From the Volta-Electrometer to the Electronic Coulometer John T. Stock University of Connecticut, Storrs, CT 06269-3060
The "pile", a crude form of electric battery, was described by Volta, its originator, in 1800. Areport ofthe first electrochemical process, the dewmposition of water into its elements by means of a "pile", appeared in the same year. The appearance of Volta's pioneer device opened the way to the development of large, powerful batteries. With these, electrolysis, electromagnetism, and other phenomena such as electrical heating could be explored, at least qualitatively. With no standards and no real means for electrical measurements, the development of the quantitative aspects of electricity was severely handicapped. For example, the "strength" of a battery was sometimes assessed by the length of thin platinum wire that could be heated to redness. Ideas concerning the intensity, strength or flow rate, and amount of electricity (nowadays measured in volts, amperes, and coulombs, respectively) were confused. In fact, voltaic, frictional or "ordinary", animal, and some other kinds of electricity were thought to be separate species. Further, there was wnsiderahle belief in the "two-current" theory, meaning that electricity existed in positive and in negative forms. A breakthrough occurred in 1820, when Oersted reported that the magnetized needle of a compass could be deflected when a current flowed through an adjacent wire. Almost immediately, Amphre began to work out the principles that underlie our understanding of electromagnetic effects. Further, the Oersted effect was soon utilized in the development of the galuanometer, a device that could be used to compare, or even measure, current strengths ( 1 ) . The present-day device is the ammeter.
Figure 1. Voita-electrometers. meter (4). Passage of current between platinum electrodes immersed in dilute H2S04solution decomposed water. The total amount of electricity passed through the system was determined from the resulting volume of hydrogen, oxygen or, in the forms of voltameter shown at (a), (b), and (c) in Figure 1, of both gases. In 1839, Carlo Matteucci (18111868), then Professor a t Ravenna, described a voltameter in which gas liberated by the electrolysis of acidified water displaced its own volume of liquid. This was collected and measured (5). A few vears later. Johann Christian Poeeendorf (17961877) (Fig. 2) imp;oved the type of watgvoltam&er in which hydrogen and oxygen were collected separately. Poggendoffs aim was to shorten the path in the electrolyte solution, so that a battery of only two cells would produce
The Identities of All Forms of Electricity Realizing that the uncertainties concerningthe nature of electricity handicapped its quantitative study, Michael Faraday (1791-1867) set out to prove the identities of all forms of electricity. He then demolished the "two-current" theory. The way was then clear to his extensive studies on the phenomenon of electrolysis (2). Having alreadv noted his strong belief that the extent of to the quantity of an elect~odecompositionis electricity passed through thc system, Faraday went on to study the passage of the same current through a series of solutions. He found that, within the limits of his experimental techniques, the amounts of chemical reactions in the systems were in proportion to the respective chemical equivalents. The second law of electrolysis, published in 1834, implies that a fixed quantity of electricity (now termed the Faradav or the Faradav constant. and usuallv given the symbol ~ j w i lbring l aboui electrochkmical actioh to the extent of one gram-equivalent (3). Devices Used to Measure Electricity Devices for the measurement of quantities of electricity were, therefore, vitally important in the development of electrochemistq The present paper deals mainly with one of these devices, the hydrogen-oxygen or water voltameter. Voltameter The pioneer contribution was Faraday's introduction of the uolta-electrometer, a term he later shortened to uolta-
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Figure 2. Johann Christian Poggendolf.
quite rapid electrolysis. One version of his voltameter is shown in Figure 3 (6). The tube used to collect hydrogen had twice the cross section of the oxygen tube. The lower ends of these graduated tubes were cemented into unglazed ceramic cylinders, as shown. Each of these contained a platinum plate electrode, with a connecting wire passing through the wall of the tube. The tubes were inverted for filling and t h e n immersed close together in the solution to the depth NN, as shown.
