabilities of the respective electrode. To remove any air, grip the pHionics STs Series™ device tightly near the cable (not by the cable though – don’t go swinging the transmitter around like a lasso) and flick the device down to force solution back into the bulb.
3. Perform a visual inspection of the electrolyte fill level in the electrode. If the signal is highly erratic, the electrolyte may be low and only periodically contacting the reference wire. If using a pHionics STs Series™ device, please see our guide on Electrode Removal and Replacement or watch the video to ensure the proper procedure is being followed during disassembly. Once the electrode is removed, check if the electrolyte level is low (less than 1/2″ or 1.27cm). Low electrolyte solution means the electrode is at the end of its life and must be replaced.
4. Verify the electrode has been cleaned properly based on characteristics of the solution being measured by following instructions in the table below. If these do not work, then it may be possible to follow the regeneration steps below the table and get more value from the pH electrode. Regeneration is not guaranteed, however, and the electrode will require replacement eventually.
Old electrodes with mineral deposits.
Last Resorts (pH SPECIFIC)
Mechanical cleaning
Put a drop of dishwashing soap on a soft toothbrush – and brush the bulb and junction lightly for about a minute. Rinse with water and calibrate the sensor. If there is no change in the slope and offset or stability, rinse with tap water and scrub once more vigorously. This process has allowed us to revive many old electrodes out in the field but may result in slightly reduced accuracy.
Regeneration
The following section is for pH ONLY. Contact of hydrofluoric acid to a dissolved oxygen electrode destroys it.
Regeneration is the term used for bringing an electrode back into working order after being unusable. This can be due to desensitization of the glass bulb from drying out or damage from a harsh application process. The regeneration theory is based upon removal of the damaged/dry outer surface of the bulb using hydrofluoric acid to uncover a clean new glass layer.
Be very careful in following all recommended safety procedure while handling hydrofluoric acid and consult the MSDS. Eye protection, gloves, and a good lab coat are necessities to provide crucial seconds before any splashed or spilled acid reaches your skin and starts to burn. Confirm that a hydrofluoric-resistant container (e.g. NOT glass) is being used.
Dip the bulb (and only the bulb) into 0.1M hydrofluoric acid for at most two minutes. Rinse the electrode with distilled water and place in pH 7 buffer for 1 hour. Store electrode in electrolyte solution (3M KCL) overnight before calibration.
Freshly cleaned STs Series Dissolved Oxygen sensor.
Next Steps
Hopefully, the electrode is now as good as new! If, however, none of these steps lead to acceptable functioning of the electrode then the electrode must be replaced. If the electrode failed significantly faster than expected, please email us with the details of the application process being measured as we may be able to recommend alternate solutions or provide advice on extending electrode life in the future. You deserve the best and we will provide that.