Industrial Use and Applications of Ion Selective Electrodes - Journal of

Waters Symposium: Ion Selective Electrodes

waters symposium: ion selective electrodes

Industrial Use and Applications of Ion Selective Electrodes Truman S. Light Consultant in Analytical Chemistry and Instrumentation, Retired The Foxboro Company, 4 Webster Road, Lexington, MA 02173-8222 Young researchers do not give much consideration to the effect of management decisions on the course of their research and their careers. This was true 30 years ago when I started at The Foxboro Company and I am afraid it is true again today, when economic conditions are downsizing many corporate research programs. Managers are usually not well known in scientific circles, probably because their names do not appear on the research and application papers that emanate from the company’s laboratories. However, when I joined The Foxboro Company in 1964 I became acquainted with two managers who did influence the development of industrial analytical instrumentation in that Company. They were John Dobson and Charles V. Cooper. John Dobson was at first an enigma to me—one of his business cards said that he was President of Foxboro, Mexicana (he later told me he carried three different business cards), and I knew him as someone sitting in the Chairman or the President’s Office. He had a longstanding interest in chemical analytical instrumentation, as I did. Charles Cooper came to The Foxboro Company at the same time that I did as a marketing and research manager. Before coming to this Waters Award Program, I reached him at his retirement home in Florida and he wondered whether Jim Waters remembered when Charlie Cooper at the Rohm and Haas Company had bought industrial gas analyzers that Jim Waters had developed at MSA. Shortly after I arrived at Foxboro, Dobson and Cooper came to my lab telling me that they were interested in a company (no names given to me) that had invented something called “specific ion electrodes” and would I evaluate them “blind”. (Ion selective electrodes, ISEs, did not come into the vocabulary until later.) I was told this company had electrodes that could dip into a solution and immediately measure many ions. One of these was for calcium. Would I make some samples to be taken to this company and see how well they did. I entered into the spirit and prepared one pure calcium solution, a second with a fair amount of acid in it, and another with the nearest relative in the periodic table that I thought would interfere—barium. I later learned that they (Orion) carefully examined these solutions by every analytical means at their disposal, and did well in analysis of the first two solutions but not so well on the third with barium. Later, this electrode was brought on to the market as an electrode for divalent cations and has been quite successful as the “water hardness” electrode. I think I neglected to tell Dobson and Cooper that the research community is composed of scientists with like interests; and I had known Jim Ross several years earlier when I had resided temporarily at MIT to finish my doctoral thesis, and he had helped me sufficiently to earn my acknowledgment in a publication that developed from my thesis (1). I had even talked with Jim about joining Orion at my career juncture when I joined Foxboro and

Martin Frant joined Orion. It is indeed too bad that Jim Ross is not well enough to be with us at this Pittcon/ Waters Symposium. Foxboro invested in Orion. My recollection is that they owned 100,000 shares. I thought it would be interesting to learn how the world of finance functioned and bought 15 shares of Orion when it first came onto the market under the name of “Geochron” (which was a very small geological consulting company that John Riseman, the entrepreneurial cofounder of Orion, had bought because it represented an inexpensive way to get his company listed on the stock market). Geochron stock was later converted to the name of Orion. I attended stockholders meetings and found them very entertaining. One day when I walked into the annual meeting John Riseman called down from his presidential podium “Ted, have you got the proxy for the 100,000 shares of Foxboro?” and I brought down the house when I replied “No, but I do have the proxy for my own 15 shares”. The association between the two companies brought us into close professional contact with Jim Ross and Martin Frant, and we went about our research business of modifying electrodes and applications to fit into the industrial process monitoring mold. You have heard Martin Frant describe the growing up of Orion. Industrial research thrives with the cooperation of many people. As I recall those times in the Foxboro lab, I immediately think of the splendid research and engineering team we had and have to mention names that appeared on publications (2–7): names such as Mickey Oliver, a former chairman of Pittcon who is sitting in the audience someplace; Jim Swartz, who did a doctoral

Figure 1. Industrial Research Laboratory, C.V. Cooper and T. S. Light, ca. 1968.