Instead of a magnetized needle, Weber used a movable wire coil that was suspended by two fine parallel wires. This bifilar suspension not only provides the torsional restoring force when the coil deflects, but also allows for the entry and exit of current. A mirror on the coil, a distant scale, and a telescope were used for the precise measurement of the angle of deflection. The current flowing for a given time through the bifilar instrument also flowed through acidulated water, thereby producing 'Imallgas", the mixture of hydrogen and oxygen. Weher calculated the weight of water thus decomposed from the volume of this gas, obtaining the value 0.009376 for the ECE of water. (Except where mentioned, all ECE values are given in milligrams per weber, the usual 19thcentury practice. One weher = 10 coulombs. Nowadays, such values are expressed in grams per coulomb). Around 1840, Robert Wilhelm Bunsen (1811-1899) (Fig. 5), then Professor of Chemistry a t Marburg, developed the Electrochemical Equivalent carbon-zinc power cell that bears his name. This led to his The term "electrochemical interest inbattery-operated arc lamps. To examine the ecoequivalent" (ECE), implying nomic aspects, he needed to know the relationship bethe amount of electrochemical tween the amount of zinc consumed in the battery and the action produced by one unit of current-dependent intensity of the light. (Incidentally, this current in unit time (nowaled to his development of photometry!) He used the direct days, one ampere second or weighing of an amalgamated zinc rod anode to determine one coulomb) appears to have the ECE of zinc. To con6rm the result, he determined the I been introduced by Wilhelm ECE of water. From this, and the known chemical equival:J, 2' Eduard Weber (1804-1891) lents of zinc and of water, his direct measurement of the (Fie. 4) in 1840 (7). ECE of zinc could be checked. . . At that time he was a private investiThere are some points of interest concerning this work. Figure 3. Poggendortwater gator. His professorship a t It was reported by Bunsen's friend, the French scientist voltameter. Gottingen (which he resumed Jules Reiset (1818-?). Emphasis in this report was heavily in later years) had been terminated. He was re~ortinathe on the Bunsen cell and on the arc lamp (8).The amount of first electrochemical equivalent, that oi'wate; He water decomposed by the current was determined directly was of the opinion that the tangent galvanometer, then in by weighing the small voltameter before and aRer the excommon use, could be used to compare current strengths periment. Corrections, such as for temperature and presbut could not measure these absolutely. In this instrusure of a gas volume were thus avoided. A description of ment, a small compass needle positions itself according to this voltameter appeared several years later, when Bunthe resultant of two forces, the horizontal component of the sen had moved to Heidelburg (9). earth's magnetic field and the field produced by the fixed Wilhelm Theodor Casselmann (1820-1872) obtained his coil through which the current passes. doctorate a t Marburg in 1843, and remained there as Privat-docent until 1845. According to Friedrich Wilhelm Kohlrausch (18401910) (10)(Fig. 6), who obviously had access to Casselmann's dissertation (111, Casselmann made careful determinations of the ECE of water, making use of a weight voltameter. The results lay between 0.009295 and 0.009434, with a n average of 0.009371. Peculiarly, Casselmann's account i n the open literature made no reference to this work (12). Instead, the arc light that so interested Bunsen was dealt with. Admittedly, Casselmann indicated that this account was an addition to his dissertation. Early i n 1843, J a m e s Prescott Joule (1818-1889) read to t h e Literary and Philosophical Society of Manchester a n account of heat evolution during the Figure 5. Robert Wilhelm Bunsen Figure 4. Wilhelm Weber. electrolysis of water. This
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Volume 70 Number 7 July 1993
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Values of the Electrochemical Equivalent of Watera
Worker
Original
Adjusted
Weber
0.009376
0.009396
Casselmann
0.009371
0.009391
Bunsen
0.009266
0.009624
Joule Kohlrausch Averaae
0.009222
0.009331
0.009222 0.009476 0.009476
'Milligrams per weber 'see text.
