16 Bull. Hist. Chem., VOLUME 29, Number 1 (2004)

Early Potentiometry

Early workers in the field ofpotentiometry faced a seriousproblem. The type of cell com-monly used in potentiometricstudies could supply only a tinycurrent and often had a high in-ternal electrical resistance. If theelectromotive force (emf) of sucha cell is to be measured accu-rately, the apparatus must drawessentially no current from thecell. One obvious approach is tooppose this emf with another thatis exactly equal, and is generatedexternally. This is the basis ofthe “compensation method,” de-vised by Johann ChristianPoggendorf in 1841 and laterimproved by others (1).

The problem here lies in thedetection of balance, that is, ofthe so-called “null point.” Theelectromagnetic galvanometer isessentially a current-measuring device, but a galvanom-eter of the mirror type may serve when the cell resis-tance is not too high. However, this high-sensitivitygalvanometer is not very convenient and is easily dam-aged by overload. The development of the nearlycurrentless capillary electrometer solved the problem.

GABRIEL LIPPMANN AND THECAPILLARY ELECTROMETER

John T. Stock, University of Connecticut

This device had the added ad-vantage of simplicity and, withsome later forms, of compara-tive robustness.

Lippmann

The studies that led to the inven-tion of this electrometer and tothe formulation of the associatedtheory were not the work of alifetime, but of that of a begin-ner, Gabriel Lippmann (Fig. 1).He was born near Luxemburg onAugust 16, 1845. His parentsmoved to Paris and eventuallyhe was admitted to the ÉcoleNormale. Pursuing only thetopics that aroused his interest,Lippmann was not an ideal stu-dent. He failed in the examina-tion that would have qualifiedhim as a teacher (2). Neverthe-less, his latent abilities were rec-ognized and he was given the

opportunity to study in Heidelberg, where the celebratedphysicist, Gustav Robert Kirchhoff (1824-1887) wasprofessor.

Lippmann saw a then well-known demonstrationthat involves a drop of mercury covered by dilute H2SO4.When touched by an iron wire, the drop contracts but

Figure 1. Gabriel Lippman, Proc. R. Soc.London 1922, 101A, i-iii.

Bull. Hist. Chem., VOLUME 29, Number 1 (2004) 17

regains its original shape on removal of the wire. Theagitation of a mercury drop in contact with a voltaic cellwas first observed by William Henry in 1800. BetweenHenry’s observation and the work of Lippmann,Partington cites seven additional reports of the samephenomenon, the most important being John Draper’s1845 observation of the depression of an electrifiedthread of mercury confined in a capillary tube (3).

Lippmann, recognizing that the effect must be dueto a connection between electric polarization and sur-face tension, developed the concept that led to the de-sign of the capillary electrometer in Kirchhoff’s labora-tory. Having been granted a Heidelberg Ph.D. in 1873,Lippmann returned to Paris, where he obtained a sec-ond doctorate in 1875. After various appointments hebecame professor of mathematics at the Sorbonne in1883, and then of physics in 1886. He held the latterposition for the remainder of his life.

The Capillary Electrometer

Lippmann’s publications on electrocapillarity began witha brief note in French (4). He explained that the con-traction of the mercury drop was due to the electricalpolarization of its surface, thereby changing its “capil-lary constant.” Among other comments was that, if amercury-dilute H

2SO

4 interface was formed in the cap-

illary tip of a tube containing mercury, the observationby microscope of the displacement of the interface pro-vided a sensitive measure of the emf applied to the sys-tem. Fortunately, while in Kirchhoff’s laboratory, hehad been able to use an electrostatic electrometer of thetype invented by William Thomson (later, Lord Kelvin)(1824-1907) and described by him in 1867 (5). Whenthis instrument was connected in place of the polarizingsource, it could be shown that a mechanical displace-ment of the interface resulted in a deflection of the elec-trometer. The effect was found to be independent ofthe shape of the surface, but proportional to the changein its area. The note concluded with intriguing remarksabout an electrocapillary motor that had been con-structed. Almost simultaneously, Lippmann publishedin German a description of this motor and of the prin-ciples outlined above (6). Two years later an even moreextended French paper appeared (7).

