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Select The Proper pH Sensor

Oct. 26, 2021
Consider a number of key factors including the value of digitalization

Accurately measuring the levels of acidity and alkalinity in chemicals production is vital to ensuring the highest standards of product quality, safety and environmental performance. Thus, pH is one of the most widely checked process parameters. However, it also is one of the most difficult to get right. The choice of a suitable pH measurement device depends upon the nuances of the particular process and the nature of the substance involved.

Chemical processors face many issues that require managing pH. For example, reactions and other production steps often demand careful pH control. In addition, the need for pH measurement usually extends to effluent streams; the wrong pH value can lead to serious corrosion in the pipework carrying the waste, especially at older facilities built with less-corrosion-resistant materials. Furthermore, lack of correct balancing of pH levels at the point of discharge could cause damage to the aquatic environment, potentially resulting in prosecution and stiff penalties.

For process engineers, the wide range of sensor options available (producers of pH electrodes often have extensive and complex portfolios) can make selection difficult. Exacerbating the situation, the retirement of more-experienced engineers has left many operating companies with less expertise on pH measurement.

The latest developments in the digitalization of pH measurement help address all these issues. Today, pH devices are easier to install, commission, operate and maintain. The latest ones offer “plug and play” technology that allows fast connection of the sensor to a digital transmitter, cutting the time needed for installation and removing any uncertainty during commissioning.

Digital technologies allow operators to check the pH performance of their processes from a tablet (Figure 1) or another smart device. The inclusion of features that help simplify operation and maintenance also enables engineers to quickly get to the root of any functional issues. For example, many instruments can produce a dynamic QR code to indicate faults. A technician can use a smart phone to scan this code to get diagnostics information and the possibility of online help from the vendor.

Digitalization also can provide a better overview of processes, ensuring more informed and objective decision-making. Integrating the readings of pH sensors and others into digital management systems makes data collection more efficient, saving both time and costs, while affording a more practical way to control overall plant operations.

Getting Started

A key preliminary for selecting the right pH electrode for an application is understanding the process itself. Because processes often involve fluids that range from mild to highly caustic or acidic as well as conditions that can vary in temperature, a device should offer the best balance of durability in the particular operating environment and performance. Therefore, knowing exactly what is being measured can help set sensible criteria for device selection.

In addition, it aids to have a grasp of the factors that can affect pH measurement performance. This requires a basic understanding of how a pH electrode functions.

Today’s devices work on an electrochemical principle. A sensor known as a glass pH 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 bulb or membrane made from a special glass formulation that is responsive to hydrogen ions. The pH measurement results from 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 then is transferred through the membrane where it is picked up on the inner surface.

The glass electrode (Figure 2) contains an aqueous internal filling solution of a known pH along with a silver wire coated with silver chloride (AgCl), which is called an internal element. The immersed element allows for electrical continuity with the inner surface, thus affording an electrical connection back to the pH meter.

To complete the electrical circuit, a reference electrode provides a return path to the sample solution. Reference electrodes come in various designs but a typical construction uses an AgCl-coated silver wire immersed in a potassium chloride (KCl) solution. This offers a stable environment for the reading but, equally important, allows for an electrical continuation between the pH electrode and the sample, thus completing the circuit.

More-demanding applications, such as those involving sulfides, require use of a double reference electrode. Such a reference electrode usually consists of a AgCl sealed electrode with its own junction, fitted into a second chamber with a junction in contact with the sample. The main advantage of this electrode is that the reference solution in the second chamber, usually just KCl, can be chosen to be compatible with both the “inner electrode” solution and the sample. This electrode can have a slurry-filled sealed outer chamber or a reservoir-fed arrangement to suit the application.

Once the fundamentals of a pH electrode are understood, it becomes easier to grasp what can go wrong. In general, pH electrodes have several potential weak points that can limit both their effectiveness and overall service life if not considered at the outset. So, let’s look at three key aspects:

1. The electrode glass. The formulation of the glass used for the electrode can significantly impact its performance, both in terms of accuracy and ability to withstand extended exposure to the inherent process conditions. Some processes involve substances that can be very aggressive, subjecting the glass pH electrode to prolonged attack that can accelerate wear. For example, glass and semiconductor processing often relies on solutions containing hydrofluoric acid (HF). HF etches the surface of glass membranes, eventually dissolving them away completely. In this situation, opting for an HF-resistant glass will lengthen the working life of the electrode.

Glass Electrode

Figure 2. The pH sensor contains an aqueous internal filling solution of a known pH along with a coated silver wire called an internal element.

Caustic processes with pH levels of pH 12 or above also can pose problems. Here, sodium ions can exceed the concentration of hydrogen ions, causing a sodium error that results a reading lower than the true pH value. The resolution is to choose a glass type that offers a low sodium error.

In addition, the temperature of the sample being measured can affect the performance of the glass. In situations where either the medium itself is at a low temperature or the sensor is installed in low-temperature conditions, opting for a low-temperature glass will help ensure a fast response to changes in pH. Conversely, in high-temperature processes where media are more aggressive, using a high-temperature glass will aid in protecting against premature ageing of the glass that can quickly degrade the performance of electrodes with general-purpose pH glass.

