John T. Stock Department of Chemistry University of Connecticut Storrs. Conn. 06268
anturies of Qyantitative Electroc Conductivity -0
I n 1776, Henry Cavendish (17311810) described to the Royal Society his attempts to imitate the effects of the torpedo, a fish that can deliver an electric shock ( 1 ) . He stated: “It appears from some experiments, of which I propose shortly to lay an account before this Society, that iron wire conducts about 400 million times better than rain or distilled water. . . . Sea-water, or a solution of one part of salt in :30 i i f water, conducts 100 times, and a saturated solution of sea salt. about 720 times better than rain water.” Allowing for temperature differences, Cavendish’s conductivity ratios are surprisingly close to modern values. Apparently, no one questioned Cavendish as to how he obtained these results, and he never published the promised account. Only static or Srictiirnal electricity was available to him. Fortunat~ly,his personal records were preserved and were examined and edited by Maxwell in 1x79 (2). From these records, it appears that ( h e n dish used a method shown in principle in Figure 1. The solutions to he compared were placed in long glass tubes A and B, which had been calibrated by means OS mercury. The effective lengths of 0003-2700/84/0351-561A$01.50/0 0 1984 American Chemical Society
the liquid paths could be altered by sliding wire electrodes X and Y in the corks that closed the horizontal portions of the tubes. Corks in the bent ends of the tubes carried fixed electrodes. These were connected to each other and also to the outside coatings of six Leyden jars of equal size. By temporarily joining together the knobs connected to the inner coatings of the jars, these were all charged to equal intensities. Cavendish then touched electrode Y with one hand and the
knob uf jar 1 with the other. He did the same with electrode X and jar 2 and then alternated between Y and X until all six jars had been discharged. In this way, he took six shocks, alternately through the first and through the second tube. If the shocks ohtained through tube A were greater than through tube B, the effective liquid column in A had the greater conductivity. He then adjusted X to make the shocks, and hence the conductivities, more nearly equal and repeated
ANALYTICAL CHEMISTRY, VOL. 56. NO. 4, APRIL 1984
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Figure 1. Cavendish’s meulod for comparison of electrolytic conductivities the entire experiment. Further adjustments led to two close settings; a t one, tube A gave a slightly greater shock than tube B, while the reverse was true a t the other setting. The adjustment needed to equalize the shocks was then estimated. Twenty years were to elapse hetween Cavendish‘s measurements and the invention of the first electric battery (the “Volta pile” of 1796). Oersted’s observations of 1819-20 led to the development of galvanometry and hence to precise and convenient methods of electric measurement (3). Early attempts to measure electrolytic conductivity (or its reciprocal, resistivity) were made by direct current (dc) methods that are satisfactory for measuring the conductivities of metals. A major problem with solutions is that polarization effects a t the electrodesolution interfaces can cause massive errors. “Attackable” electrodes, such
Figure 2. Horsfords “trough” c o n o w tivily cell 562A
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as zinc for zinc sulfate solutions or copper for copper-containing solutions, were pressed into service in an attempt to minimize polarization effects. Obviously, such a palliative is of little use in systems in which gases are evolved a t one or both of the electrodes. Overviews of the early de methods ( 4 , 5 ) indicate that one of the more successful approaches was by an American, E. N. Horsford (6).His conductivity cell was a well-varnished rectangular wooden trough 30 X 7.5 X 7.5 cm. Two parallel wooden partitions were inserted, as shown in Figure 2. One partition could be moved, so that the separation of the plate electrodes, which entirely covered the inner faces of the partitions, could he adjusted. By filling the trough to various depths, a liquid column of known length and cross section could be defined. Horsford set the interelectrode distance a t 2.5 em, then connected the cell in series with a battery, a galvanometer, and a variable resistor. The latter was adjusted to a value R1, to yield a convenient galvanometer reading. Then the interelectrode distance was increased to d cm and the original galvanometer reading was regained by decreasing the value of the resistor to R2. Then (R1 - R2) represented the resistance of a column of solution of length (d - 2.5)cm and of cross section defined by the width of the trough and the depth of the liquid. The aim of this difference method was the cancellation of polarization effects. Like most other early workers, Horsford compared the resistance of the solution with that of the material of the wire from which the resistor was made. A notable advance occurred in 1869, when Kohlrausch (5)described the
ANALYTICAL CHEMISTRY. VOL. 56, NO. 4, APRIL 1984