Vol. 74 No. 2 February 1997 • Journal of Chemical Education

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Table 1. Partial List of Some Ion Selective Electrodes and Industrial Applications Electrode

Industrial Use

Ammonia

Pollution

Cadmium

Plating, waste

Calcium/Hardness

Boilers, H2O treatment

Chloride

Desalination, pharmaceutical

Cyanide

Plating, waste

Cupric

Circuit boards, mining

Fluoride

Potable H2O, stack, HF

Fluoroborate

Plating

Nitrate

Water, farm effluent

Silver

Photographic

Sodium

Ultra pure H2O, food, ion exchange

Sulfide

Paper pulp, ore, Stack

Figure 3. Ion selective electrode process control system with reagent addition.

Figure 4. Continuous process titrator, ion selective electrode endpoint detector. Figure 2. Ion selective electrode process control system.

thesis on computer corrections for on-line measurement using ISE’s; Ken Fletcher, Les Negus, Dick Mannion, and Carleton Cappuccino. And while not a co-worker, nor did he appear on any publications, Rich Danchik of Alcoa, the co-chair and organizer of this symposium, was well known to me and participated greatly in giving us understanding of process analytical needs. Industrial Process Analysis with ISEs Let me tell you about industrial process analysis with ISEs. In Figure 1 we have a photo of Charlie Cooper, whom some of you might know, with me in the Research Lab. I show this not because of the personalities, but to have you notice the equipment of this industrial research laboratory, and how it differs from an academic research laboratory. There is an on-line flow-through cell, rugged industrial electrodes with built-in ultrasonic cleaner. In an industrial plant environment, this equipment could be washed down with a hose at the end of a working day. Somewhat incongruously, on the bench is an Orion laboratory pH/ISE meter that had dual impedance input circuits, a first in its day. I would like to give you an idea of how ion-selective electrodes are incorporated into process control measurement systems. This is not a lecture on the existence and construction of ISEs,

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Figure 5. Continuous gas analyzer using ion selective electrode.

but Table 1 shows how they are applicable to the process industry and some of the industries in which they have occurred. Several figures are taken from Durst’s National Bureau of Standards book on ISEs (8). Figure 2 shows the basic ion selective system—a measuring electrode, reference electrode, and temperature sensor being monitored by a suitable process meter and documented by a recorder. The “unknown” process, stream A, is shown flowing past the electrodes, and this stream may be al-

Journal of Chemical Education • Vol. 74 No. 2 February 1997


Figure 6. Multiple ion selective electrodes process control system.

Figure 9. Continuous fluoride monitor for potable water supplies.

Figure 7. Differential analysis cell with ion selective membrane.

Figure 8. Amplifier with dual high-impedance inputs.

ternated with another, stream B, or a standard solution. This is accomplished without operator attention by a timing device in the analytical controller. Sometimes it is necessary to add a reagent to the sample stream to make conditions suitable for the ISE measurement, as might be the case when a buffer solution is added. This is shown in Figure 3. Where a direct ISE is not available, or more accuracy is required than can be obtained from direct measurement, a process titrator may be used as shown in Figure 4. Here, reagents for the titration

