Electrodeless Conductivity

Chapter 29

Electrodeless

Conductivity

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Truman S. Light1 The Foxboro Company, Corporate Research Center (N01-2A), Foxboro, M A 02035 The history of electrodeless conductivity is discussed from its practical beginning in 1947 to the present. This technique for measuring the concentration of electrolytes in solution utilizes a probe consisting of two toroids in close proximity which are immersed in the solution. The toroids may also be mounted externally on insulated pipes carrying the solution. One toroid generates an alternating electric field in the audio frequency range and the other acts as a receiver to pick up the current from the ions moving in a conducting loop of solution. Fouling coatings, suspensions, precipitates or oil have little or no effect. Applications are reviewed for continuous measurements include the mining, pulp and paper, and heavy chemical industries. The measurement of electrical conductivity of ions in solution has long provided useful quantitative chemical composition information. As a tool for physical chemical studies, it is used for determining the degree of dissociation of weak electrolytes and their ionization constants. Similarly, it is used for the study of precipitation and complex formation reactions and for determination of solubility product and formation constants. Analytical chemistry applications include direct quantitative analysis of strong and weak acids, bases and salt solutions, aqueous and nonaqueous conductometric titrations, and a variety of continuous monitoring determinations such as oceanographic salinity, aluminum and pulp industry processing liquors, pickling, plating, anodizing and degreasing baths and chromatographic detectors (1-3). The classical technique measures the electrical resistance (or its reciprocal, the conductance) between two inert conducting electrodes contacting the solution. Low frequency alternating current, 10 to 50,000 hertz, is usually employed to minimize

1Current address: 4 Webster Road, Lexington, MA 02035 0097-6156/89/0390-0429$06.00/0 © 1989 American Chemical Society

Stock and Orna; Electrochemistry, Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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ELECTROCHEMISTRY, PAST A N D PRESENT

electrolytic reactions and polarization at the electrode/solution interface. Physical chemical studies of conductivity and its theory and practice are discussed in many standard reference works (1-9) . The chief difficulty with this measurement is associated with electrodes contacting the solution. In addition to the more subtle errors of electrode/solution capacitance and polarization, coatings are an obvious and serious problem. Insulating or diffusion hindering layers may be formed from oils, bodily fluids, metal precipitates and waste streams. Coating is especially prevalent in alkaline solutions where heavy metal anions such as phosphates, carbonates, hydroxides and sulfates, are precipitated, and in suspensions such as latex and the liquors of the pulp and paper industry. There are two frequency domains in which conductance measurement have been made without electrodes in contact with the solution. The first is a high-frequency method in the megahertz region. The electrodes take the form of a pair of metal sheets or bands on the outside of the sample cell, which is made of an insulating material such as glass. Alternatively, the glass encased sample may be placed inside an induction coil which is part of the circuit. The glass plays the part of a dielectric in a capacitor. The impedance of a capacitor is so low at high frequency that the alternating current passes freely into the sample and the impedance of the solution becomes a complex function of the resistance of the solution, its dielectric constant and the capacitance of the circuit. High-frequency conductometry, also called oscillometry, has been treated extensively by Blake (10), Sherrick et al (11) and Pungor (12) and appears to be seldom used now. The most recent paper known to the author was concerned with the design of a high-frequency oscillometer and appeared in 1981 (13). Oscillometry will not be discussed further in this paper. A second method of measuring the conductance without the use of contacting electrodes has become popular, especially in the chemical process industries. Usually referred to simply as "electrodeless conductivity", it has also been called "inductive" or "magnetic" conductivity. This method is the subject of this paper and is described below. Although instruments for electrodeless conductivity measurement have been commercially available since the 1950's for process industry applications, literature review of this subject is lacking. This paper will review and discuss the history and applications of electrodeless conductivity. Instrumentation The electrodeless conductivity measuring system utilizes a probe consisting of two encapsulated toroids in close proximity to each other, as shown in Figure 1. One toroid generates an electric field in the solution, while the other acts as a receiver to pick up the small alternating current induced in the electrolytic solution as illustrated in Figure 2. The equivalent electrical circuit may be compared to a transformer with the toroids forming the primary and secondary windings and the core replaced by a coupling loop which is the conducting solution.



29.

LIGHT

Electrodeless Conductivity

Figure 1.

Figure 2.

Principle of the electrodeless conductivity cell and instrument.

Simple representation of an electrodeless conductivity measuring circuit.