A detailed description of the motor, shown in Fig.2, is beyond the scope of the present article. Batterypower is applied alternately to the pistons of bunchedcapillaries that work in cylinders containing mercury.These stand in a trough containing dilute H2SO4. Ver-

tical movement ofthe pistons leads toleft-right oscilla-tion of overheadpiece v and hence tothe rotation ofwheel R, throughthe agency of crankx. The motor dem-onstrated that (i)electrical energycould be convertedinto mechanical en-ergy by use of theprinciples thatLippmann had de-veloped and (ii) thate lectrocapi l laryforces were by nomeans insignificant.

Lippmann pointed out that, hitherto, determinationsof capillary constant or “tension superficielle” had notbeen satisfactory. He mentioned a paper by GeorgHermann Quincke (1834-1924), then professor of phys-ics at Würzburg (8). Quincke had attempted to refutethe results given by Lippmann in 1873. Quincke at-tributed the changes in superficial tension to the pres-ence of impurities and concluded that capillary phenom-ena could not provide a measure of electrical effects.Lippmann’s comment on this paper was that it showedthe state of affairs when he began his work.

Both experiment and theory were used by Lippmannto establish his two “laws:”I. The capillary constant ex-tended at the surface ofseparation of mercury andsulfuric acid is a function ofthe electrical difference thatis at this surface. II. When,by mechanical means, a liq-uid surface is deformed, theelectrical difference at thissurface varies in a sensesuch that the “tensionsuperficielle” developed byvirtue of the first law op-poses continuation of themovement.

Figure 2. Electrocapillary motor;Ref. 5, p 522.

Figure 3.Electrocapillary source of

current; Ref. 5, p 512

18 Bull. Hist. Chem., VOLUME 29, Number 1 (2004)

The assembly shown in Fig. 3 provided “une expe-rience curieuse,” based on the above principles. Dropsof mercury fall from the very narrow tip of the funnelthrough dilute H2SO4 and into a pool of mercury. Wiresa and b connect the masses of mercury to a galvanom-eter, which at once indicates that electricity is flowing.This flow continues indefinitely, provided that mercuryfrom the growing pool is restored to the funnel. Whena drop of mercury forms and grows, the electrical dif-ference at its surface increases and the mercury in thefunnel becomes negative with respect to the mercury inthe pool. The electrical effect is enhanced when thedrop of mercury reaches the pool.

Fig. 4 shows Lippmann’s high-sensitivity capillaryelectrometer. The tip of the vertical tube A is drawn outto a very fine capillary and bent as shown. The tip dipsinto dilute H2SO4 contained in vessel B, at the bottomof which is a pool of mercury. The height of the col-umn of mercury in A is such that the liquid interface inthe capillary can be satisfactorily viewed through a mi-croscope with an eyepiece scale. Wires a and b con-nect the two masses of mercury to the source of the emfto be measured, or rather to be compared with that ofanother source. For example, it was common practiceto use the zinc-copper Daniell cell as a standard and toassign unit value to its emf. Lippmann provided sev-eral examples ofsuch comparisons,such as of the emfsof the Daniell and ofthe Leclanché cell.

Users of theelectrometer soonbecame aware of theimportance of pro-viding a fresh mer-cury surface beforethe next observation;otherwise the re-sponse might havebeen irregular. Withelectrometers of theLippmann type,pressure may betemporarily in-creased, so that a drop of mercury is expelled from thecapillary.

In a paper of 1880, Lippmann mentioned claimsconcerning the sensitivity of the capillary electrometer

that he had found in the literature (9). One claim indi-cated measurement to 1/10000th that of a Daniell cell,i.e., to about 0.1 mV, another to 1/30000th.

Almost immediately after the appearance ofLippmann’s preliminary note (4), a “capillary galvano-scope” made by Werner Siemens (1816-1892) was re-ported (10). This device, intended for testing ratherthan for measuring, was obviously less sensitive thanthe Lippmann instrument. A 0.5-mm diameter hori-zontal capillary that curves slightly upwards joins twovertical mercury-containing tubes. A small drop of di-lute H2SO4 is situated at the middle—the highest point—of the capillary. The application of an emf moves thedrop to an extent indicated by an attached scale. How-ever, the convex form of the capillary hinders extensivedisplacement. On breakingthe circuit and short-circuit-ing, the drop returns to themid position.