application of alternating current (ac) to the measurement of electrolytic conductivity. Slight electrolysis that may occur in one half-cycle should reverse during the next half-cycle; ideally, polarization effects should he eliminated. For several decades, Kohlrausch and his eo-workers studied the theory and practice of electrolytic conductivity measurements, producing high-precision results (7).Kohlrausch used several ac sources; an approximately 1000-Hz signal provided by a buzzer or a small induction coil became the most popular. This enabled a telephone receiver to act as halance detector in a modified Wheatstone bridge arrangement, which was adjusted to give zero or minimum sound. Although this ac source was very simple, the output was highly unsymmetrical and far from the desirable sine-wave form. In 1913 the now classic paper by Washhurn and Bell on high-precision measurements appeared (8).These workers used a 1000-Hzalternator, tuned the telephone detector to this frequency, used the equivalent of a very long bridge slidewire, and worked a t carefully stabilized temperatures. The fact that impedance, rather than mere resistance, governs the balance of an ac bridge was already well-known. The electrode-solution interfaces in an electrolytic cell act as capacitors, the effects of which can he balanced out by adjustment of a variable capacitor connected across the variable resistor. To minimize capacitance and inductance effects inherent in wire-wound resistors, Washhurn and Bell recommended platinum film on glass resistors. The important theoretical aspects of electrolytic conductivity and the subsequent refinements of technique that led to higher precision and greater ease of measurement are described in several monographs (9-11 ). It is perhaps sufficient to mention two events that occurred around 1920. One was the study by Kraus and Parker (12, 23) of the design and calibration of conductivity cells. The other was the introduction of the vacuum tube oscillator as a source of symmetrical ac (14).
The specific conductivity of an electrolyte solution depends both on the solute and the solvent. Although nonaqueous systems have been studied extensively ( 9 , 1 5 ) ,water is of course the most important solvent. Methods of solvent purification have received much attention, so that the “solvent correction” needed for precise measurements on electrolyte solutions can he minimized. Despite exhaustive purification, the specific conductivity does not become zero, hut tends to a lower limit that may he very small-in
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limits of the precision of either method. In 1860, Beetz attempted to measure electrolytic conductivity without the use of electrodes (5). His idea was to induce a current in the solution by the motion of a nearby magnet. Some years later, Guthrie and Boys (21) used a powerful rotating magnet, in the field of which the vessel containing the solution was suspended hy a torsion wire. The conductivity was found by measurement of the force required to oppose the rotation of the vessel. These workers make it clear that the technique is not an easy one. Suppose that an approximately 0.001 M solution of HCI is progressively titrated with 0.1 M NaOH, so that essentially no change in volume occurs: HsO+ t CI- t Na+ t OH2H20 t C1- Na+ Highly conducting HsO+ is replaced by less highly conducting Na+ and the conductivity falls linearly, reaching a minimum when the solution has hecome one of NaCI. Continued titration then results in a rise in conductivity, largely because OH-, which is a very good conductor, is no longer being destroyed. The endpoint is obtained by the intersection of two straight lines that are drawn through a suitable number of points obtained by measurement of conductivity after each addition of titrant. The history, technique, and early applications of conductometric titration have been surveyed by Kolthoff (ZZ),who has made major contrihutions to the subject. He points out that the first analytical use of this form of titration was Whitney's 1895 determination of free sulfuric acid in complex chromium sulfate solutions. The major applications of conductometry are to acid-base titrimetry (9)and to certain precipitation and other ion combination titrations. Conductometric redox titrations are uncommon, principally because such titrations are usually made in acid or other strongly conducting media. This serves to emphasize that electrolytic conductivity is nonspecific; all ions present in a solution contribute t~ its conductivity. When applicable, conductometric titration is particularly valuable a t low titrand concentrations. Absolute conductivity measurements are not required, so that cell calibration is unnecessary. Routine titrimetry can often be carried out with line frequency ac (9).This can be regulated to compensate for line voltage fluctuations and to permit direct readout on a suitable meter (23). The advent of electronics permitted the construction of stable sources of high-frequency (HF) ac. At radio
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conductivit! the case of water, approximately 0.04 pS cm-' at 18 OC. This is due to the slight but definite self-ionization of water:
2n20 H ~ O +t onThe conductivity is greatly increased by even mere traces of dissolved electrolyte. This fact, noted by Cavendish over two centuries ago, is of great practical use in the measurement of low concentrations of electrolytes and for the assessment of the purity of solvents. Even small water stills or ion exchange water purifiers are usually equipped with a simple form of conductivity monitor. Nonelectrolyte solutes make no contribution to the conductivity, and the presence of such solutes cannot be monitored in this manner.