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Several configurations for the sensing cells are possible. Cells designed for immersion and available in several sizes are shown in Figure 3. The toroids are covered with a chemically resistant fluorocarbon or other high-temperature resistant non-conducting thermoplastic material. Any precipitates or coatings adhering to this probe have little or no effect on the measured conductance as long as they do not displace a significant fraction of the solution. The generating toroid is energized from a stable audio-frequency source, typically at 20 kHz. The pick-up toroid is connected to a receiver which measures the current through this secondary winding. The current is then amplified and displayed on a meter or an analog strip-chart recorder. It may also be used to control a valve or an alarm. The output is a direct function of the conductance of the solution in the loop, in a manner completely analogous to the traditional measurement with contacting electrodes. Range and Temperature Effects. The useful range of commercially available instruments extends from 0 to 100 μs/cm to 0 to 2 S/cm, with relative accuracy of a few tenths of one percent of full-scale. (The Siemens, S, is the SI unit for conductance and is identical with the "mho" or the reciprocal ohm.) Conductance is temperature dependent and a temperature sensor is incorporated in the toroid probe with a compensation circuit which corrects to the standard reference temperature of 25°C. Many salts have conductivity temperature coefficients of the order of 2 percent per degree Celsius. This temperature coefficient is non-linear and in some cases may vary from 2 to 7 percent per degree Celsius over a 100 degree range (14). Using a microprocessor-based electrodeless conductivity instrument, compensation for this non-linearity may be provided (15). Limitations. The electrodeless conductivity technique using low-frequency inductive cells is available for analysis and control in the chemical process industries and in other continuous monitoring applications. Although its stability, freedom from maintenance, and accuracy, are superior to contacting conductivity techniques, lack of bench models of this type has hindered its laboratory use and application to date. One of the reasons that electrodeless conductivity is not favored as a laboratory tool is due to the size of the probes and the sample size requirement. The smallest electrodeless probe is about 3.6 cm in diameter and has an equivalent cell constant of 2.5 cm" 1 . It requires a minimum solution volume of several hundred milliliters to ensure a complete solution loop without wall effects which distort the apparent cell constant. For a large probe of 8.9 cm diameter, the cell constant is 0.45 cm - 1 and an effective solution volume of several liters may be needed. For the lower conductance ranges, which require a smaller cell constant, the diameter of the probe and measuring container must increase. Accurate measurement below approximately 10 μS/cm is not practical. History Patent Literature. In 1947, Matthew Relis at the Massachusetts Institute of Technology wrote a thesis titled "An Electrodeless Method for Measuring the Low-Frequency Conductivity of Electrolytes" (16). He had begun his work at the Naval Ordnance Laboratory,


Figure 3.

Various types of immersion cells for electrodeless conductivity with an associated instrument (Courtesy of The Foxboro Co.)


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Washington, D.C. and subsequently continued it at the Woods Hole Oceanographic Institution in Massachusetts where an instrument based on his work was used as an in situ salinometer. He described an electrodeless conductivity instrument which was based on the immersion of coupled generating and pickup toroids in an electrolyte solution and used audio frequency alternating current. Figure 4, illustrating this principle, is taken from the patent titled "Method and Apparatus for Measuring the Electrical Conductivity of a Solution" that was subsequently granted to Relis in 1951 (17) . The patent was licensed to Industrial Instruments, Inc. which was acquired by Beckman Instruments in 1965. Relis described the principle and an apparatus that measured conductivity with an accuracy of better than 1% over the conductance range of 10 micromhos to 10 mhos (10 microsiemens to 10 Siemens). Within experimental error, the electrodeless method yields data identical to that obtained via the conventional contacting method of measuring conductivity but without the interference caused by minor deposits on or polarization of the contacting electrodes. Relis reported that the only earlier description of the principle of this method was given in an account of experiments conducted in 1920, "Demonstration of Induction Currents Produced in Electrolytes without Electrodes", at the Federal Polytechnical School of Switzerland by Piccard and Frivold (18) . However, a patent was issued to Ruben in 1927 titled "Electrochemical Sensing Device" based on similar principles (19) . Stock (20) , in discussing "Two Centuries of Quantitative Electrolytic Conductivity, 1776-1879-1984", has noted that electrodeless conductivity measurements were made much earlier by inducing current in solution from adjacent magnets. These measurements were reported in 1860 by Beetz (21) and in 1880 by Guthrie and Boys (22). The latter workers suspended a solution in a vessel from a torsion wire in the field of a strong magnet and measured the force required to overcome the rotation of the vessel. Following the Relis patent, several workers and instrument companies patented improvements on electrodeless conductivity equipment (23-32). In 1955, Fielden received a patent which was assigned to the Robertshaw-Fulton Controls Company (23) . He described a method for mounting the toroids on a nonconducting pipe external to the electrolyte solution to be tested. This method, illustrated in Figure 5 by the Sperry patent (24), required that the solution be in a loop so that complete electrical contact is maintained between the primary and secondary toroids. Although seemingly an attractive way to measure process solutions, this method has not been extensively used because it created the equivalent of very large cell constants due to the large solution loop path and because there was the possibility of loss of some of the generated current in the pipelines and ground paths of the process streams. This latter problem was addressed in 1968 by patents issued to Sperry (24), Kidder (25) and Rosenthal (26) and assigned to Beckman Instruments. Gross (27-28) in patents assigned to Balsbaugh Laboratories and The Foxboro Company eliminated anomalies in the permeance of the cores of the toroids. Then the only coupling is effected by the fluid loop and results in greater sensitivity and improved linearity. A design for a high temperature and pressure electrodeless conductivity probe is described and


