William Francis Giauque: An Adventure in Low-Temperature Research Anthony N. Stranges Department of History, Texas ABM University, College Station, TX 77843
A comprehensive history of 20th-century physical chemistry remains largely unwritten, although we can find numerous historical accounts on eminent individuals and important ideas scattered through the literature. Considerably less has appeared on the history of late 19th- and 20thcentury thermodynamics, and it is in even greater need of synthesis. William Francis Giauque (1895-1982) made pioneering and exhaustive investigations on entropy and lowtemperature chemistry, and he undoubtedly will occupy an important place in these histories. Of his many important investigations, four of these are particularly noteworthy. Giauque contributed significantly toward establishing the third law of thermodynamics as a fundamental scientific principle and invented the adiabatic demagnetization cooling technique. He also demonstrated the natural occurrence of 0-17 and 0-18 isotopes and molecular hydrogen's orthopara forms. His experimental researches were meticulous, most of them definitive, with improvements on his results coming only from refinements in technique. They earned Giauque the 1949 Nobel Prize in Chemistry. As recipient of several prestigious scientific awards and teacher of many excellent chemists, Giauque's career resemhled the careers of other outstanding 20th-century chemists. Yet, in some ways i t differed. Indeed, Giauque almost chose a career in engineering instead of chemistry. He made a good choice, one that led to a life enriched even more by an
Figure 1. William F. Giauque a1 age one (1896). Courtesy of Robert Glauque.
episode of near-simultaneous invention and a Kekul6-like dream. Early Llfe
Giauque was born in Niagara Falls, Ontario, Canada, on Mav 12. 1895. the eldest in a familv of two sons and a daughter (Fig.'l). His parents were ~ i e r i c a ncitizens, automatically making their children citizens according to the laws in effect at that time. Giauque received his elementary school education mainly in Michiaan where his father was a weightmaster and station agent for the Michigan Central Railroad. Upon his father's death in 1908, the family returned to Niagara Falls, and, despite family opposition, Giauque enrolled in a two-year commercial course a t the Niagara Falls Colleeiate and Vocational Institute intendine to acquire enough business training to help support them (Fig. 21. Bv this rime. Ciauque's mother had become a part-time s&m&ess andtailor 'for J. W. Beckman, a chemist with American Cyanamid Company. Her employment was most fortunate because convincing Giauque to switch to the fivevear general college-preparatory) course the next year took the comhined effort of Giauque's mother and the Heckman family. Elecrricai engineering was Giauque's first choice, hut, lacking both finances and engineering experience, he planned to work a short time in one of NiagaraFalls's powergenerating plants. Unable to find any engineering openings,
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Figure 2. William F. Giauque at age 13 (1908)holding pet pigeon. Counesy Roban Giauque.
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Giauque accepted a position with the Hooker Eledrochemical Company across the Niagara River in Niagara Falls, New York. Hooker's well-organized laboratory impressed Giauque greatly, and the two years he spent there convinced him to study chemical engineering ( I ) . By 1916, Beckman had transferred to Berkeley, and hearing of Giauque's new interest in chemical engineering he encouraged Giauque to attend the University of California a t Berkeley. G. N. Lewis (1875-1946) had arrived there in 1912 to serve as the Chemistry Department's chairman and dean of the College of Chemistry, which included chemical engineering. He also attracted a first-rate faculty, among them Joel Hildebrand (1881-1983), George E. Gibson (188P 19591, William Bray (1879-1946), and Gerald E. K. Branch (1886-1954) (2).Beckman spoke highly of Lewis's investigations on the electron valence theory, thermodynamics, and free energy and praised the research program Lewis had established. His recommendation of Berkeley's program combined with the university's $10 total semester fee easily convinced Giauque to enroll in the College of Chemistry rather than attend the more expensive