Figure 6.Friedrich Kohlrausch. account, together with other of Joule's experimental data, came into the hands of William Thomson (1824-1907), later Lord Kelvin, who was Professor of Physics a t Glasgow. Thomson used this information to calculate the ECE of hydrogen, and hence of water (13).Converted to metric units (lo), the Joule-Thomson value for water was 0.009222. In his study of electrochemical equivalents, Kohlrausch used both the Bunsen water voltameter and a silver deposition voltameter (10). The value given for the ECE of water, 0.009476, was actually obtained by multiplying the ECE found for silver by the ratio of the then-accepted atomic weights of hydrogen and of silver. Kohlrausch made careful measurements of the earth's magnetic effect (14). He was then able to adjust the values given by earlier workers for the ECE of water. The table, taken from Kohlrausch's electrolysis paper (101, includes his own result. The numerical average, 0.009309, of the first column of values differs from the "average" given by Kohlrausch, because he gave less weight to the Bunsen and the Joul~Thomsonvalues. These were based on galvanometric data obtained without local magnetic intensity measurements. This intensity varies from place to place. Kohlrausch was particularly critical of the Joule-Thomson work, considering this to be unsuited for ECE determination. For example, the coil diameter, a parameter that govems the response of a tangent galvanometer such as that used by Joule, was given as "one foot". This can mean "exactly one foot", or that this measurement is known to only one significant figure. In 1886, Kohlrausch and his brother Wilhelm (1855-1936) redetermined the ECE of silver (16). This resulted in the calculated revised value of 0.009327 for the ECE of water. The Faraday Constant One of Kohlrausch's footnotes suggests the term "the chemical equivalent of electricity" (13). I n effect, Kohlrausch was anticipating the modern term "Faraday constant". The value of this constant can be obtained from the relationship F = gram-equivalentlECE*,where ECE*
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Journal of Chemical Education
is the electrochemical equivalent expressed in grams per coulomb. Electrochemical equivalents are purely experimental quantities, but gram-equivalents depend upon the prevaiiing values of atomic weights. With progressive refinements of methods of ECE determination and of atomicweight assessment, the value of the Faraday, 96486.18 coulombs/gram-equivalent,now has an uncertainty of only a few parts per million. It should be stressed that this value was not obtained from electrolyses of water. However, the means developed for accurate measurements of current (and, of course, of time), needed for the precise determination of the ECE of water, were, of course, applicable to other ECE determinations, such as that of silver. The water voltameter, thus, has an important place in the history of electrochemistry. Weber began the development of the electmdynamometer in the 1830's (17). In this type of instrument, the interacting magnetic fields are produced by two adjacent coils, one fixed, one capable of moving. The coils are connected in series, so that the current to be measured passes through both of them. Measurements then require no knowledge of the earth's magnetic field, although the presence of the latter must be cancelled out. Weber used a bifilarly-suspended vertical movable coil. Figure 7 shows a refined form of this type of instrument, made for the British Electrical Standards Committee around 1865 (1). Ahorizontal arrangement of the coils is more convenient, because gravity, instead of torsion of a suspension, can be used to oppose the shift of the movable coil when the current flows. In effect, the owration becomes one of "current weighing", because the movable coil ran then form the load "Current wdghinp", on one arm of a balance (18,. - begun in 1837, gradually developed into a highly precise tecGique. Joule's current balance, diagrammed in Figure 8, shows the horizontal arrangement of the fixed and the movable coils. According to Alfred Terquem (1831-1887) (19), the French scientist Achille August Cazin (1832-1877) developed a precise ''current weigher" in 1863 and used it to obtain 0.009372 as a value for the ECE of water. Elenthiere Elie Mascart (1837-1908), Professor of Physics in Paris, further examined the theory and practice of "current weighing" (20). Turning to the water voltameter, he noted ozone formation as a possible source of error. He claimed that such formation became negligible if an Hap04 electrolyte solution replaced the usualone of H2S04. He obtained a value nf0.009373 for the ECE of water, but later revised this value to 0.009325 (21). As art of his investization of the ECEs of oxwen and hydrogen, Robert ~lfred'khfeldt(1868-1937) examined a number of aqueous solutions (221.He found that the most suitable ele&olytes were neither acids nor alkalis, but NazSOa or K2Cr207.Lehfeldt named his apparatus a coulometer. In 1902, Theodore William Richards (18W1928) had suggested that this term should replace the older one,
Figure 8. Coil arrangement in Joule's current balance.
electronic coulometers (30-32), the water coulometer again receded into the background. Acknowledgment Part of this work was carried out under the Research Fellowship Program of the Science Museum, London. Literature Cited Figure 7. Bifilar electrodynamometer.