Applications

It is doubtful whether any-one appreciated the utilityand simplicity of the capil-lary electrometer more thanWilhelm Ostwald (1853-1932). He developed theform shown in Fig. 5 whilestill at the RigaPolytechnicum (11). Nonew principles were in-volved, but the device waseasy to construct and to use.An externally threaded brasstube is cemented to glasstube A, which contains a column of mercury. A collarM, which is screwed onto the brass tube, rests upon asmall stand ring. Rotation of M then raises or lowerstube A. The capillary, which is drawn out from ther-mometer tubing, is cemented into the lower end of A.A second small stand ring restricts the lateral movementof this end, and a rubber stopper supports tube P. Thiscontains dilute H

2SO

4, into which the capillary dips, and

also a pool of mercury. The microscope is aligned byadjustment of screws on the platform, and electrical con-nections are made through sealed-in platinum wires.

Following Ostwald’s move to the University ofLeipzig in 1887, his students and associates made muchuse of the capillary electrometer, either of the original

Figure 4. Lippmann capillaryelectrometer; Ref 5, p 532

Figure 5. Ostwald’s versionof the Lippmann

electrometer; Ref. 5, p 404

Bull. Hist. Chem., VOLUME 29, Number 1 (2004) 19

form or of later ones.Ostwald may have sug-gested the more compactform shown in Fig. 6. Thisform was used in a poten-tiometric study of mercury(12) and also by other work-ers in Ostwald’s laboratory.The application of the elec-trometer to acid-basetitrimetry was the subject ofan extensive investigation(13). Fig. 7, based on asketch given by Max LeBlanc (1865-1943) in hispaper on amalgams (14),

shows a sloping-capillary high sensitivity form of elec-trometer. Le Blanc and also Anton Robert Behrend(1856-1926), who used this type of instrument in a studyof potentiometric titration (15), attribute the device toOstwald..

Robert Luther, one of Ostwald’s assistants who be-came sub-director of physical chemistry in 1901, de-vised the totally sealed form of electrometer shown inFig. 8 (16). Obviously this is easily portable and canbe stored in any position. Transfer of liquid throughcross tube b permits the adjustment of the position ofthe liquid interface in capillary c.

Historical Perspective

Practical electronics began with the invention of thevacuum triode in 1917. By then, any reference to thecapillary electrometer might be expected to be one ofmere mention. A brief scan of the first four decennialindices of Chemical Abstracts shows that this was notso. For example, the value of the capillary electrom-

eter in numerous titrimetricanalyses was pointed out in1919 (17). Then a new form ofthis device was described in1942 (18). This was mid-yearof a war, so that easily-damagedgalvanometers could not be re-paired quickly. The deviceshown in Fig. 9 was intended tobe a user-safe substitute for agalvanometer. A commoncause of trouble with capillaryelectrometers in general hadbeen the precipitation ofHg

2SO

4 within the capillary. In

experiments with a Luther-typeelectrometer, it was found thatthis kind of trouble did not oc-

cur if the usual dilute H2SO

4 was replaced by 20%

HClO4. This improvement was incorporated in the new

device shown. An en-largement K is joined at Ato capillary C. If an ex-cessive emf is applied,electrolysis takes place inK, rather than in C. Thusthe mercury-acid interfacein C is so little disturbedthat, after brief short-cir-cuiting, it returns almostexactly to the initial posi-tion. Yoke I

1 is merely a

support, while I2 is a wide

capillary that ensures theequalization of pressures inthe vertical limbs.

Lippmann did notabandon his interest inelectrocapillarity after hehad published the work thatled to the development of the electrometer. However,his research spread to other topics, such as to the deter-mination of electrical units and to the exact measure-ment of time. He had been thinking about the possibil-ity of photography in color for some years before hepublished a note on this topic in 1889 (19). Thereafter,the bulk of Lippmann’s publications was concerned withthis form of photography. In 1891 he demonstrated amethod for producing permanent color photographs. Hewas awarded the 1908 Nobel Prize for Physics “for hismethod of reproducing colors photographically based

Figure 6. Compact capillaryelectrometer; Ref. 11, p 555.