Massachusetts Institute of Technology or Rensselaer Polytechnic Institute. Giauque's family relocated shortly. Giauque graduatedwith highest honors in 1920 receiving a BS degree in chemistry that contained 25% engineering courses (Fig. 3). Hildehrand, who taught Giauque, described him as simply outstanding. Two years later in 1922, Giauque enmed the PhD in chemistry with a minor in physics. Gibson directed his dissertation, :though ~ i a u ~ u e w o r k eclosely d with the ohvsicist Ravmond Hirye (1887719803 throughout his graduate studies. ~ e c a u s of e Giauque's obvious promise, Lewis immediately offered him a faculty position. Giauque still had hopes of an engineering career, but the excellent research environment that Lewis had created finally led him after several months of ambivalence to pursue a career in chemistry. Giauque remained a t Berkeley for the rest of his life ( I ) . ~~~~
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Low-Temperature Entropy lnvesllgatlons Giauque's earliest investigations were on low-temperature entroov and the third law of thermodynamics. Walther (1864-1941) first enunciated the third law, then called the Nernst heat theorem, in 1906. According to Nernst, in any reaction involving only solids and liquids (including solutions), the change in entropy approached zero as the temperature approached absolute zero (3).Five years later, Max Planck (1858-1947) in the third edition of his Thermodynamik (1911) argued that Nernst's theorem did not hold for solutions and required modification. Planck suggested assigning zero entropy to each element a t absolute zero and interpreted the third law to mean that all pure solids and liquids had zero entropy a t absolute zero (4). But Lewis and Gibson pointed out that entropy measured the randomness of a macroscopic state, and even in a pure solid or some randomness existed in its structure. Onlv a - - liauid perfect crystal of a pure substance lost its entropy a t absolute zero. In 1920Lewis and Gibson gave the definition of the third law accepted today: the entropy of a perfect crystal is zero a t absolute zero (5). Giauque demonstrated the correctness of Lewis and Gibson's third law interpretation in his doccoral dissertation of 1922 and in his first publication the following year. He showed experimentally from heat capacity and heat of fusion measurements that glycerol glasi~supercooledglycerol) at 70 K had 5.6 cal mol-K-: more entropy than crystalline elvcerol and concluded that this difference remained even a t i&olute zero (6). His third law research continued in the 1920's and early 1930's with a series of investigations on diatomic gases in which he calculated their entropies from ahsor~tionband spectra and compared the spectroscopic entropies with values determined &orimetricdly. Giauque and several graduate students, among them R. Wiebe, H. L. Johnston, and J. 0. Clayton, calorimetrically measured lowtemperature heat capacities and changes of state to obtain entropies for molecules such as hydrogen chloride, hydrogen bromide, hydrogen iodide, oxygen, nitrogen, nitric oxide, and carbon monoxide For the s ~ e c t r o s c o ~entropies. ic they used quantum-st&tical equations developed within the last 20 vears from absor~tionhand spectra studies on gaseous moiecules. These inciuded Otto ~ a c k u r ' sand Hans Tetrode's eauations of 1911-1913 for gaseous volumes and translational energies, equations that Richard Tolman (1881-1948) and Harold Urey (1893-1981) had derived independently in 1923 for rotational energies, and Hervey Hicks and Allan Mitchell's equations of 1926 for vibrational energies (8). All of these experiments, the results of which Giauque began publishing in 1928, showed his thoroughness and a high degree of accuracy and precision. The close agreement of his spectroscopic and calorimetric entropy values clearly supported the third law of thermodynamics. Giauque provided further verification of the law when he showed that his exoerimental entropies and those calculated from entropies of'formation were also in excellent agreement. At the same time his experiments demonstrated the usefulness of quantum statist& and the partition function in calculating entropies. As a result of calorimetric studies in 1852 by Julius Thomsen (1826-1909) in Copenhagen and Marcelin Berthelot (1827-1907) in Paris, most chemists a t that time believed the quantity of heat evolved in an exothermic reaction measured the reactants' affinity for each other (9). While this correlation often seemed to hold, some reactions that were clearly endothermic occurred spontaneously. Confusion also surrounded the meaning of entropy after introduction of the concept in 1854 by Rudolf Clausius (1822-1888) in Bonn. The confusion persisted until J. Willard Gihbs (1839-1903) at Yale University showed in a brilliant series of publications in 1875-1878 that both heat (enthalpy) and entropy changes