Figure 4.

Relis patent showing first illustration of modern principle of electrodeless conductivity (17).


29. LIGHT Electrodeless Conductivity


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ELECTROCHEMISTRY, PAST AND PRESENT

Figure 5.

Sperry patent illustrating electrodeless conductivity cells externally mounted to the solution being tested (24).



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437

patented by Koski (29-30). The employment of a pair of electrodeless conductivity probes for differential measurement as part of a continuous process feed forward control technique for washing a slurry stream is described in a patent by Rosenberger (31) assigned to Nalco Chemical Co. A German patent issued to Diebel (32) also discusses an electrodeless conductivity apparatus for determination of electrolyte concentration. Instrumentation Literature. Starting in 1956, papers describing various aspects of electrodeless conductivity instrumentation have appeared. Gupta and Hills (33) described a transformer bridge circuit which was also discussed by Calvert et al (34) and Griffiths (35). Other papers modifying and discussing the instrumentation aspects have appeared (36-48) . Johnson and Hart have described a simplified analyzer (41). Another analyzer capable of measuring dilute solutions of 10 μS/cm with a precision of ±2% has been described (42). The subject of electrodeless conductivity was reviewed in 1969 and 1971 (49-50), and discussed briefly in a few books (1-3, 51) . Modern instrumentation using a microprocessor-based electrochemical monitor which provides temperature compensation, curve characterization and calibration, flexible ranging, output expansion and damping and self-diagnostic capability has been described by Queeney and Downey (15). Application Literature. An excellent dissertation by Martin , "Electrodeless Conductance Measurements Using Toroidal Inductors", (50) reviewed the theory and applications of electrodeless conductivity to 1971. This thesis covers variables affecting the inductive response mechanism, useful response of the electrodeless conductivity system to the micromolar level, and extension to situations where small conductance changes are determined in the presence of large amounts of foreign electrolytes. Conductometric titrations of a redox reaction with small conductivity change in the presence of a high conductivity acid background were demonstrated. Conductometric titrations in the low microsiemens/cm region were also shown. The earliest application of electrodeless conductivity measurements appears to have been for measurement of salinity at various ocean depths (16, 17, 45, 52) . Other early uses have also included determination of the equivalent conductances of salts at high concentrations (33, 34, 46, 53) and the monitoring of nitric acid concentration in radioactive waste (41) . Attempts to extend response to solutions of low conductance seem to have met with doubtful response (47-48). As commercial availability of electrodeless conductivity equipment increased, this technique became accepted for the hostile environments of the chemical process industries. Corrosive, elevated temperature, particle and oil or grease laden solutions with suspended solids and fibers, did not interfere with the measurement as they might with contacting conductivity. Applications in the mining and metallurgical extraction industries were reported starting in 1960 (37,54,55) and in control of lime slurry and alkaline processes in the pulp and paper industry at about the same time (44,56-62). Sulfuric acid and oleum have had special attention (63, 64) and a measurement system for oleum described by Shaw and Light is shown in Figure 6 (64).



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ELECTROCHEMISTRY, PAST AND PRESENT

Figure 6.

Continuous analyzer, using electrodeless conductivity, for oleum over the range 100 to 102 + 0 . 0 2 equivalent percent H2SO4 (Reproduced with permission from Reference 64. Copyright 1982 ISA Transactions.)


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439

Conclusion


Electrodeless conductivity is a technique for measuring the conductance of a solution using the electrical inductance principle at low frequencies. This method does not use contacting electrodes thereby eliminating maintenance and other errors due to surface effects created by coatings and fouling. The measurement enjoys widespread acceptance in the chemical process industries. Its history and a review of the literature have been presented here. Literature Cited 1.

2.

3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

19. 20. 21. 22. 23. 24.

