Instrumentation: Maximize pH Response, Accuracy and Reliability

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Gregory McMillan is a Control columnist, blogger and a member of the Process Automation Hall of Fame. Read McMillan's Control Talk Blog.

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The pH electrode has improved a great deal since the development of the pH concept over 100 years ago and the first extensive use of industrial pH electrodes over 50 years ago. While glass measurement electrodes are still used to provide the highest accuracy and cover the widest range, significant improvements have been made in the glass formulation, reference design, electrode structure, transmitter capability and installation and maintenance methodology. Measurement and reference electrode problems have been largely addressed, improving performance and life expectancy. Diagnostics and practices have been developed to help deal with tough applications. Here we look at some best practices to keep the electrode response fast, efficiency high, and offset and drift low.

Great Expectations and Practical Limitations

The pH electrode offers by far the greatest sensitivity and rangeability of any industrial measurement. A pH measurement with a 0-14 pH scale can cover 14 orders of magnitude change in hydrogen ion activity (hydrogen ion concentration for dilute solutions), and can respond at the popular 7 pH setpoint to changes in hydrogen ion concentration in the eighth digit after the decimal point. But, to achieve their full potential, challenges need to be addressed. Except for unusually harsh streams in terms of high temperatures, low water content or chemical attack of glass, suppliers say most of the reported problems are with the reference electrode.

Measurement Electrode

The measurement electrode depends upon the outer glass surface layer exposed to the process being clean, hydrated, with the same activity as the inside glass surface in contact with the internal 7 pH buffer solution. The activity of the glass surface depends upon the concentration of key alkali ions and the number of active sites for the hydrogen ion exchange with water molecules in the glass surface. The millivolt potential develops from an exchange that can be visualized as a jump of the hydrogen ion between a hydronium ion in the aqueous solution and the hydrated glass surface. Abrasion, aging and dehydration of the glass surface can cause a loss of active sites that results, first, in a slowing of the pH response and, finally, as a decrease in the efficiency (span) of the pH measurement electrode. The glass resistance can become too high, resulting in noise. A slight imperceptible coating can prevent the jump of the hydrogen ion, resulting in a dramatic slowing of the glass response. Just a one-millimeter coating can cause the 86% response time of the glass electrode to increase from seconds to minutes. A loss of active sites can cause the response time to increase to hours.

Reference Electrode

The reference electrode must provide electrical continuity from the internal electrode (e.g., silver-silver chloride) through the reference electrolyte fill (e.g., silver chloride), reference junction and process fluid to the measurement electrode. To accomplish this continuity, there is either a very tiny open junction (aperture) or a porous liquid junction to allow ions in the reference internal electrolyte fill to migrate into contact with the process fluid. Unfortunately, process ions can migrate through the liquid junction pores or opening, clogging the junction and contaminating (poisoning) the internals. Additional liquid junctions are added internally to provide additional barriers to prolong the time until the contamination by process ions reaches the inner sanctum of the silver-silver chloride electrode. Also, the internal fill is changed from a liquid to a gel to slow down migration of process ions.

A porous reference such as Teflon provides the ultimate barrier for slowing down poisoning. The electrolyte and internal electrode are changed when the process fluid excessively interacts with the internal electrode or fill. For example, the fill and internal electrode are changed when the process has cyanide because cyanide ions cause precipitation of the silver and attack the internal electrode. The precipitate plugs the liquid junction. A plugging or a coating of the junction slows down ion migration and increases the potential at the junction, causing an increasing offset (drift) in the measurement. Concentrations of reference electrolyte ions build at the reference junction until an equilibrium potential is reached, opposing further migration. Similarly, process ions accumulate at the liquid junction until an equilibrium potential is reached. The potential developed is called a liquid junction potential. For process streams with a large concentration of ions (high ionic strength) the liquid junction potential can result in an offset as large as 50 millivolts. The equilibration rate increases and size of the junction potential decreases as the area and porosity of the liquid junction decreases. Thus, there is a tradeoff between plugging rate and equilibrium rate. Solid reference electrodes greatly reduce plugging and essentially eliminate positioning. However, reference equilibration might take days in a process with high ionic strength. Buffer calibration may not be able improve the accuracy beyond 0.1 pH because repeatable readings may only be repeatable to 0.1 as electrodes are moved from one buffer to another.

Instrumentation Best Practices

Figure 1 - New High Temperature Glass Stays Fast

Glass electrodes get slow as they age. High temperatures cause premature aging.For greatest measurement electrode accuracy, use a semi-spherical glass bulb with the best glass formulation for operating conditions. For high-temperature applications, make sure a high-temperature glass prevents premature aging seen as a dramatic increase in response time (see Figure 1). For cleaning and sterilization in place (CSIP), make sure the electrode structure design minimizes the offset and drift after repeated cleanings and sterilizations (see Figure 2). Use low-sodium ion error glass if excursions occur above 10 pH when caustic is present. Use HF-resistant glass if excursions occur below 8 pH when HF is present. Note that high temperatures greatly accelerate chemical attack, reducing glass life. Also realize that the benefit of HF-resistant glass is marginal. A few ppm of HF at low pH can dramatically reduce the life of the best glass. The best bet is to insure the process stream never drops below 8 pH.

To minimize plugging and contamination problems, use a solid reference electrode or a readily replaceable reference junction. The entire sleeve of some solid reference electrode designs is the liquid junction helping to ensure electrical continuity minimizing the effect of coatings. Most solid references eliminate the contamination problem. Some liquid reference designs offer a removable reference junction, so the junction and electrolyte can be easily replaced.

Figure 2 - New Design Eliminates Drift after Sterilization

Consider a flowing reference junction as a last resort to quickly equilibrate to a constant minimum junction potential. If you must measure accuracies of 0.02 or better pH in a stream with a propensity for plugging or a high significant ionic strength, realize that a flowing junction can quickly establish a small constant liquid junction potential, and prevent clogging and contamination. This solution is a last resort because the installation requires the pressurization of an external reservoir of electrolyte for the reference to keep a small flow of electrolyte. Excessive electrolyte flow will contaminate process samples and even buffer solutions.

Use wireless pH transmitters to eliminate electrical interference and spikes. The use of wireless transmitters eliminates the ground path that is the cause of much noise and spikes. Wireless transmitters tend to have the latest improvements in transmitter design, including better diagnostics and signal resolution. Wireless transmitters also offer flexibility for testing electrodes in the lab at process conditions and analyzing the results in the data historian.

Compensate or be cognizant of changes in solution pH with temperature. The standard pH electrode temperature compensator corrects for changes in the millivolts generated by the glass with temperature, but not for changes in the solution pH, which will change with temperature due to a change in the acid, base and water dissociation constants with temperature. The effect is greatest when the pH is near these constants. The actual effect of temperature on a solution pH should be quantified by tests on representative lab samples. Smart transmitters offer solution pH temperature compensation.