erns st
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Figure 3 Wllllam F Glauque at age 22 (1917) Courtesy of Robert Glauque
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Journal of Chemical Education
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were necessary t o establish a reaction's spontaneity a t constant temperature and pressure. T o measure spontaneity, Gihhs in 1875 defined a new function called a potential and expressed its dependence on enthalpy, entropy, and temperature with the equation (10) d f =dx-tdn f
x 7
t
(1)
: potential : enthalpy : :
entropy abaolute temperature
Seven years later, Hermann von Helmholtz (1821-1894) in Berlin derived a similar relation in which he introduced the term "free energy" synonymous to Gibbs's potential (11). For this reason chemists and physicists today speak of the Gibbs-Helmholtz equation when describing the thermodynamic process A 0 =AH-TAS A C : change in Gibbs (free) energy AH
TAS
: :
(2)
enthalpy change product of the entropy change and absolute temperature
Gihhs (1876), Helmholtz (1882), and the Dutch chemist Jacohus van't Hoff (1886) showed that equilibrium constant (K)and electromotive force ( E ) measurements provided additional ways of calculating standard Gihbs (free) energies (12). The equations, using modern symbols, are AGO = -RTln K
R K
(3)
: universal gas constant :
equilibrium constant (a dimensionless, constant, function only of T)
and AG" = -nFAE" n : number of moles
F
: :
(4)
Faraday change in electromotive force fora reaction
Bythe 1920'sBerkeley's chemists hadcarried out animpressive program of Gihhs (free) energy determinations. Lewis, Gibson, and Merle Randall (188&1950) obtained Gihbs (free) energy values of many reactions from chemical equilihrium studies and electromotive force measurements. Giauque and his students calculated Gihhs energies from enthalpies and entropies measured thermodynamically, and from spectroscopic data (13). Orfh* and ParaHydrogen
Giauoue's research on the e n t r o ~ vof eases showed in 1928-1632 that molecules such as h&ogen, carbon monoxide, nitric oxide. and nitrous oxide crvstallized with a definite amount of residual entropy as-the temperature approached absolute zero. The entropy of solid hydrogen results from a disordered nuclear-spin alignment of the molecule's two protons. The alignment is parallel in the ortho form. antioarallel in the oara form (141. Disordered crystalline k & g e m e n t s cause'the entropy found in the other molecules. Werner Heisenbere (1901-1976) a t Niels Bohr's Institute for Theoretical phy&s in Copenhagen first sueeested in 1927 that hvdroeen . - and other elementary diatomic molecules existed in symmetrical (para) and antisymmetrical (ortho) forms (15). That same year while a t Bohr's Institute Friedrich Hund (h. 1896) pointed out that nuclear spin accounted for an element's hyperfine spectrum, and spectral analysis, therefore, provided information on the two spin states (16). Nuclear soins. like electron spins. are difficult to reverse. When a hydrogen molecule intkracts with electromagnetic radiation the resulting electronic hand spectrum contains ~
two different sets of lines, one for each spin alignment. The more intense set belongs to ortho-hydrogen and corresponds to odd rotational levers (odd rotationai quantum numbers J),while the fainter set represents para-hydrogen and even rotational levels. The two forms produce a regular pattern of lines that alternate in intensity, and from the intensities molecular hvdroeen anneared to he a 3:l ortho:nara mixture .. a t ordinarytemperatures. Edward Condon (1602-1974), a t that time a National Research Fellow in Germanv. had carriedout theoretical calculations on the hydrogen ~ o l e c u l in e 1927 showine that the ortho-para equilibrium was temperaturedependent though estabiished only slowly. In a letier to the Berkeley