Loveland, J.W. Conductometrv and Oscillometry in Treatise on Analytical Chemistry; Kolthoff, I.M.; Elving, P.J.; Eds.; Wiley Interscience: New York, 1963; Part I, Vol. 4, Chap. 51. Loveland, J.W. Conductometrv and Oscillometry in Instrumental Analysis, 2nd ed.; Christian, G.D.; O'Reilley, J.E.; Eds.; Allyn and Bacon: Boston, 1986; Chap. 5. Light, T.S.; Ewing, G.W. The Measurement of Electrolytic Conductance in Handbook of Analytical Instrumentation; Ewing, Ed.; Marcel Dekker: New York, (in press). Jones, G.; Bollinger, D.M. J. Amer. Chem. Soc. 1935, 57., 280. Glasstone, S. Introduction to Electrochemistry; Van Nostrand: New York, 1942. Lingane, J.J. Electroanalvtical Chemistry 2nd ed.; Interscience: New York, 1958; Chap. IX. Harned. H.S.; Owen, B.B. The Physical Chemistry of Electrolytic Solutions, 3rd ed.; Reinhold: New York, 1958. Fuoss, R.M.; Accascina, F. Electrolytic Conductance; Interscience: New York, 1959. Robinson, R.A.; Stokes, R.H. Electrolyte Solutions; Academic Press: New York, 1959. Blake, G.G.; Conductimetric Analysis at Radio-Frequency; Chemical Publishing Co.: New York, 1952. Sherrick, P.H.; Dawe, G.A.; Karr, R.; Ewen, E.F. Manual of Chemical Oscillometry; E.H. Sargent: Chicago, 1954. Pungor, E. Oscillometry and Conductometry; Pergamon Press: Oxford, 1965. Sher, A.; Yarnitzky, C. Anal. Chem. 1981, 53, 356-358. Light, T.S.; Licht, S.L. Anal. Chem. 1987, 59, 2327-2330. Queeney, K.M.; Downey, J.E. Advances in Instrumentation, 1986, Vol. 41 Part 1, pp 339-352. Relis, M. M.S. Thesis, Mass. Institute of Technology, Cambridge, Mass., 1947. Relis, M. U.S. Patent 2 542 057, 1951. Piccard, A.; Frivold A. (Swiss) Archives des Sciences Physiques et Naturelles, 1920, Ser. 5, Vol. 2, pp 264-265, May-June. Ruben, S. U.S. Patent 1 610 971 1926. Stock, J.T. Anal. Chem., 1984, 51, 561A-570A. Kohlrausch, F.; Holborn, L., Das Leitvermoqen der Elektrolyte; Teubner: Leipzig, 1898, p 5. Guthrie, F.; Boys, C.V. Phil. Mag., 1880, 10, 328. Fielden, J.E., U.S. Patent 2 709 785, 1955. Sperry, E.A. U.S. Patent 3 396 331, 1968.


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25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

45. 46. 47. 48. 49. 50. 51.

52. 53. 54. 55.

56.