Despite the success of the audio-frequency ac approach, interest in dc methods did not vanish. Marie and Noyes ( I S ) successfully measured the conductivity of dilute solutions of acids by using two carefully adjusted hydrogen electrodes and an ordinary dc bridge. Another approach is to measure the potential drop E across a pair of identical auxiliary electrodes AA that define a fixed length of the column of solution through which a known current 1 is passing. This approach, first tried more than a century ago (5),was developed by several workers (17-20). The principle is shown in Figure 3. Gordon and Gunning (20)carefully examined the design of the four-electrode cell. In general, their de results and those obtained by precise ac techniques agree within the
iigure 4. Cell arrangements for HF conductivity (a) inductance type; (b) capacitance type
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ANALYTICAL CHEMISTRY. VOL. 56. NO. 4. APRIL 1984
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Figure 5. Ele’ system
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frequencies, an interesting possibility arises: Certain measurements can he made without placing electrodes in the solution (24).The principles, instrumentation, applications, and extensive literature of H F (sometimes termed oscillometric) methods have been reviewed by Pungor (25).In the inductive mode, the titration vessel is placed within a coil as indicated in Figure 4a. The cell and its contents form the dielectric when the titrimeter operates in the capacitive mode. One possible electrode arrangement, that of two parallel metal rings mounted on the outside of the titration vessel, is shown in Figure 4h. Nonlinear and sometimes inverted titration curves may he encountered in HF methods. The reasons have been discussed by Loveland ( I I ) , who points out that the necessity of having to find all of the proper conditions is a limitation of the technique. If these are found, then most conductometric titrations can also he performed by H F methods. The introduction of ac bridge methodology, development of high-precision techniques for fundamental studies, use of conductometric titration as an analytical tool and, in the 1950s, great activity in the field of H F titration are high points in the long history of quantitative electrolytic conductivity. Yet progress still continues; new analytical techniques have appeared. One of these is concerned with amounts and concentrations that may he very low. Another is applicable to high electrolyte concentrations. Modern liquid chromatography is well suited to the separation and determination of substances a t trace levels. Ion chromatography is a comparatively recent development in this area (26:27). Conventional ion exchange resins are usually of the high-capacity type, so that the displacement of the 568A
Figure 6. Control of temperature and of NaOH concentration in a fruit-peeling system TIC, temperature-indicating controller: CiC. conductivity-indicating comroiler; IIP. interface beween Conductivity monitor and CIC
ions of interest requires a quite concentrated electrolyte solution as eluent. Under these conditions, the small changes in conductivity as each displaced ion reaches the detector are swamped by the massive conductivity of the ions of the eluent. If the separating column is charged with an exchanger of low capacity, a less concentrated eluent can he used. Attached to the outlet of this column is a second column, the purpose of which is to suppress the conductivity otherwise exhibited by the eluent. For example, passage of Na2C03 eluent through a suppressor column filled with the hydrogen form of a suitable cation exchanger converts this salt into H2COs. This is such a weak electrolyte that it does not interfere with the conductometric detection of the anions of strong acids. Suitable choices of eluents and exchangers permit the rapid quantitation of quite complex lowconcentration mixtures of anions or cations. With carefully chosen, very dilute eluents, the separation and conductometric detection of various ions can he achieved without a suppressor column (28,29). A system that is now of considerable industrial importance hears a superficial resemblance to one of the systems used in H F titrimetry. Coupled coils are used in both cases, hut the coils of an industrial electrodeless conductivity system are toroidally wound and are operated at a frequency that is only about 20 kHz. The greatest practical difference is that the industrial system is especially suited to the measurement of high conductivities. The industrial elect,rodeless system was developed by Matthew Relis. He began his work a t the Naval Ordnance Lahoratory and described a practical in situ salinometer in his MS thesis (30).Relis continued to work on this instrument a t Woods Hole and was
