Water and wastewater operators and laboratory technicians who measure pH for compliance and process control may encounter occasional problems that require troubleshooting. A few simple guidelines can help in resolving these problems quickly and in performing pH analyses with reliable results.

Troubleshooting a pH problem means looking at various aspects of the procedure to identify the cause. First and foremost, make sure the calibration buffers have not expired and replace them if they have.

Second, it is best to calibrate with buffers poured fresh daily or at least weekly; buffers that are reused for extended periods, especially if kept uncovered, can become contaminated and adversely affect calibration. Keep in mind that the buffers should bracket your sample values and that a three-point calibration will ensure the best results.

Third, be sure to rinse the electrode thoroughly with deionized or distilled water and blot it dry between buffers used for calibration. This will prevent small drops of one buffer from contaminating the next buffer.

Finally, you may want to consider the possibility that one or more of your buffers, though not expired, could be bad. This happens from time to time; here are a few guidelines to identify such a problem:

• Calibrate your meter with your regular buffers.
• Analyze a check standard, preferably from a different manufacturer (do not use one of the calibration buffers).
• Assess the accuracy of the check standard to see if it meets the acceptance criteria (e.g., ±0.1 pH units, from Standard Methods #4500-H+ B).
• If this fails, obtain buffers from a different manufacturer.
• Switch one of your original buffers with the new buffer, calibrate the meter and analyze the check standard.
• If it passes, measure the original buffer you switched out as a sample — if it fails the same acceptance criteria, that indicates a problem with it.
• If the check standard passes, repeat the procedure with the other buffers one by one and determine which if any of them is bad.
• Keep in mind that if you are not analyzing check standards from a separate source or if you rerun a calibration buffer as a check standard, you may not encounter this problem.

Calibration

Troubleshooting a pH calibration entails evaluating the slope (if displayed on your meter) to assess the efficacy of the electrode. Manufacturers should provide an acceptable slope range (as a percent or in millivolts) for their specific electrodes, so be sure to follow those criteria. If your slope is outside that range after a calibration, you can pour fresh buffer, clean the electrode (if needed) and recalibrate to get a good slope.

If your meter does not display slope, you should at least follow the manufacturer’s recommendations about assessing a calibration. Either way, unacceptable calibrations can result in unstable or unreliable sample results.

A good troubleshooting step after calibration is to verify it using a check standard, preferably from a second source or at least a different lot number. If this standard fails to meet the acceptance criteria, that could be a sign the calibration did not hold and you may need to recalibrate the meter.

pH meters

Today’s pH meters are rarely at fault when troubleshooting a pH problem. Essentially, they are glorified voltmeters that use an algorithm to convert millivolts to pH readings on a scale from 1 to 14. Short of a manufacturing defect or damaged from dropping or misuse, they are dependable and work for many years with no problems.

If you do suspect your meter is at fault, consult the user manual or the manufacturer’s technical support department for help. One tip that can determine a problem with your meter is to unplug the electrode and plug in the shorting cap (if so equipped). This will enable you to check the millivolt reading to ensure that it meets the manufacturer’s requirements. Another useful tip is to remove your electrode and replace it with another suitable, working probe. If both probes fail to work, then your meter may need repair.

Temperature probes

Since pH is temperature dependent, you need to use either an automatic temperature compensation (ATC) probe or measure the temperature with a thermometer and adjust the meter accordingly. Whether you use a separate ATC probe or a triode with built-in temperature probe, be sure it is undamaged and plugged in tightly (if applicable), and that the meter is not displaying a default temperature that indicates a possible problem with the ATC probe.

pH electrodes

As for electrodes, first be sure you are using the correct type of electrode for you application. Second, make sure the measuring electrode is not damaged (cracked or scratched). If using a new electrode, remember that they can be faulty from manufacturing defects or damaged in shipment.

Also be sure to follow the guidelines in the user manual for properly conditioning a new electrode. Remember, too, that a pH electrode has a limited lifespan of six to 18 months, depending on the application (water or wastewater) and the quality of care and maintenance. Two crucial maintenance tips are to rinse the electrode with deionized or distilled water between uses and to store it in an appropriate solution, such as storage solution or pH 4 or 7 buffer.

If you see sluggish or erratic pH readings, you should clean the junctions and the rest of the electrode following the manufacturer’s guideline. Most electrodes can be cleaned with a lab detergent or with a very dilute hydrochloric acid solution (such as 0.1 N). Allow the electrode to equilibrate in a buffer or storage solution after cleaning. A clogged junction may take a little more effort to clean, but cleaning is necessary for proper electrode operation.

For any type of electrode, begin by inspecting it and the cable for signs of damage and replace it if any are found. Depending upon the type of electrode, a few troubleshooting tips will help.

For gel-filled electrodes:

• Look for air bubbles, and if you find them shake the probe downward as in shaking a thermometer to reset the liquid level. This will force the bubble back up to the top of the probe. Rinse and dry off the electrode tip.
• Look at the round disk (often made of Teflon) that serves as a seal to retain the gel inside the electrode body.
• If there are signs of leakage you will need to replace the electrode.

For refillable electrodes:

• Make sure the electrode is kept filled with the proper solution or gel.
• If the wrong filling solution is used (such as deionized or distilled water), the electrode may be permanently damaged.
• If the electrode has been dry for an extended period, it may be permanently damaged.
• The small opening near the top (for refilling) should usually be left open for measuring and closed during storage. Check your user manual.

Samples

Although you should be using temperature compensation, this process is not always foolproof. If you are measuring a cold sample, you may find its temperature changing too quickly to allow the meter to stabilize. In that event you may want to let it equilibrate for a few minutes — but remember the 15-minute holding time. In addition, readings will stabilize faster if you swirl or stir samples. Keep in mind that SM #4500-H+ B states, “…bring sample and buffer to same temperature…” Some state regulators may want you to follow this, even if you are using ATC.

For more information, consult manufacturers’ user manuals. Another recommended resource is Standard Methods for the Examination of Water and Wastewater, Method 4500-H+ B (Electrometric Method), latest method revision: 2000.

About the author

Peter Strimple is a process specialist with Severn Trent Services. He can be reached at 321/229-7747 or peter.strimple@stservices.com.