Lahuratory he suprgested that evidence for the two forms might result from keeping hydrogen a t liquid air temperatures for two or three months. Condon expected to see marked difference in hydrogen's heat capacity if a transition occurred between the two forms (17). Giauque began the suggested experiments late in 1927. He and Johnson obtained 20 g of pure hydrogen by electrolyzing water. After keenine the hvdroeen in a steel container a t liquid air tempe&tuYes (85 K)foF197 days, from October 19, 1927, to May 3,1928, they observed a small decrease of 0.04 cm (0.4 Torr) in hydrogen's vapor pressure at the triple point. The change occurred because as the temperature fell, fewer and fewer molecules had sufficient rotational energy to remain in the higher ortho or odd rotational energy levels. More &olecules reversed their n u c l e a r ~ s ~ i ntos - - ~ and - - more ~ ~ become the lower energy para form and occupy the lower or even rotational levels. Giauque and Johnson's 197-day experiment indicated an entropy difference of 4.39 cal mol-'K-' for the two forms. Their results clearly supported the third law interpretation that Lewis and ~ibson-gavein their 1920 paper (18). Karl Bonhoeffer (1899-1957) and Paul Harteck (19021985) a t the Kaiser Wibelm Gesellschaft in Berlin first separated ortho- and para-hydrogen in 1929 when they used 10 g of charcoal t o adsorb a small amount of hydrogen gas. The charcoal acted as a catalyst in establishing the equilibrium between ortho- and para-hydrogen. They succeeded in keeping the charcoal a t the temperature of liquid hydrogen (20 K) for about 20 min. After pumping off the gas they showed from its much hieher thermal conductivitv that i t consisted of 99.7% ~ a r a - h i d r o g e n(19). That same year Arnold Eucken (1884-1950) and Kurt Hiller a t the Technical University in Breslau detected a change in the heat capacity of hydrogen they had cooled to 90 K for periods of 4 to 14 days (20). By the 1930's low-temperature research estahlished beyond doubt that molecular hydrogen existed in two forms and that a catalytic conversioi of ortho to para produced almost pure para-hydrogen. T o account for the e n t r o ~ vvalues of CO. NO. and N10. Giauque believed that in thecrystal state some of the moie: cules had head-to-head arraneements (CO..CO. NO-NO. N,O-N?O) and others head-to-tad arrangements ICO-OC, NO-ON. N10-0,N) Such disorder in the crvatal could account fo; the small entropy at 0 K. For CO the entropy was 1.1 cal mol-'K-1. In a completely random crystalline arrangement of CO, or NO, and NzO, the entropy increased to a maximum of S = R in 2 or 1.38 cal mol-'K-I. While the hehavior of these compounds seemed a t first to fall outside the third law, Giauque's results proved again the law's validity only for perfect crystalline order in the lowest energy state (7,14). lnventlon of Coollng by Adiabatic Demagnetlzatlon Giauoue's third law investieations led to his most sienificant ac~omplishment,the in;ention in late 1924 of c;oling hv adiabatic demaenetization. His new cooline method ena k e d scientists :t understand better the phncip~esand mechanisms of electrical and thermal conductivity, to determine heat capacities, and to investigate the hehavior of superconductors at extremely low temperatures. Michael Far-
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aday (1791-1867) in London had conducted the first systematic low-temperature research. Beginning in 1823 he used compression and cooling with ic6-salt mixtures to liquefy gases such as chlorine, sulfur dioxide, ammonia, and carbon dioxide. By 1880 new cooling methods had produced even lower temneratures. In 1877 Carl Linde (1842-1934) in Munich developed a commercially practical refrigeration process based on the exnansion of ammonia eas. In 1877-1878 Louis Cailletet (1832-1913) a t hilti ill on-&seine reached temneratures lower than 80 K and liauefied the "nermanent gases" oxygen, nitrogen, nitrogen dioxide, carbon monoxide, and acetylene. Linde and Cailletet used the Joule-Thomson effect (1852). In thismethod acompressed andcoolgas, after expansion through a small opening, cools further because the expanding gas expends some of its kinetic energy to overcome intermolecular attractions. Almost simultaneously in 1877 Raoul Pictet (1846-1929) in Geneva developed a cascade process in which each gas in a group of several gases with decreasine