ANALYTICAL CHEMISTRY, VOL. 56. NO. 4. APRIL 1984
granted a patent for it in 1951 (31). This patent recognizes that a dual toroidal coil system had been demonstrated as early as 1920 (32).Relis devised means for the elimination of capacitive coupling and stray field effects, so that accurate measurements could he made. He licensed his patent to Industrial Instruments, Inc.; this firm merged with Beckman Instruments, Inc. in 1965. Various modifications to Relis’s design have been made by others (33), but the basic principle remains as shown in Figure 5. An oscillator A energizes the primary toroid B, which along with secondary toroid C is immersed in the solution to he examined. The greater the conductivity of the solution, the greater the current induced in C. This current is handled hy handpass amplifier D, which drives meter or recorder E. In practice, B and C are arranged parallel to one another and mounted coaxially close together in a watertight epoxy or similar corrosionresistant housing. Dip-type sensors (34)and units for in-line mounting for continuous monitoring of conductivity in a flowing system (35) are commercially available. The ruggedness of electrodeless conductivity equipment enables measurements to he made on corrosive, abrasive-containing, or other hostile fluids. A list of typical applications includes seawater, raw sewage, cement slurries, hydrofluoric acid, and radioactive solutions (36). An application of an electrodeless conductivity monitor is in “caustic peeling,” the process used to remove the skins of apples, peaches, tomatoes, and the like before they are canned. A conveyor belt carries the fruit through a lye solution that is 12-20% NaOH and is maintained at 60-90 “C, depending on the particular application. The skins are released and can he suh-
How big is a laboratory that can analyze 100 samples a day for Blcohol, caffeine, calories, cellurose, fat, hardness, hydroxyl lumber, iodine value, latex, lignin, menthol, moisture, molecular ight, nicotine, oil, protein, resin, starch, sugars, film thickness and composition, and more? of i
posltlon Analyze? msker.ll mese m e a s I u e m U s , m d hers, rapldty, a c c u m w , d non-destructhfy. Called a "sIeeper"am0ng spectmscoplc techniques, Ni . pidly becoming a standard in many anafytlcaf :oydratorles. To find out how your labora%owcan benefit from NIR. call
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sequently washed away by jets of water. Continuous circulation of the lye is required, and dilution by the fruit being processed must he overcome by the addition of “makeup caustic,” a concentrated NaOH solution. Figure 6 depicts the layout of a lye peeler (37). The electrodeless conductivity monitor operating through a controller provides the proper throttling signal to a valve that regulates the flow of makeup caustic and thus maintains a constant concentration in the peeling bath. A thermal element, shown adjacent to the conductivity sensor, maintains the correct temperature by regulating the flow of steam to the header. A recent and striking application of the electrodeless technique is to the on-line analysis of oleum that contains from 2% to 7% free SO:, (33).The H20-SOa system has a very sharp conductivity minimum at a composition that corresponds to 100%H~SOI. With appropriate temperature control, an accuracy of fO.029’0 was achieved in monitoring the SO:, content of the liquid as it flowed through the installation. Acknowledgment This work, carried out under the Research Fellowship Program of the London Science Museum, is intended as a tribute to the great scientist and teacher Piet Kolthoff. Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. References 11) Cavendish, H.Phil. Trans. 177646, 196.
12) Maxwell,J. C.: Larmor. J., Eds. ”The Scientific Papers of the Honourable
(12) Kraus, C. A,: Parker, H.C. J. Am. C h u m Sot. 1922,44,24‘22. (13) Kraus,C. A.:Parker, H.C. J . Ani.