Equally as important is the design of the glass electrode. For example, a glass with a self-cleaning flat profile will help reduce the risk of fouling in applications with high levels of particulate matter. Alternatively, bullet glass sensors are the prime choice for any application up to 140°C and 10 barg. Their robust construction suits in-line, dip and retractor-type installations in a variety of applications.

2. The reference electrode. To ensure accurate performance, it is essential that the reference electrode potential is very stable and not affected by chemical changes in the solution. Most pH sensors use an Ag/AgCl reference electrode containing a chloridized silver wire immersed in an electrolyte solution of 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 AgCl to help stop the coating on the reference element from dissolving.

A common problem with reference electrodes is poisoning caused by the ingress of chemicals such as sulfides and bromides from the sample being measured. Over time, poisoning can change the chemistry of the reference electrode, creating reference potential instability and reducing the accuracy of the pH measurement. It can decrease the lifetime of the electrode, necessitating early removal and replacement with a new one.

3. The reference junction. This provides the interface point between the reference electrode solution and the process sample. To ensure effective measurement, the solution must 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 reduce the risk of electrode poisoning and providing prolonged stability and resistance against fouling. Making the path between the sample and the reference as long and complicated as possible can significantly extend the operational life of a sensor. Some electrode types offer designs such as multiple junctions — for example, ABB’s 500PRO electrode boasts a triple junction design — and options such as solid-state reference technology, where a material such as wood charged with KCl serves as the reference, can aid in avoiding problems with plugging and poisoning that can affect liquid-, gel- or slurry-filled electrodes. Other options include designs that feature a longer path between the reference junction and the electrode as a way of delaying the impact of any poisoning from the sample.

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 become either blocked or fouled, restricting the flow of the solution and impeding the measurement.

Various options are available to help minimize the risk of blocking. For instance, junctions made from polytetrafluoroethylene (PTFE) offer good protection against the formation of particulate matter; PTFE junctions are ideal for most applications except those involving hydrocarbons. For those types of applications, a better alternative is a solid reference junction using a substance such as wood impregnated with KCl. Less prone to becoming blocked by hydrocarbons, a solid reference can help prolong an electrode’s lifetime and improve its long-term performance.

Installation Considerations

Many manufacturers offer different installation options for their pH instruments. So, knowing the measurement location can be useful. Ensuring that a pH electrode is placed in the right part of the process can make a material difference to its performance. In particular, to prevent the sensor from drying out, it should be in constant contact with the sample medium.

Given the variables that can affect a pH device’s performance, it’s sensible to locate the sensor to enable easy access for inspection and carrying out maintenance tasks such as cleaning and calibration.

How a sensor is installed also can massively impact its performance and operation. For example, mounting the sensor to a tank or other vessel can prompt problems because flow within the vessel can be omnidirectional and cause accelerated fouling. Locating the sensor in a recirculation line 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.

Depending on the application and the medium being measured, a variety of installation options are available to help facilitate access; these include retractable systems for high pressures, flow cells and dip-type sensors.

The Importance Of Diagnostics

Many pH electrodes now offer extensive diagnostic data on the status of the device. This information can help identify both deteriorating performance and its possible root causes.

The advent of digital pH systems, in particular, has enhanced these diagnostic capabilities. Because an application typically requires a large number of pH sensors, being able to quickly identify the particular device that is failing can help save both time and engineering resources. Modern digital pH instruments have built-in software, making them very quick and easy to install, configure and maintain. It now increasingly is possible to dial into devices to find out everything you need without having to physically visit them. Many advanced digitally enabled measurement devices can check internal connections and electronics as well as warn of sensor memory failure.

Digitalization has markedly enhanced the process of diagnosing a fault. For instance, issues such as electrode poisoning now are easier to trace thanks to features like perpetual impedance diagnostics (found on ABB’s next generation pH and ORP sensors), which analyzes the resistance and impedance between the reference and measuring electrode; and smart reference electrode monitoring, which provides early warning of electrode poisoning, enabling quick diagnosis of problems.

Condition monitoring can lead to the optimum scheduling of maintenance and allow advance preparation to avoid process impact. By connecting the pH sensor to a digital transmitter, the unit’s data output from its diagnostic functions can help accurately determine the root cause of a pH measurement error. A plant can use these data to identify what went wrong and derive strategies to prevent the same problems from recurring. Operations and maintenance staff can make an informed decision on whether the fault is fixable or the sensor needs replacing — minimizing downtime and reducing the need for unnecessary inventory.

Make The Right Choice

With so much to consider when it comes to pH electrode selection and the extensive array of different options offered by manufacturers, determining the optimum choice can seem daunting. Hopefully the pointers in this article will help. Fortunately, some manufacturers are trying to ease decision-making.

For example, ABB recently launched a simplified portfolio with just five distinct ranges of dedicated electrode options designed for specific duties, from general purpose through to harsh and ultra-pure applications. Each range incorporates the necessary features to address many of the challenges outlined earlier, enabling users to find the right device for their requirements more easily and quickly.

NIKODEM SIWEK is continuous water analyzer global product manager for the Measurement & Analytics Division of ABB, Warminster, Pa. Email him at [email protected].

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