Figure 7. High-sensitivity sloping-capillaryelectrometer; Redrawn from Ref. 13

Figure 8. Luthercapillary electrometer;

Ref. 15, p 426

Figure 9. Uhl electrometer;Ref. 17, p 326

20 Bull. Hist. Chem., VOLUME 29, Number 1 (2004)

on the principle of interference.” In his Nobel Lecture,he demonstrated that colors were indeed produced byinterference in a nonpigmented emulsion. He admittedthat the exposure, one minute in sunlight, was too slowfor portraiture. With modern dyestuffs-based color pho-tography, the necessary exposure is of course only a frac-tion of a second.

Lippmann died aboard ship on July 12, 1921, whilereturning from a visit to Canada; but by no means didinterest in the development and use of the capillary elec-trometer die with him.

REFERENCES AND NOTES

1. E. Hoppe, Geschichte der Elektrizität, Barth, Leipzig,1884, 393-396.

2. A. Schuster, “Gabriel Lippmann, 1845-1921,” Proc. R.Soc London, 1922, 101A, i-iii.

3. J. R. Partington, A History of Chemistry Macmillan, Lon-don, 1964, Vol. 4, 707.

4. G. Lippmann, “Relation entre les phénomènes électriqueset capillaries,” C . R.. Séances Acad. Sci., Ser. C 1873,76, 1407-1408.

5. W. Thomson, “Report on electrometers and electrostaticmeasurements,” Report of the British Association for theAdvancement of Science, Dundee, 1867, 489-512.

6. G. Lippmann, “Beziehungen zwischen den capillaren undelektrischen Erscheinungen,” Ann. Phys., 1873, 149, 546-561.

7. G. Lippmann, “Relations entre les phénomènesélectriques et capillaries,” Ann. Chim. Phys., 1875, 5,494-549.

8. G. Quincke, “Ueber elektrische Ströme beiungleichzeitigem Eintauchen zweier Quecksilber-Elektroden in verschiedene Flüssigkeiten,” Ann.Phys.,1874, 153, 161-205.

9. G. Lippmann, “Bermerkungen über einige neuereelectrocapillaire Versuche,” Ann. Phys., 1880, 11, 316-324.

10. Anon., “Ueber ein von Hrn. Siemens construirtesCapillar-Galvanoscope,” Ann. Phys., 1874, 151, 639-641.

11. W. Ostwald, “Das Kompensations-Elektrometer,” Z.Phys. Chem. (Leipzig), 1887, 1, 403-407.

12. H. Brandenburg, “Abnorme elektromotrische Kräfte desQuecksilbers,” Z. Phys. Chem. (Leipzig), 1893, 1I, 552-576.

13. W. Böttger, “Die Anwendung des Elektrometers alsIndikator beim Tittrieren von Säuren und Basen,” Z.Phys. Chem. (Leipzig), 1897, 24, 253-301.

14. M. Le Blanc, “Ein Beitrag zur Kenntnis der Amalgame,”Z. Phys. Chem. (Leipzig), 1890, 5, 467-480.

15. R. Behrend, “Elektrometrische Analyse,” Z. Phys. Chem.(Leipzig), 1893, 11, 466-491.

16. C. Drucker, Ed., Ostwald-Luther Hand- und Hilfbuchzur Ausfuhrung physikochemischer Messungen,Academische Verlagsgesellschaft, Leipzig, 4th ed., 1925,426.

17. J. Pinkhof, “Das Elektrometer als Titrierindicator,”Pharm. Weekblad, 1919, 56, 218-234.

18. F. A. Uhl, “Ein neues, dauernd betriebssicheresCapillelektrometer als Galvanometerersatz,” Z. Anal.Chem., 1942, 124, 324-327.

19. G. Lippmann, “Sur l’obtention de photographies en val-ues justes par l’emploi der verres colorés,” C .R. SéancesAcad. Sci., Ser.C, 1889, 108, 871-873.

ABOUT THE AUTHOR

Dr. John T Stock, recipient of the Dexter Award in theHistory of Chemistry, is Professor Emeritus of Chemis-try, University of Connecticut, Storrs, CT 06269-3060.

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