are seen; and instead of a technician or Ph.D. to add them at a suitable time and then write the data in a notebook, a signal processor with logic and a recorder performs this task. A gas analyzer may be designed as shown in Figure 5. Here a scrubbing tower has been added to obtain the gaseous sample in suitable solution form. Multiple ion-selective electrodes may be used in appropriate cases; several analyses being reported simultaneously on the chart recorder are illustrated in Figure 6. The principles of these process methods are the same as analytical chemical applications in laboratory practice. It is possible with the differential analysis system of Figure 7 to maintain a desired process solution as the reference electrode. Not only is there no liquid junction potential, but a zero potential difference exists as an optimal control condition when the process is on target. The differential analysis system goes together with the introduction of amplifiers with dual high-impedance inputs of Figure 8, contrasting with the single high-impedance input of classical potentiometers. One well-known application for monitoring fluoride in drinking water supplies is shown in Figure 9. The water sample is taken from the main distribution pipe and taken away to waste, complying with requirements of not introducing any chemicals, such as the reference electrode filling solution, into a food system. Figure 10 is a calibration curve for the industrial fluoride electrode system. Here may be seen the maintenance of optimum control at zero mV, which corresponds to the desired public health standard of 1.0 + 0.05 mg/L for fluoridated water and also shows the sensitivity to the low levels of fluoride (ca. 0.1 mg/L) in ground waters in New England. Users of pH and other ISE measurements may be familiar with frequently encountered problems of drift and less than optimal accuracy. Most of these problems may be attributed to the reference electrode rather than the measuring electrode, and in particular to the liquid junction potential existing at the tip of the reference electrode. Figure 11 shows a typical process reference electrode, which contains a nearly saturated potassium chloride solution that is also saturated with silver chloride. A ceramic junction, or other porous tip, makes contact with the solution under test. This tip may absorb some of the test solution—or if the reference electrode is not filled sufficiently, may even permit the test solution to back-diffuse into the reference electrode, thereby con-


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Figure 10. Fluoride electrode calibration.

Figure 12. Continuous flowing industrial reference electrode.

Figure 11. Conventional industrial reference electrode.

taminating the fill solution and defeating the optimum junction potential offered by concentrated potassium chloride solutions. Some reference electrodes have designs to mitigate this error. In one, called double junction construction, two compartments are maintained inside the reference electrode. Another design conveniently permits manual flushing after each measurement. A special design for process reference electrode is shown in Figure 12. Here the tip is relatively fast-flowing (2–5 mL/ day), and has a large reservoir (0.5–1 L) hoisted 2–3 m high and holding several months’ capacity, thus furnishing the low maintenance requirement of process measurements. For the case of pressurized process streams, a device called a DP (differential pressure) cell is used. It senses the process pressure and always maintain the reference electrode at a higher pressure, ca. 3 psi. A typical process measurement system using ISEs might look like the industrial installation photograph of Figure 13. Flow-through chambers, reference elec-

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trodes, ultrasonic and mechanical cleaners for dirty process solutions, differential cells, process titrators, reagent addition systems, differential cells, and gas analyzers may be incorporated and integrated with computers, monitors, and process control systems. The problems of temperature compensation and computer corrections for interferences and impurities have been addressed. The housings are heavy duty and sealed against dirt, fumes, and moisture. Other installations might look like the sodium analyzer of Figure 14. Somewhat different in physical appearance, this sodium ion analyzer might be used on an ultrapure water system for washing semiconductors during their manufacturing. Here, levels of sodium in the parts-per-trillion range are detrimental. Note the five-gallon reservoir to hold one month’s supply for reagent addition. This analyzer employs an ingenious system for reagent addition. The sodium ion electrode has interference from other univalent cations, and hydrogen ion of course may be the predominant one in solutions. The hydrogen ion interference may be overcome by making the solution alkaline, but the neutralizing agent should not contain even traces of other univalent cations. Figure 15 shows an ingenious reagentaddition system in which the neutralizing solution, such as ethylamine, is diffused into the sample stream through silicon rubber tubing, which permits the amine to pass as a neutral species but blocks any ions (9). Ion Selective Field Effect Transistor (ISFET) One of the newer ISEs suggested for process work is the ion-selective field effect transistor (ISFET). Sometimes referred to as a solid-state device, the ISFET has been known and studied for many years (10–12) The ISFET for pH has recently become commercially available in both laboratory and on-line versions (13–17). The features advertised (in addition to being ‘revolutionary’) include unbreakable (no glass), presoaking not required,



Figure 13. Installed ISE process analyzer.