Kidder, R.J. U.S. Patent 3 404 335, 1968. Rosenthal, R. U.S. Patent 3 404 336, 1968. Gross, T.A.O., U.S. Patent 3 806 798, 1974. Gross, T.A.O., U.S. Patent 4 220 920, 1980. Koski, O.H. U.S. Patent 3 867 688, 1975. Koski, O.H.; Danielson, M.J., Rev. Sci. Instrum. 1979, 50, 1433-36. Rosenberger, R.R., U.S. Patent 4 096 028, 1978. Diebel, H., German Patent 1 598 075, 1974. Gupta, S.R.; Hills, G.J. J. Sci. Instrum. 1956, 13, 313-314. Calvert, R.; Cornelius, J.A.; Griffiths, V.S.; Stock, D.I. J. Phvs. Chem., 1958, 62., 47-53. Griffiths, V.S.; (a) Anal. Chim. Acta 1958, 18, 174; (b) Talanta 1959, 2, 230. Salamon, M. Chem. Techn. (Berlin) 10. Jg. Heft, 1958, 207-210. Eicholz, G.G.; Bettens, A.H. Can. Min. Metall. Bull. 1960, 53, 901-907. Rosenthal, R.; Kidder, R.J. Industrial and Engineering Chem. June, 1961, 53, 55A. Fatt, I. Rev. Sci. Instrum. 1962, 33, 493-494. Williams, R.A.; Gold, E.M.; Naiditch, S. Rev. Sci. Instrum. 1965, 36, 1121-1124. Johnson, C.M.; Hart, G.E. Anal. Instrum. 1967, 4, 23-30. Hackl, A.E.; Deisinger, H. Allgemeine und Praktische Chemie 1968, 19, 229. Gross, T.A.O.; Sawyer, P.B. Measurements & Data Nov.-Dec., 1975, 102. (a) Timm, A.R.; Liebenber, E.M.; Ormrod, G.T.W.; Lombaard, S.L. An Electrodeless Conductivity Meter of Improved Sensitivity and Reliability, report no. 2003, Nat. Inst. Metallurgy, Johannesburg, Feb. 28, 1979; (b) Ormrod, G.T.W., Electrodeless Conductivity Meters in the Measurement and Control of the Amount of Lime in Alkaline Slurries, NIM-SAIMC Symposium on Metallurgical Process Instrumentation, Nat. Inst. Metallurgy, Johannesburg, 1978 Brown, N.L.; Hamon, B.V. Deep-Sea Research 1961, 8, 65. Lavagnino, B.; Alby, B. Ann. Chim. 1959, 49, 1272. Lopatnikov, V.I. Soviet Physics (Eng. trans.) 1961, 6, 505. De Rossi, M. Sci. Tec. 1962, 6, 31; Chem. Abstr. 1964, 61, 3754. Pazsitka, L.; Z. Anal. Chem. 1969, 215, 103. Martin, R.A. Ph.D. Thesis, Univ. of Pittsburgh, 1971. Smith, D.E.; Zimmerli, F.H. Electrochemical Methods of Process Analysis; Instrument Society of America: Research Triangle Park, NC, 1972, pp 138-141. Park, K. Anal. Chem. 1963, 35, 1549. Davis, R.L.; Le Master, E.W. Texas Journal of Science, 1972, XXIII, No. 4, 497-501. Kelly, F.J.; Stevens, C.S. Canadian Mining Journal, Jan. 1964, pp 42-43. G.D. Fulford, Use of Conductivity Technigues to Follow Al2O3 Extraction at Short Digestion Times, in Light Metals; Bohner, H.O., ed.; The Metallurgical Society of AIME: 1985; pp 265-278. Gow W.A.; McCreedy H.H.; Kelly F.J. Canadian Mining and Metallurgical Bulletin, July, 1965.


29. LIGHT 57. 58.


59.

60.

61.

62.

63.

64.


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Moon, A.G.; Vaughn, R.L. Soc. Mining Engineers of AIME, Transactions, 1978, 264, 1727-1730. Farrar, D.; Khandelwal, B. Effective Alkali Measurement Improves Continuous Digester Performance, presented at PAPPI (Pulp and Paper Institute) Conference, Vancouver, Spring, 1983. The Foxboro Co., Foxboro, Mass. 02035, Product Application Data, (a) PAD B2000-001 Electrodeless Conductivity Systems for Clean-In-Place (CIP) Caustic Dilution; b) PAD B2000-007 Electrodeless Conductivity Systems for Interface Detection In Clean-In-Place (CIP) Systems; c) PAD B2000-008 Electrodeless Conductivity Systems for Acid Concentration Control in CIP Systems; d) PAD G2600-006 Reclamation of Black Liquor Spill in the Pulp and Paper Industry; e) AID G2600-007 Digester Alkali Concentration Control; f) PAD G2600-014 Continuous and Batch Kraft Digesters; g) PAD J2200-009 Caustic Concentration Control in Caustic Saturator; h) PAD K2200-011 Controlling the Neutralization of Caustic in Textile Mercerization; i) PAD Q9900-014 Oleum Strength Analyzer System; j) PAD B2030-001 Caustic Control for Vegetable Peeler. Musow, W. On-Line Causticity Sensor and Programmable Monitor Applied to Slaker Lime Addition Control, Canadian Pulp and Paper Assoc., Montreal, 1986. Musow, W.; Bolland, A. Toroidal Conductivity Sensor Technology Applied to Cvanidation of Flotation Tailings Circuits, Instrument Soc. of America 12th Annual Mining and Metallurgy Industries Symposium, Vancouver, 1984. Musow, W.;Montgomery, J. Recausticizing Control Utilizing Toroidal Magnetic Sensor Technology and Controller with Artificial Intelligence Atlanta, Ga., Pulping Conference/TAPPI, Sept., 1985. del Valle, J.L. Measurement of the Concentration of H2SO4 Using an Electrodeless Conductivity Method, Technisches Messen Atm. 1977, Vol. 7/8, 263-265. Shaw, R.; Light, T.S. ISA Transactions 1982, 2_1, 63-70.

RECEIVED August 9, 1988


Electrodeless Conductivity

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