critical temperatures and trinle points liauefied the next member (see table( he process liquefied a gas by compression a t the critical temperature, the highest temperature a t which the gas existed as a liquid, and cooled it to its triple point, the lowest temperature a t which it existed as a liquid, by boiling under reduced pressure. Because no liquids have critical temperatures and triple points between nitrogen's boiling point (77 K), hydrogen's critical point (33.2 K) and triple point (14 K), and helium's critical point (5.2 K). .. the cascade nrocess failed to liquefy hydrogen-and hklium. A solution to the problem of reaching these temperatures finally appeared near the turn of the century. In 1895 Linde considerahlv imnroved Joule-Thomson coolina with the invention of his regenerator or heat-interchanger cyclic coolina. James Dewar (1842-1923) at the Royal Institution in ~cyndonin 1898 combined the Linde and cascade processes to liquefy hydrogen (20.4 K), and Heike Kamerlingh-Onnes 11853-1826) at the Crvoeenic Lahoratorv in Leiden in 1908 used the combined proc&s to liquefy hklium (4.2 K) (21). These cooline nrocesses made temneratures of 5.0-0.8 K accessible in ;he laboratory and led :n 1911 to KamerlinghOnnes's discoverv of su~erconductivitvin metals such as mercury, tin, andlead. Kamerlinah-Onnes also studied the mametic suscentibility of the paramagnetic compound gadolinium sulfate octahydrate, Gd2(S04)+H20, at liquid helium temperatures. These measurements became Giauque's starting point in calculating the effect of amagnetic field on the octahydrate's entronv. Thev enabled him to show theoreticallv that the appli&ion a i d subsequent adiabatic removal of the field a t liauid-helium temneratures nroduced additional cooline. ~ i i a h a t i cdemagnetization suggested a new method i f reaching temperatures near absolute zero. Two years later, in a theoretical paper published onDecember 11,1926,Peter
Debye (1884-1966) in Zurich used the same gadolinium compound to describe in detail the principle of adiabatic demwnetization cooline. Debve's paver anneared 8 months after%endell Latimer fi893-i955j on ~ ~ r i i 9 , 1 9 2publicly 6, discussed for the first time the work of his Berkeley colleague a t the American Chemical Society's ~aliforniaSection Meetine- .(22). . In 1924 it was well known from Nernst's heat theorem and the Gibbs-Helmholtz equation that the heat capacities of substances became very small and approached zero a t temperatures below 10-15 K. At these temperatures a substance had lost practically all its thermal entropy, and magnetization/demagnetization should have produced no further significant cooling. But paramagnetic compounds, such as gadolinium, cerium, and dysprosium salts, have thermal and magnetic entropy. In the absence of a magnetic field a t low temperatures they no longer have appreciable thermal entronv entropy .. because their atom..but still nossess mametic ic magnets have an irregular arrangement. Applying a powerful magnetic field forces the atomic magnets to line up with the field reducing the magnetic entropy. A cooling bath removes the heat generated by the entropy decrease. Giauque recognized that if he insulated the compound thermally and removed the field under adiabatic conditions, the total entropy must remain constant. By removing the field the atomic magnets return to their random arrangement, and increase the maenetic entronv. .. Temnerature measures " thermal motion. Therefore, the accompanying decrease in thermal entropv, which corresponds to a decrease in motion, results in the i&peratures lowering. Because magnetic entropy isafactor in cooling only a t low temperatures, Giauque pointed out that cooling by adiabatic demagnetization is most effective a t temperatures produced by the evaporation of liquid helium (1 K). The sequence of steps in magnetic
In-
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Some Crltlcal Temperatures and Trlple Polnts of Gases
Gas C3H8 CZHB CHn Ar CO NI 0 2
FI Ne Hz
He4
190
Critical Temperature (K)
Trlpie Point (K)
370.0 305.4 191.1 150.7 132.9 126.1 154.8 144.0 44.7 33.2 5.2
85.5 89.9 90.7 83.8 68.1 63.2 54.4 53.5 24.6 14.0
Journal of Chemical Education
Figure 4. Fmm lefi to right: Paramagnetic Substance in vacuum jacket immersed in Dewar vessel 01 liquid helium: solenoid magnet with adiabatic demagnetization apparatus. Les Prix Nobel 1949.