stable readings in a few seconds, small sample volume, and handles dirty samples. pH glass specifications such as slope and sodium ion error are not offered. The membranes are said to be low impedance compared to the glass electrode, but a special measuring circuit is required. This circuit is not compatible with other modern potentiometry analytical instrumentation. The ISFET is shown in Figure 16. It is a threeelectrode semiconductor device with a source, gate, and drain. The gate is a surface-sensitive insulating material, which develops an electrochemical potential that is measured in comparison with a built-in conventional reference electrode. This gate, which for pH measurements may be made of materials such as silicon oxides or nitrides or aluminum oxides or nitrides, is comparable to the membrane of an ion-selective electrode. The gate modulates the electrical signal that exists between the source and drain. The potential between the reference electrode and gate is a Nernstian function of the ionic activity of the solution. This makes the ISFET interpretable in a similar manner to the potentiometric signal obtained from pH or other ion-selective electrode analysis performed with classical potentiometry. I became interested in the ISFET about ten years ago (Light, T.; Fung, C., unpublished results, The Foxboro Co., 1988) and obtained some pH ISFETs that were then commercially available; some others were constructed in our laboratory. By this time, the myth that a reference electrode could be abolished had been put to rest. We were curious to understand what the pH ISFET system offered that was equivalent to or better than the classical glass electrode pH meter that shone as an example of a virtually specific analytical instrument for a moderate cost. At that time, we had examined two commercially available ISFET pH electrodes as well as some from our own microfabrication laboratory. This study revealed major problems with the packaging of this moisturesensitive electronic device. No ISFET survived for more

Figure 14. Sodium analyzer.

than a week when used to measure alkaline solutions. Secondary effects, such as the sodium ion error or susceptibility to oxidation–reduction solution interference (which had long been documented with glass electrodes) (18) had never been reported. For the commercially available ones, there were no data given on the Nernst response. For the present paper, I examined two modern ISFET pH electrode systems recently introduced to the market—one for laboratory applications from Orion Research (Model 620/6165), and the other for process measurements from Leeds & Northrup (Model 079290/ 079230). These new ISFET electrode systems performed much better than those we had examined ten years ago. pH response and sodium ion error were measured quantitatively by a critical electrode evaluation method (19). This method, which employs a constant ionic medium of BaCl2 and produces an acid–base swing of approximately 700 millivolts, results in great accuracy in measurement of the slope response of the pH electrode. As shown in Table 2, The Orion electrode response was slightly greater than the Nernst theoretical value of 100.0% and the Leeds & Northrup was slightly less. While not a large deviation from the Nernst theoretical


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value, this might indicate that neither electrode is really a glass membrane or it might also indicate liquid junction effects at the reference electrode. More about the reference electrode is discussed below. Another parameter of interest when measuring pH in alkaline solutions is the so-called “sodium ion error”, which has never been reported or discussed with FET electrodes. Again, the Orion electrode is slightly over and the L&N slightly under the theoretical 100% mark (which corresponds to the theoretical Nernst response of 59.16 mV/ decade at 25 °C). These results are good, but not quite as good as the best glass electrodes, as discussed by Bates (18). Another test that was run on these FET electrodes was the sensitivity to oxidation–reduction reactions, a parameter which also has not been reported for FETs but which we found was a problem ten years ago. Like glass electrodes, the FETs examined in this paper were not found to be affected by the presence of oxidation reagents such as chromate or ferric salts. The FETs survived six months of testing, including alkaline solutions at varying temperatures, with no change in characteristics. The reference electrodes were interesting in the two ISFET electrodes examined. The tip of the reference electrode is where the solution under test is contacted and where arises the well-known liquid-junction potential (18) whose difficulties were discussed above. This liquidjunction potential accounts for most of the complaints concerning drifting and inaccuracy in pH electrode measurements. The tip of the Orion reference electrode was completely renewable by flushing with a simple thumb motion on the top of the electrode. The result was a very stable pH measurement and no drifting, even when making the transition from pH 1 to pH 13 solutions. However, this requires manual attention, a feature not appreciated in process electrode measurements. The L&N electrode system had a well built reference electrode with a large-area ceramic tip, but required more time to come to equilibrium and may have accounted for the slightly less than 100% acid–base slope reported in Table 2. This process reference electrode could be rebuilt, but would require “down time”. Note elsewhere in this paper, a design for a continuously renewable process reference electrode requiring attention only at 6-month intervals. Another new development that has been utilized industrially and whose progress is interesting to follow is called flow injection analysis (FIA); a still later development is called sequential injection analysis (SIA). Gary Christian and the Process Analysis Center at the University of Washington have been publishing work in this field (20). An excellent review of many aspects of industrial applications for ion-selective electrodes has been written by Bailey (21).

