Greg McMillan and Stan Weiner bring their wits and more than 66 years of process control experience to bear on your questions, comments, and problems. Write to them at [email protected].
Stan: This is the hundredth anniversary of the glass pH electrode. News of the invention of the pH glass electrode was published in 1909, but it wasn’t until Dr. Arnold O. Beckman developed the first high-impedance amplifier (Model G) in 1939 that industrial measurement of pH become possible. Dr Beckman persevered despite being told the total market was 500 instruments. By 1956 Beckman had sold 126,000 Model G pH meters. Today about 100 pH electrodes are sold every hour.
Greg: The glass electrode is more important than ever because of its extraordinary rangeability and sensitivity to hydrogen-ion concentration. What other measurement can cover 14 orders of magnitude of concentration and detect changes as small as 10-14 (0 to 14 pH scale)? This incredible capability is the key to why the glass pH electrode is as important today as it was when invented 100 years ago and why something better hasn’t displaced it. Yet not much has been published about application problems and practices. The life of the pH electrode is largely secret. The books and academic studies on the effect of process conditions on the glass electrode are over 15 years old. The chapter on electrodes in my book, Advanced pH Measurement and Control, (available online from Amazon.com, Textbooks R Us and A1 books, among others) is largely based on these old publications and my application experiences at Monsanto and Solutia.
Stan: The pH measurement application expertise resides primarily in a few old hands at the pH electrode manufacturers. All of these guys could be headed for greener pastures (golf courses and ranches), so there is a sense of urgency on our part in capturing some of this expertise. We interviewed Scott Broadley and Bob Garrahy at Broadley-James Corporation (BJC) and Jim Gray and John Wright at Rosemount Analytical Inc (RAI) to get the inside story.
Greg: We are also getting input from anyone interested in pH by means of an on-line survey available at www.zoomerang.com/Survey/?p=WEB228KQLJS4KT. The survey is quite detailed. There is some value in completing the survey just in terms of exposure to various issues and possibilities. If you don’t know the answer to any particular question, just click the 0-1% button on the survey.
Stan: The typical pH system available in the 1960s and 1970s consisted of three separate probes (measurement, reference and temperature). The user has a choice of lots of glasses (rugged, general-purpose, low-pH, high-pH, low-temperature and high-temperature). The reference electrode was typically a flowing junction, which prevented clogging and contamination, promoted faster equilibration and regulated a more constant potential at the reference junction. These electrodes are now relics. What happened?
John: There was a big jump in industrial pH measurements in the 1970s. Maintenance became a bigger issue. There were no diagnostics, and users were left to their own devices to figure out whether the problem was with the measurement or reference electrode. Also, installing and maintaining the fill and pressure of a flowing reference was an increasing aggravation. As a supplier, we seek to identify and alleviate the biggest points of pain to the customer, and maintenance was the hot button in those days.
Bob: The Clean Water Act of the 1970s, which required the neutralization of wastewater and removal of heavy metals prior to discharge to the sewer, was a huge boon to the pH market—enough to support the startup of a half-dozen companies. All of these electrode suppliers offered a combination electrode where the measurement and reference electrodes and often a temperature sensor were contained within one probe. Various reference designs were used to reduce loss of electrolyte or process flow into the reference junction to the point where the reference did not need to refilled or pressurized: If the probe was perceived to have problem, it was simply replaced.
Greg: The throwaway concept was immensely popular, but in some cases, the probe life was a matter of days or, in some extreme cases, a matter of hours. In some nasty streams, coating, abrasion and premature aging of the glass, and coating, plugging and poisoning of the reference were shortening the life of the probes. These problems lead to more creative designs and advertising and to the “Top Ten” list at the end of this article.
Stan: Why do some users swear by the performance of new features?
John: There is a placebo effect where, if one believes the electrode has new exceptional qualities, one is less likely to blame the electrode for what are actually operational or process problems. The reality in most control rooms is that the measurement is the easiest thing to blame.
Stan: Studies show that more than 65% of the removal and recalibration of flow, level, pressure and temperature measurements is unnecessary. I suspect the percentage is higher for pH due to its extraordinary range and sensitivity. The removal and buffering of electrodes upsets the equilibrium of the reference and leads to calibration adjustments chasing calibration adjustments. Also, the biggest cause of electrode breakage is handling.
Jim: One user I encountered would hit the electrode on the side of the tank to shake loose any stuff hanging on the electrode and then wondered why the electrode didn’t work.
Bob: I tell people to think of the construction of a pH probe as similar to a chemical glow stick. If you bend or strike it, you break the internal glass element. But in the case of a pH probe, it doesn’t glow—it just stops working! The resulting crack or short may have no visual indication, and the problem is tough to catch online because the signal fails at close to 7.0 pH—the most common pH setpoint.
Greg: While only 10% of the applications had real problems, the consequences were so significant in terms of poor plant performance (plant capacity, product quality and environmental violations and permitting) that the point of pain drove customers to niche solutions that became corporate standards. For example, large chemical companies standardized on solid references because they solved severe reference contamination problems. Other electrode suppliers came up with double and triple junctions, less porous junctions and gel fills to slow down the process fluid getting into the inner sanctum of the internal silver-silver chloride element of the reference. Most suppliers eventually developed their own version of the solid reference by using porous wood, Teflon or a polypropylene and glass composite saturated with electrolyte.
“Top Ten Uses of Old pH Electrodes”
(10) Christmas Tree ornaments. You will need to get a bigger tree.
(9) Jewelry for your spouse. Nothing says “I love you” like a personal pH electrode.
(8) Knick knacks. Electrodes are much more interesting and fun than figurines.
(7) Party favors. Electrodes make great conversation pieces.
(6) Horny Toad costumes. Just be careful you don’t mistakenly poke someone.
(5) Disco décor. Go for glitter and glamorous.
(4) “Price is Right” prizes. See what customers are willing to pay.
(3) E-bay sales inventory. Claim they were used in celebrity pools.
(2) Electrodes on the skids. Most of them are probably better than the existing ones.
(1) Adorable Automators’ Association Hall of Fame inductees. We’re gonna need a bigger building.
