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Dec
22

The Secret Life of pH Electrodes

This is 100th anniversary of the glass pH electrode yet we still do not know much about what affects their life. Not much has been published about application problems and practices. There have been a few academic studies on the effect of process conditions on the glass electrode but these are over 20 years old. The best book on the theory of pH measurement The Determination of pH is over 35 years old. 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 0.00000000000001 (0 to 14 pH scale).

While the fundamentals of the glass pH electrode have not changed in a hundred years, there have been significant improvements in the glass formulation and construction so that it can handle high pH and high temperature fluids and repeated sterilizations. For example a new high temperature electrode increases the life expectance by 100% as seen in the attached test results. HighTemperatureGlassElectrodeLife The same company has developed a new electrode for the biopharmaceutical industry that can withstand 50 sterilizations. These new glass electrodes maintain a response time of seconds whereas other electrodes develop response times of several hours due to the premature aging of the outer glass surface.

As the number of pH applications increased dramatically due to the Clean Water Act and the growth of biopharmaceuticals and specialty chemicals, maintenance costs became a bigger issue. Filling and pressurizing reference electrodes and troubleshooting separate measurement electrodes became points of pain. In response, electrode suppliers developed the throwaway combination electrode that built a sealed reference, glass measurement, and temperature sensor into a single rugged probe body. Maintenance simplified to calibration and replacement. Some users unfortunately skipped the calibration part.

Reference electrodes maintain a low resistance path to the process for electrical continuity of the pH measurement by an internal electrolyte that is contact with the process fluid through a porous reference junction. The elimination of the flowing reference junction can lead to coating and contamination because there is no flushing action to prevent back flow (migration) of the process through the reference junction into the interior. To deal with these potential problems there have been major improvements in the construction of the reference electrode to reduce contamination (poisoning) by the use of multiple internal junctions and by the use of a porous solid with tortuous paths instead of a gel to slow down the migration of process ions and prolong the time it takes for the process to reach the internal silver-silver chloride element.

There have been attempts to develop alternatives to the glass electrode, such as the Iridium oxide, ISFET, and optical sensors. Yet we have not seen significant use.

The secret life of pH electrodes will be exposed in a series of Control Talk columns starting in the February issue of Control magazine. The series will include very candid views about the history and future of pH measurement from the most experienced people at major electrode manufacturers and the results from an online survey to find out what is really going on with pH applications. You can participate in the survey via the following link: http://www.zoomerang.com/Survey/?p=WEB228KQLJS4KT

If you want to learn more about pH measurement and control consider attending a short course. If you have a pH control problem, you can get it solved in the course and learn how to create a virtual plant of the pH system to demonstrate and prototype process control improvements. You can get info on my course via the following link:

http://www.emersonprocess.com/education/catalogrev/automationsystems/9060.asp

For more info on pH measurement check out previous entries in the pH and plant design categories on this web site. You can get more information on pH modeling and control from recent articles “Virtual Control of Real pH,” Control, p. 47 (Nov. 2007) http://www.ControlGlobal.com/articles/2007/385.html

and “Virtual Plant Provides Real Insights,” Chemical Processing, (Jan 2009).

In the future we can look forward to new electrode diagnostics, interrogation of the history of calibration adjustments for the electrode, and wireless transmission. CD Feng, receiver of the 2007 ISA Arnold O. Beckman Founder Award for his technical contribution and innovation has agreed to provide key chapters on pH sensors, diagnostics, and new technologies for the 4th edition of my book Advanced pH Measurement and Control. Maybe the life of the pH electrode won’t be so secret.