Figure 15. Reagent diffusion system.

Figure 16. FET ion selective electrode circuit for pH .

Conclusion Table 2. Response Characteristics of FET (Field Effect Transistor Electrodes)a Acid–Base Slope Na+Slope Electrode Ox. Response (%) (%) Orion pHuture Model 620/6165 Leeds & Northrup Durafet Model 079290/079230

102.7

101.7

NIL

98.0

98.4

NIL

a Test Method: BaCl 2 Constant Ionic Media; Light, T.; Fletcher, K. Anal. Chem. 1967, 39, 70–75.

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In this Waters Symposium for recognizing Developments of Analytical Instrumentation, I have shown the interdependence between researchers and management in bringing instrumentation out of the idea stage and onto the floor of the Pittsburgh Conference. We are indebted to Jim Waters and his program committee for his sponsorship of this symposium. This presentation has had the opportunity to examine the difference between the world of laboratory and process ISEs and touch on some recent developments in the field showing that researchers seem to be always able to come up with new ideas to find their testing ground in the market place.



Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Liberti, A.; Light, T. S. J. Chem. Educ. 1962, 39, 236–239. Swartz, J. L. Ph.D. thesis, Northeastern University, 1974. Light, T. S.; Fletcher, K. S. Anal. Chim. Acta 1985, 175, 117–126. Negus, L. E.; Light, T. S. Instrument. Technol. 1972, 19, 23–36. Light, T. S.; Mannion, R. F. Talanta 1969, 16, 1441–1444. Light, T. S.; Cappuccino, C. C. J. Chem. Educ. 1975, 52, 247–250. Queeney, K. M.; Downey, J. E. Adv. Instrument. 1986, 4(Part 1), 339–352. Light, T. S. In Ion-Selective Electrodes; Durst, R., Ed.; NBS Special Publication 314, U.S. Government Printing Office: Washington, DC, 1969; Chapter 10. Frant, M. S.; Baer, C. S. ULTRAPUREWATER, 1992, July/August, 21–26. Bergveld, P. IEEE Trans. Biomed. Eng. 1970, 17, 70 (197); 1972, 19, 342 . Janata, J.; Huber, R. H. Ion-Selective Electrode Rev. 1979, 1, 31–78. Moir, S. Sensors 1988, July, 39–44. Bergveld, P.; Sibbald, A. Analytical and Biomedical Applications of Ion-Selective-Field Effect Transistors; Elsevier: New York,1988; Vol. 23. Gennet, T.; Purdy, W. C. Am. Lab. 1991, 23(2), 60–64; 23(4), 60–66. Connery, J. G.; Baxter, R. D.; Gulczynski, C. W. Pittsburgh Conference on Analytical Instrumentation, paper no. 561; New Orleans, March 1992. Gray, D. M. On-line High Purity pH Measurement; Electric Utility Chemistry Workshop, Champaign, IL, March 1994. Frant, M.; West, S.; Turpin, R.; Ewing, C. 45th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy; Chicago, March 1994. Bates, R. G. Determination of pH, 2nd ed.; Wiley: New York, 1973. Light, T.; Fletcher, K. Anal. Chem. 1967, 39, 70–75. Christian, G. D. Analyst, 1994, 119, 2309–2314. Bailey, P. L. Ion-Selective Electrode Rev. 1979, 1, 81–136.


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