Determination of pH is critical wherever water is used in processing industries. Accurate measurement and control of pH and conductivity can help optimize plant and process performance. It can also improve the resilience of systems, minimize energy consumption and reduce waste. However, pH measurement is often perceived as complex and requires 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. They make real-time pH measurement more accessible for a wider range of applications while increasing efficiency, reducing costs and extending the life of detection equipment.
Overview of challenges
Although pH is one of the most important measurement parameters, it is also difficult to obtain correctly. No two industrial processes are the same, so there is no one solution that can be easily applied to every pH measurement application. Instead, the choice of pH measuring device will vary depending on the nuances of the process in which it is used, with different solutions required depending on the nature of what is being measured, where and for what purpose.
Getting the wrong pH levels can be expensive. Extreme pH values can corrode pipelines, leading to leaks and failure of components such as valves and pumps. Additionally, if pH levels are not properly balanced, it can impact the natural environment, potentially resulting in financial penalties.
For process engineers unfamiliar with pH electrodes, an added complication can also be the wide range of options available. This situation is further exacerbated by the growing skills gap in various industrial sectors.
The latest developments in the digitization of pH measurement are helping to solve these problems. PH devices are now easier to install, commission, use and maintain. The latest generation of devices includes plug-and-play technology, allowing quick connection of the sensor to a digital transmitter. This reduces the time required for installation while removing the uncertainty during commissioning. Digitization also brings other benefits, such as devices now having the ability to generate dynamic rapid response (QR) codes to indicate condition or faults. This is fueling a broader trend to improve visibility, accessibility and integration of data across the plant to better inform operational decision-making. It also makes it easier to switch to 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, allowing for save time and money.
Fundamentals of electrodes
Today’s devices work on an electrochemical principle, using a sensor called a glass pH electrode. This electrode is used in conjunction with a reference electrode to complete an electrical circuit which produces a pH value for a measured sample.
A basic glass electrode consists of an inert glass rod sealed to an ampoule or glass membrane made from a special glass formulation that reacts with hydrogen ions. The pH measurement is produced as a result of an ion exchange process that takes place between the hydrogen ions in the solution and the ions on the surface of the glass membrane. This develops a charge on the surface of the membrane which is then transferred through the membrane where it is picked up on the inner surface.
Inside the glass electrode is an aqueous internal filler solution of known pH along with a silver wire coated with silver chloride, called the internal element. The submerged element provides electrical continuity between the interior surface, providing an electrical connection 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 different designs, but a typical construction uses a silver wire coated with silver chloride immersed in a solution of potassium chloride (KCl). This provides a stable environment for the reading but, just as important, allows electrical continuity between the pH electrode and the sample, thus completing the circuit. For more demanding applications, such as those involving sulfides, a double reference electrode would be used.
PH electrodes are not one-size-fits-all devices, and some types can have potential weak spots that can limit both their effectiveness and overall lifespan if not taken into account from the start:
1 – 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 ability to withstand prolonged exposure to inherent process conditions. While some substances will be relatively benign, others can be aggressive, subjecting the glass pH electrode to prolonged attack which can lead to faster wear.
Glass performance can also be affected by the temperature of the sample being measured. In situations where the medium itself is at low temperature or when the sensor is installed in low temperature conditions, the use of a low temperature glass will help ensure a rapid response to changes in pH. Conversely, in high temperature processes where the media are more aggressive, the use of a high temperature glass will make it possible to guard against the premature aging of the glass which can quickly degrade the performance of the electrodes using a pH glass at general use.
2 – The reference electrode
To ensure accurate performance, it is essential that the potential of the reference electrode is stable and not affected by chemical changes in the solution. Most pH sensors use a silver / silver chloride reference electrode containing a chlorinated 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 prevent the coating of the reference element from dissolving.
A common problem with reference electrodes is the problem of poisoning caused by the ingress of chemicals such as sulfides and bromides from the sample being measured. Over time, this can lead to changes in the chemistry of the reference electrode, making the reference potential unstable and reducing the accuracy of the pH measurement. When this occurs, the life of the electrode may 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 able to flow freely 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 providing extended stability and resistance against fouling. By making the path from the sample to the reference as long and complicated as possible, the operational life of a sensor can often be extended.
Under certain circumstances, for example when the reference system is contaminated with salts evaporating from the electrolyte or when the sample itself contains substances which can form salts, the reference junction may become blocked or fouled. , limiting the flow rate of the solution and hindering the measurement.
To help minimize the risk of blockage, a variety of options are available, including polytetrafluoroethylene (PTFE) junctions. Providing good protection against particle formation, 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 likely to get blocked by hydrocarbons, the solid reference can help extend the life of an electrode and improve its long-term performance.
Choose the right installation option
With many manufacturers offering different installation options for pH instruments, it can also be helpful to know where the measurement will be taken. Making sure that a pH electrode is placed in the right part of the process can make a real difference in performance. In particular, the sensor must be in permanent contact with the sample medium to prevent it from drying out. The sensor should also be located to allow easy access for inspection and to perform maintenance tasks such as cleaning and calibration.
The way a sensor is installed can also impact performance and operation. For example, mounting on a tank or ship can be very demanding, as the flow inside the vessel can be omnidirectional and cause accelerated fouling. If the sensor is mounted in a recirculation line, it can provide the benefits of a “self-cleaning” mechanism due to the unidirectional flow of the sample, which will help keep the sensor operational for longer.