cooling, he said, was comparable t o the three steps in the refrigeration process using an idealized expansion engine (23). When Giauque began calculating the low temperatures achievable with magnetic cooling, he had neither the expensive large-scale equipment to conduct experiments nor the thermometer to record the readings. He planned not merely to measure low temperatures but t o use magnetic cooling in his low-temoerature thermodvnamics research. Parmaenetic salts werk ideal because of their high heat capabilities a t low temoeratures, thoueh Giauque later experienced difficulty inmaking good tgermal contact with t h e cooled salt. The equipment he required included a magnet with a strong homogeneous field (8,000-20,000 gauss), a hydrogen and a helium compressor, a purification system for removing oil, air, and other gases from the helium, and vacuum pumps for hydrogen recovery and reduction of liquid helium's temoerature. Because this e a.u i.~ m e n twas not immediatelv available, Giauque and his graduate student, D. P. ' ~ a c ~ o u eall. succeeded in carrvine out the fin1 adiabatic demametiiati'on cooling that prid&ed temperatures below 1K; only on March 19, 1933. The value recorded was 0.53 K. Nine years had passed since Giauque first conceived of cooling by adiabatic demagnetization. The Leiden group had known all along of his research, but to Giauque's astonishment they never attempted the cooling experiments before he did (Fig. 4). (25). .-~, In the experiments Giauque and MacDougall placed a 61e -samole . of oaramaenetic eadolinium sulfate octahvdrate in a copper caiorimete; tub el^ vacuum jacket filled with helium eas to conduct heat from the compound surrounded the tube. The tube and jacket rested inside a copper-lead Dewar flask to which they added liquid helium (through a vacuumFigure 5. Adiabatic demagnetization apparatus. Les Prix Nobel 1949. jacketed transfer~tube)to a height of one m&er and then placed the flask within the copper coils of asolenoid magnet. Low-viscosity cooling oil (kerosene) pumped rapidly over bare copper conductors removed heat and promoted effiLOW TEMPERATURE RESEARCH cient heat transfer which, Giauque found, was the principal problem in designing solenoid magnets (Fig. 5). The cooline oroeressed in three staees: (1) the electric current through the copper coils causedihe atomic magnets to line UD. releasing heat, and decreasing the entropy of the compoundj (2) when the cooling stopped, the comoound was insulated against heat flow by evacuating helium gas from the surrouiding jacket; (3) the electric current was turned off, quickly demagnetizing the compound which did magnetic work by inducing an electric current in the copper coils. Because no heat entered, the atomic magnets absorbed thermal energy and cooled the compound (25). T o measure such low temoeratures oresented a problem. The commonly used constant volume gas thermom&er, even onecontaining helium.deviated from ideal behavior at these temperaturesand was in error. An alternative was to measure the salt's magnetic susceptibility which according t o Curie's law (1905) varied inversely with the absolute temperature. Giauque made the first susceptibility measurements with a coil of several thousand turns of fine copper wire wound around the insulating vacuum jacket. As the Figure 6. Change of thermodynamic temperature with magnetic field at cow temoerature decreased. the alternatine current flowine stantentropyfor gadolinium phosphomolybdate hidecahydrate.Les PrixNobel through the coil also decreased while &e salt's magnetic 1949. susceptibility increased. From the relation between current and susceptibility Giauque obtained magnetic susceptibility values and then calculated the absolute temoerature from magnetization and therefore conflicted with the accepted Curie's law. For gadolinium compounds the curie constant view that the decrease was finite. Giauque obtained true is 7.880, giving the equation thermodynamic temperatures by plotting the change in enthalpy between zero field and some constant field (H) T = 7.880/Magnetic Susceptibility (5) against entropy dHfldS = T.The graph shows the variation of T with magnetic field strength and Giauque's calculation The Curie equation provided good low temperature values. But uoonaooroachineabsolute zero (1K) it failed because i t of the absolute temperature from the equation T = d H H S a t allowed entropy val