Chrm. Sac. 1922,44,24‘’Y. (14) Hall, R.E.;Adams, L. H. J. Am. Chem. Soc. 1919.41,1515. (15) Geary. W. J . Courd. Chvni. Hru. 1971. 7 YI
(16iuMarie.C.: Noyes. W.A. J . Am. Chem. SOC. i921.43,1095. (17) Newbery.E. J. Chmi. Soc. 1918.113. ~. 701. (18) Eastman, E.D.J. Am. Chem. Sor. 1920,42,1648. (19) Cuthbertsun. A. C.; Maass. 0.J. Am. Chcm. Soe. 1930,52.489. (20) Gunning, H. E.: Gordon, R. A. J. Chem. Phys. 1942. 10, 126. 121) Cuthrie, F.: Boys, C. V. Phil. Mag.
1880, 10,328.
(22) Kolthoff. 1. M. “Konduktumetrische
Titration”: Steinkopf:Dresden, Germa-
ny, 192:1. 123) Stock, J. T. J. Chem. Educ. 1967,44, C?n
U,,,.
(24) Denina. E.;De Paalini, F. S. Gazz. (‘him. Ital. 1934.64.675. 125) Pungor, E. “Oscillometry and Cun-
ductumetry”; Pergamon Press: London, E.ngland. 1965.
(26) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chrm. 1975,47,101. ( 2 7 ) Small, H. Anal. Chem. 1983,55. 235 A.
(28) Fritz. J. S.:Gjerde, D.‘r.;Becker, H. M. And. Chem. 1980,52,1519and
references cited.
(29)Okada, T.;Kuwarnoto. 1’. Anal. (‘hum. 1983,55,1001. (801 Relis. M. J. MSThesis. Massachusetts Institute of Technology. 1947. (31) Helis. M. J. U.S. Patent 2 542 057, 1951. (321 . . Piecard. A,: Frivold. 0. E. Arch. Sci. Phys. Not.’ 1920,2,264. I3B) Shaw, R.: Light, T. S. ISA Trans. 1982,21(4),63. (34) Foxboro Analytical, Plymouth. Mass.; Catalur Sheet I’SS 6-3B1 A.
137) Faxh,n,’AnalyticaI, Plymouth Mass.; I’nduet Application Data PAD B2030001.
Henry Cavendish. F.H.S.”; University
Press: Cambridge, England. 1921;Vul. I. pp.”:l.:l11. (:I) Stock, J. T.;Vaughan, D. “The Development of Instruments to Measure Elec-
tric Current”: Science Museum: London, England, 198:);p. 2:l. 14) ,Wiedemann, G. “Die Lehre vom Galvanismus”: Viewir: Braunsehweir, Germany, 1861: VoI. 1; p. 191. IS) Kohlrausch, F.: Holborn. L. “Dar Leitvermiigen der Elektrolyte”: Teubner: Leipzig, Germany. 1898;p. 5. (6)Horsford, E. N. Ann. der Physik 1847. 70. 238.
17) Kohlrausch. F.: Maltby, M. E. Wiss. Ahhandl. f’h,ys. H~irhonstolt1900.3. I hli. 1x1 Washburn. E. W.:Bell, J. E. J. Am. U w m . Soc. 1913.35, 177. (91 Ihvics, C.W. “The Conductivity of Solutions,” 2nd ed.: Chapman & Hall: Londun, England. 1933. I101 Harned. H. S.: Owen, 8. B.“The I’hpieal Chemistry of Electrolyte Sdulions.” 2nd ed.: Reinhold: New Yurk.
N.Y., 1YSO. Loveland. J. W. In “Treatise on Analytical Chemistry”: Kolthoff, l, M.: Elviny, 1’. .I.. Eds.; Wiley-lnterscience:New Yuck, N.Y., 396B;Part I, Vol. 4,Chapter
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John Stock recviued his P h D and D S c . degrFes from the Uniuersity of London. I n 1956, he joined the facult y at the Uniuersity of Connecticut, where he is now professor emeritus of chemistry. His interests are i n electroanalytical chemistry, microchemical analysis, the history of chemistry, and the design of apparatus and equipment for the teaching of chemistry.