"voltameter", so easily confused with "voltmeter" (23).Although not immediately accepted, the new term eventually won out. With the development of other precise but more convenient wulometers, such as those based on the electrochemistry of iodine (24) and silver (25) species, interest in the water coulometer waned, but did not vanish. A few examples may be given. Kohlrausch developed a water coulometer for currents as large as several amperes (26).The large, closely-spaced electrodes in 20% HzSOl electrolyte ensured low electrical resistance and hence low heating effect. Nickel electrodes, operating in NaOH solution, had been suggested as a replacement for the platinum-H2S04system. However, it was shown that gas production was less than expected, an effect attributed to the formation of "nickelsuperoxyd" (Ni2O3.4H20) on the anode (27). Electronic Coulometers About 40 vears aeo. e for mea- . the need for a s i m ~ l means suring quantities of electricity in controlled-potential coulometry (where the current diminishes as the process advances) revived interest in the water coulometer (28). With the advent of electromechanical integrators (29) and
1. Stock, J. T.; Vaughan, D. TheDeuelopmnf ofInstnrmonb O Memure Electric C u r rent; science Muaeum: Landon, 1961. 2. Stock, J. T. The Pathway Lo the Lous ofEletmlyslg In Miehml Fanday, Chemist 1992. ondPapvlorLmfurer; Davenport, D.,Ed.. Bull Hia.Chem.Sp1. d., 3. Faradsv.M.Phi1. %ions. Rav Soc. 1834.. 124.. 102-116. 4. Ref. 3. 85-93. 5. Matteucei, C.Ann. Chim.Phya 1839,71,90-111. 6. Poggendo~J.C.Ann.Phys. Ckem. 1842,55,277301. 7. Weber, W. I" Gews, C. F ; W e b q W .Ed*. Resu1totr ou. &m Beobochtun#e" &s magnrtischrn % r e i n im Johm 1840; Weidmannsche Buchhandlung: I e i p ~ i g , 1841, pp 91-98(Reprinted in A n n Phys. Ckem. 1842.55. 181-169). 8. Relset, J.Ann. Chim. Phys. 1MS.8, m 4 . 9. Bunsen, R. Ann. Phys. Ckem. I W , 91, 619-625. (The w e k h t wltameter is de="bed on pp 62W21.1 10. Kohlrauseh, FAnn. Phys. Chem. 1873,149. 170L186. 11. Caaselmann, W. T. Dk&unnlaeke Eahlenzinkkette:Disaertation, Marburg, 1643, p 61. 12. Caaselmann, W. TAnn. Phys. Ckem. 18M.63, 576693. 13. Thornson.WPhrl. Mag. 1851,2,42%444. 14. Kohlrauseh,FAnn. Phys. C h . 1BB9.138, 1-10. 15. %f 10, p 115. 149. 16. Kohlramch,F;KoNrausch,W.Ann.Phys. C h m . 1-27. Ed.; Springer: Berlin, 1893, Vol. 3. 17. Weber, W. In Wlkelm Wsbebe Werke; Weber, H., DO 21-92. pp 41--42. 18. ~ ;1. i 19. Terquem,A J. &Phya l 8 n . 1.383395. 1, 109-119. 20. Mascart, E. E. J &Phya., endSol lea%, 21. Macart, E. E. J dePhys., 2nd Sex 1884,4 283-286. 22. Lehfeldt,R.APhil. Mog. 1908,15. 6 1 P 4 9 . 23. Richards, T W.; Heimmd, G. W p m Ampr Amd.A* Sci 19W,37,4 1 M . 24. Washbum,E. B.;Batea,S. J.J . h x Ckem. Soc 1912,34, U41-1368. 25. Lord Rayleigh: S i d w e k , H. Phil. %ions. Rq..Soc 1884,175, 4 1 1 4 0 . 26. Kohlrauach, F.ElekLmtehn Z d t 1885,6, 190-194. 27. Reisenfeld. H. 2. Ekktmhem. 19W12, 621422. 28. Lingam, J. J.EktmrinaIytImIChhmkL'y, 2nd ed.; 1nterseience:NewYmk. 1958,pp 452-457. 29. Ref 28, pp 34M50. 30. Kramer, K.W.; Fiseheq R B.Anal. Chom. 1954,26.41&416 31. Bard, A. J.; Solon, E . h l . Chem 196%34, 1161-1183. 32. Wise, E. N.Aml. Ckem. 1962,34, 1181.
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