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pH for beginners

Dr Jon Farrington, a manufacturing chemist with ABB Instrumentation in the UK, explains how to trouble-shoot some of the most common issues.

pH probes are mysterious creatures compared to many of the other instruments commonly used in industry. Most end users are not specialists and that’s fine if everything is running smoothly. But if something goes wrong with a pH reading, the subtleties of electrochemistry mean that many engineers are not confident about diagnosing possible problems. But there are general guidelines that can help users diagnose some of the most common faults, and it’s sometimes possible to fix them without resorting to new equipment.

pH measurements need three elements: a pH electrode, a reference electrode and a temperature compensation element. These may be separate, but are often integrated into a single combination electrode. Problems can crop up with any of the elements, or may be to do with other aspects of the installation, such as faulty cables or connections.

Perhaps the most useful approach is to look at some typical faults and highlight the most likely causes behind them.

Short scaling

Short scaling (where the sensor doesn’t register the pH you’re expecting) or a slow, sluggish response can both result from contamination of the pH electrode.

The response of the probe to pH depends on the diffusion of ions from the sample to a pH-sensitive glass where ion exchange takes place. Some types of contamination slow this process down, leading to a sluggish response, while others block some of the ion exchange sites and result in short scaling.

The usual suspects are deposits of grease, proteins or scale, depending on the particular situation. For example, a manufacturing process involving machinery will often result in oil in the wash water, while food production raises the possibility of protein deposits. Scale problems are more likely in hard water areas.

If grease is the problem, a good wash with an organic solvent such as isopropanol is the answer. Alternatively, the probe might need an overnight soak in acid to remove scale, while an enzyme such as pepsin can digest protein deposits.

Although deposits from the sample are the primary cause of contamination, they are not the only possibility. Be sure that dirty fingers have not left deposits on the glass during installation, for example.

Short scaling can also be caused by problems with the cable termination. The high-impedance signal from the pH probe means that the wire from the probe must be coated in an anti-microphonic layer to prevent interference from other electrical signals. This anti-microphonic layer must be completely removed at the termination. Of course, this should not be a problem for electrodes that are supplied ready-terminated.

Erratic readings

Readings that bounce around and will not settle are usually the result of contamination or poisoning of the reference electrode. In fact, reference poisoning lies behind more than 95 per cent of the problems with pH sensors. The reference must be connected to the sample so eventual contamination is inevitable, although it can take years to take place in benign applications. The most common poisons are sulphides, which are present in biogas and in many power generation applications. Bromides can also be a problem. Once poisoning occurs, the reference electrode must be replaced, but there are other possibilities worth checking first. For example, make sure that there is continuity between the reference solution and sample solution. If not, where appropriate, a reference probe top up, or even a gentle shake might solve the problem. Additionally, a check on the liquid junction to see that it is clean is also advisable.

Calibration issues

pH probes are dynamic systems and must be calibrated regularly. Just how regularly will depend on the application, and a suitable interval could be anything from a week to a year. For a typical, non-critical process application such as effluent monitoring, three- to six-month intervals will usually suffice. Of course, it’s inevitable that some problems will show up during calibration, even though everything seemed fine during normal operations.

The theoretical response of a pH probe is logarithmic and a plot of pH against output voltage should give a linear graph with a given slope. This theoretical slope is what the probe should be aiming for and any deviation is usually expressed in percentage terms. So a perfect probe would deliver 100%, while ABB probes are sent out from the factory at 98.5 per cent or better. Around 80 per cent is good enough in most practical applications.

If you find that the probe has a high slope, the sensor glass may be coated or getting a bit old. Try cleaning it as already described, but if this fails you may need to replace it.

A low slope may mean that the electrode is lazy, which sometimes happens if it is being used in an application that normally operates across a very narrow pH range. For example, if a sensor is used in the final stage of effluent purification, it will only see samples that are almost exclusively pH neutral. If this is the case, challenging the probe by submerging it in acid will often reinvigorate it.

As well as the slope, calibration also includes a check reading, which looks at how far the sensor deviates when responding to a particular pH, usually 7. ABB sensors are shipped with a maximum deviation in the check reading of ±0.3pH. The key point about the check reading is that it shouldn’t really change. If it does, it gives users a good idea that something is going on with the reference side of the system.

Ask the experts

Although end users should be able to diagnose and treat some of the common problems that crop up with pH readings, the fact remains that pH sensors are complex, dynamic electrochemical systems. If you’re still in doubt, it may be best to check with your supplier. Reputable vendors should always offer to support their equipment with expert advice to sort out any problems.

ABB’s line of analytical instruments has been developed to meet the worlds growing demand for accurate, reliable information about process quality. Offering one of the broadest selections of instruments available today, ABB is unique in its capabilities and application knowledge.

Dr Jon Farrington is a manufacturing chemist with ABB Instrumentation in the UK.

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