The Secret Life of pH Electrodes - Part 3

John: There will be a commensurate increase in the type of diagnostics we can provide as we get smarter about detecting and interpreting the impedance changes in the glass and reference electrode. As users become more aware of the actual problem, we’re seeing an interest in being able to fix rather than replace them.

Greg: I hope we’ll also see better options for automatic compensation of the effect of temperature on the solution pH. The standard temperature compensation offered for electrodes is for the change in millivolts generated per pH by the glass electrode per the Nernst equation. Most people are unaware that the solution’s pH also changes due to changes in the water and acid and base dissociation constants with temperature. For strong acids and bases, their respective dissociation constants are well beyond the normal pH range. For these processes, what we see is mostly the effect of temperature on the water dissociation constant that can be approximated to be -0.03 pH/oC for basic solutions.

Stan: Whether a customer sees more glass or reference problems depends on the type of application, control limits, and how long the electrode has been in service. Often we hear about reference problems in the pharmaceutical industry perhaps because the control limits are so tight. For example, we have the following input from Rick Cooley after taking the pH survey.

Rick: By far the biggest pH measurement problem I encountered was electrode junction potential. From a training perspective, the biggest issues were a lack of understanding that temperature compensation only corrected for the change in the Nernst response and not the actual solution’s change in pH with temperature, and a lack of understanding of measurement uncertainty and its relationship to control limits. Just because a vendor says the electrode and/or meter is accurate to ±0.01 pH units, this doesn’t mean users can set their control limits based on that value. The actual application can have a significant affect on the system’s measurement accuracy, particularly due to the impact various process conditions have on the electrode’s performance.  

Greg: Except for some non-aqueous and high-pH (> 12 pH), and high-temperature (> 65 °C) applications that wreacked havoc with the glass, most of the problems I’ve seen are drifting reference potentials due to coatings, contamination and slow equilibration. With better diagnostics and users sharing results, we’ll all become smarter. For example, a trend of glass resistance is an indicator of a deterioration of the glass surface due to aging or chemical attack. For more info on pH, check my modeling and control website, www.modelingandcontrol.com.