Handle with Care

Nikodem Siwek explains how to get the most from your pH measurement devices

THE arduous nature of chemicals processing applications can be especially tough on pH sensors, making correct selection and maintenance especially important. This article looks at how to get the best performance out of pH measurement equipment and how developments in digital sensing technology can help.

In chemicals processing applications, pH sensors have a tough life. Constantly in contact with the process medium, they are frequently subject to extreme wear and tear that can significantly shorten their service life compared to other types of measurement instruments. For operators requiring high accuracy measurements to ensure the quality of their products, it is important to understand the key factors that can affect pH sensor performance and to put in place an effective maintenance and servicing regime that can prolong their operation. 

pH measurements need three elements, excluding the pH analyser: a pH electrode, a reference electrode and a temperature sensor (temperature compensator). These may be separate but are often integrated into a single combination electrode. Problems can arise with any of the elements or may arise due to other aspects of the installation, such as faulty cables or connections or even something as basic as choosing the wrong device for the application.

In particular, the subtleties of electrochemistry mean that many engineers are not confident about diagnosing possible problems, resulting in issues being missed or devices being prematurely or even unnecessarily replaced.  In this article, you will find some general guidelines that can help you diagnose and fix some of the most common faults without resorting to purchasing a new sensor or other equipment.

Short scaling or sluggish response

A pH sensor’s response to changes in 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, causing a sluggish response, while others can hinder the ion exchange, resulting in short scaling, where the pH reading is not the pH value expected.

The subtleties of electrochemistry mean that many engineers are not confident about diagnosing possible problems, resulting in issues being missed or devices being prematurely or even unnecessarily replaced

Common causes to look for include deposits of grease, proteins, or scale, depending on the application.

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

In addition, analogue sensors use a high impedance cable that make them susceptible to interference and sluggish response, especially over long distances. This can restrict the length of the cable connecting the sensor to the transmitter. With digital pH sensors, the issues associated with high impedance and restrictions on cable length are removed without compromising accuracy and speed of response.

Erratic readings

Erratic and unstable readings are usually the result of contamination or poisoning of the reference electrode, caused by the substance being measured mixing with the reference electrolyte. The most common poisons are sulfides and bromides.

While in most cases, the reference electrode must be replaced once poisoning occurs, it is worth checking other possibilities first. Start with making sure there is continuity between the reference solution and sample solution. If a refillable reference electrode is being used, ensure the reference probe is fully topped up with filling solution. A gentle shake to ensure the solution is in contact with the junction might solve the problem. It can also be advisable to check that the liquid junction is clean.

Some digital sensors can remove this challenge by giving an early warning of electrode failure. This relieves the headache of investigating the listed alternatives to electrode failure and helps avoid the problem of lost inventory due to unreliable readings.