The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #87.
The reference electrode should provide a fixed reference potential and a path of electrical continuity from the internal reference element via a porous surface or aperture between the reference electrolyte and the process called a liquid junction. Coating of the reference junction can cause slow equilibration. Process ions, such as cyanide, can cause precipitation of silver ions, which will plug the reference junction. Plugging of the reference junction can isolate the internal reference electrolyte, causing a loss of electrical continuity.
Contamination of the reference junction by process ions getting into the junction can cause a significant change in the liquid junction potential. If the electrolyte around the internal element is poisoned by entering process ions, the change in reference potential will be so large that the electrode will be useless. For such applications a solid-state reference or a different electrolyte must be used. All of these conditions will show up as an offset whenever a calibration check is done.
Coating and plugging of the reference junction will also show up as an increase in the reference electrode resistance. Because the reference electrode resistance is relatively small (kilohms), a reference resistance diagnostic can provide an early warning of junction coating and plugging problems. This is not the case for the glass electrode because the glass resistance is so high (Megohms). For glass electrodes, response time is a much more sensitive indicator of coating (Tip #86).
Suppliers prefer that the electrodes always be removed and buffer calibrated because the conditions and procedure are well known and are even automated in smart transmitters. The buffer is a primary calibration standard, whereas a lab pH meter is not. For applications demanding accuracies of better than a 0.1 pH, buffer calibrations may be periodically necessary.
I have always preferred an online standardization adjustment, where a grab sample is taken to make a zero adjustment to compensate for an offset. The electrodes do not have to be removed and the installation does not have to be isolated, flushed, and drained. An online standardization adjustment eliminates glass damage during handling, upsets to the reference junction equilibrium and the process, and not seeing the effect of process temperature and ionic strength on pH. However, the grab sample pH changes as the sample cools off, gets contaminated, or changes in composition. Pure water is easily contaminated by absorption of glass ions or carbon dioxide. Process fluids can change composition from reaction or vaporization or escape of dissolved gases as bubbles. Dissociation constants change with temperature. A rapidly changing process pH is not suitable for standardization because the exact time the sample was taken becomes critical. Hazardous samples require extra handling precautions and a lab pH measurement must be done under a fume hood.
Maintenance personnel often chase calibration adjustments by responding to temporary deviations. Installing an online referee electrode can eliminate unnecessary calibrations. The installation of three electrodes and middle signal selection (Tip #88) creates a reliable referee via the middle value, which offers many advantages.
Concept: Buffer calibrations provide the best calibration under standard conditions but do not take into account temperature, ionic strength, and reference junction equilibrium effects.
Details: Process streams with high ionic strengths, low water content, and changing process temperatures will have a pH measurement that is highly dependent upon operating conditions other than hydrogen ion concentration. The reference junction potential will be large and the reference electrode will take time to equilibrate with process conditions, causing a drifting pH measurement after removal and reinstallation. Some solid-state electrodes in high ionic strength streams take a day or more to equilibrate. For difficult process streams where the hydrogen ion activity or dissociation change, do an online field standardization adjustment.
Take a grab sample and immediately measure the sample pH in the field if the sample vapors do not pose a hazard. If possible use a wireless pH transmitter instead of a lab meter to get the results into the DCS historian. Verify the accuracy of the test electrode by buffer calibration before using it for standardization. Use the middle value of three electrodes as a referee to determine when calibration needs to be done. Short-term fluctuations should be ignored and the effect of deviations due to differences in response time noted. For high accuracy applications and where validation procedures are required, such as with bioreactors and pharmaceuticals, do a two-point buffer calibration, making a span and zero adjustment. Do more frequent buffer calibrations as electrodes near the end of their life expectancy.
Measure and record the measurement response time (Tip #86) for all buffer calibrations. Record the calibration adjustments for the electrode. Smart electrodes can store calibration history in an electrode chip. Monitor offset as indicative of a reference problem. Monitor reference electrode resistance to get an early warning of reference coating and plugging. If the reference electrode is coated, plugged, or contaminated, replace or refurbish it. If the reference electrode has a removable junction, replace the junction and electrolyte.
Watch-outs: A sample can change composition and temperature between the time the sample is taken and the time the sample is measured. The process can change composition and temperature between the time the sample is taken and when the calibration adjustment is done. A drifting reading in a buffer calibration or upon installation is an indication that the reference junction is still equilibrating.
Exceptions: A broken or cracked glass electrode or a broken wire or loose connection will cause the pH reading to be fixed near 7 pH. A standardization adjustment will not reveal that the offset is due to a damaged glass electrode. Monitor glass resistance to detect broken or cracked glass.
Insight: The health of the reference can be monitored online with less disruption to the electrodes and the process by reference electrode resistance diagnostics and standardization in the field.
Rule of Thumb: Use a smart transmitter and smart electrodes and do standardization and/or periodic buffer calibrations based on electrode life expectancy and application conditions and requirements.
Hunter Vegas, P.E., holds a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University. His job titles have included instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he joined Avid Solutions, Inc., as an engineering manager and lead project engineer, where he works today. Hunter has executed nearly 2,000 instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms.