Determination of pH is a critical operation wherever water is used in process industries. Accurate measurement and control of pH and conductivity can help to optimize plant and process performance. It can also improve the resilience of systems, minimize energy use and reduce waste. However, pH measurement is often perceived to be complex and require high maintenance, with plant operators often relying on external laboratory analysis of process samples.
New technologies and solutions are emerging to make pH measurement easier than ever before. They make real-time measurement of pH more accessible for a wider range of applications while increasing efficiency, reducing costs and extending the lifetime of sensor equipment.
Overview of the Challenges
While pH is one of the most important measurement parameters, it is also difficult to get right. No two industrial processes are exactly the same, so there is no single solution that can conveniently be applied to every pH measurement application. Instead, the choice of pH measurement device will vary according to the nuances of the process in which it is being used, with different solutions required according to the nature of what is being measured, where and for what purpose.
Getting pH levels wrong can be costly. Extreme pH values can cause corrosion in pipework, leading to leaks and failure of components like valves and pumps. Furthermore, if pH levels are not correctly balanced, this can impact the natural environment, potentially resulting in financial penalties.
For process engineers unfamiliar with pH electrodes, an added complication can also be the sheer range of options available. This is exacerbated further by the growing skills gap in various industrial sectors.
The latest developments in the digitalization of pH measurement help to address these issues. pH devices today are easier to install, commission, operate and maintain. The latest generation of devices includes plug-and-play technology, allowing fast connection of the sensor to a digital transmitter. This cuts the time needed for installation while removing uncertainty during commissioning. Digitalization also brings other benefits, such as devices now having the ability to produce dynamic quick response (QR) codes to indicate status or faults. This feeds into a wider trend of improving the visibility, accessibility and integration of data across the plant to better inform operational decision-making. It also facilitates the shift toward remote monitoring—continuous analysis of equipment to detect potential faults and prevent them before they turn into failures, without having to manually inspect the equipment, potentially saving money and time.
Today’s devices work on an electrochemical principle, using a sensor known as a glass pH electrode. This electrode is used in conjunction with a reference electrode to complete an electrical circuit that produces a pH value for a measured sample.
A basic glass electrode comprises an inert glass stem, sealed to a glass bulb or membrane made from a special glass formulation that is responsive to hydrogen ions. The pH measurement is produced as the result of an ion-exchange process that takes place between the hydrogen ions in the solution and the ions at the surface of the glass membrane. This develops a charge on the membrane surface that is then transferred through the membrane where it is picked up on the inner surface.
Within the glass electrode is an aqueous internal filling solution of a known pH along with a silver wire coated with silver chloride, called an internal element. The immersed element allows for electrical continuity between the inner surface, providing an electrical connection back to the pH meter.
To complete the electrical circuit, a reference electrode is used to provide a return path to the sample solution. Reference electrodes come in various designs but a typical construction uses a silver wire coated in silver chloride immersed in a potassium chloride (KCl) solution. This provides a stable environment for the reading but, equally important, allows for an electrical continuation between the pH electrode and the sample, completing the circuit. For more demanding applications, such as those involving sulfides, a double reference electrode would be used.
pH electrodes are not a one size fits all device, and certain types may have some potential weak points that can limit both their effectiveness and overall service life if not considered at the outset:
1 - The Electrode Glass
The formulation of the glass used for the electrode can have a major impact on its performance, both in terms of accuracy and its ability to withstand prolonged exposure to the inherent process conditions. While some substances will be relatively benign, others can be aggressive, subjecting the glass pH electrode to prolonged attack that can cause it to wear more quickly.
The performance of the glass can also be affected by the temperature of the sample being measured. In situations where either the medium itself is at a low temperature or where the sensor is installed in low-temperature conditions, using a low-temperature glass will help to ensure a fast response to changes in pH. Conversely, in high-temperature processes where mediums are more aggressive, using a high-temperature glass will help to protect against premature aging of the glass that can quickly degrade the performance of electrodes using general purpose pH glass.
2 - The Reference Electrode
To ensure accurate performance, it is essential that the reference electrode potential is stable and is not affected by chemical changes in the solution. Most pH sensors use a silver/silver chloride reference electrode containing a chloridized silver wire immersed in an electrolyte solution of potassium chloride (KCl). This solution slowly seeps out of the sensor through a reference junction to provide an electrical connection between the reference element and the sample. The solution also includes silver chloride (AgCl) to help stop the coating on the reference element from dissolving.
A common problem with reference electrodes is the issue of poisoning caused by the ingress of chemicals such as sulfides and bromides from the sample being measured. Over time, this can cause changes in the chemistry of the reference electrode, causing the reference potential to become unstable and reducing the accuracy of the pH measurement. When this happens, the lifetime of the electrode can be reduced and require premature replacement.
3 - The Reference Junction
The reference junction provides the interface point between the reference electrode solution and the process sample. To ensure effective measurement, the solution must be free to flow through the junction to mix with the sample and establish the electrical circuit.
The design of the reference junction can play a major role in helping to reduce the risk of electrode poisoning and offering prolonged stability and resistance against fouling. By making the path between the sample and the reference as long and as complicated as possible, the operational life of a sensor can often be extended.
Under certain circumstances, such as where the reference system becomes contaminated by salts evaporating out of the electrolyte or where the sample itself contains substances that can form salts, the reference junction can either become blocked or fouled, restricting the flow of the solution and impeding the measurement.
To help minimize the risk of blocking, various options are available, including junctions made from polytetrafluoroethylene (PTFE). Offering good protection against the formation of particulate matter, PTFE junctions are ideal for most applications except those involving hydrocarbons. For these types of applications, a better alternative is to opt for a solid reference junction using a substance such as wood impregnated with potassium chloride. Less prone to becoming blocked by hydrocarbons, solid reference can help to prolong an electrode’s lifetime and improve its long-term performance.
Choosing the Right Installation Option
With many manufacturers offering different installation options for pH instruments, it can also be useful to know where the measurement will be made. Ensuring that a pH electrode is placed in the right part of the process can make a real difference in performance. In particular, the sensor should be in constant contact with the sample medium to prevent it from drying out. The sensor should also be located to enable easy access for inspection and for carrying out maintenance tasks such as cleaning and calibration.
How a sensor is installed can also have an impact on performance and operation. For example, mounting to a tank or vessel can be very demanding, as flow within the vessel can be omnidirectional and cause accelerated fouling. If the sensor is mounted in a recirculation line, it can deliver the benefits of a “self-cleaning” mechanism due to the unidirectional flow of the sample, which will help keep the sensor operational for longer.