Henry's Law A Historical View John J. Carroll Depaltment of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6
Henry's law is an important part of the modern science of physical chemistry. However, it is used in all branches of science where the solubility of a gas in a liquid is an important phenomenon. Notwithstanding, many people are surprised by the age of Henry's law. This paper places Henry's law in a historical context. Henry's law is as old as the science of modem chemistw. In its simplest form, Henry's law states that the solubility of a gas in a liquid is proportional to the partial pressure of the gas. That is
where Pi is the partial pressure of component i in the gas; H, is the Henry's law constant for solute i in solventj; and xi is the mole fraction of component i in the liquid. Other concentration units are often used for the liquid phase (e.g., molarity, molality, and weight fraction). The Roots of Modem Chemistry At the turn of the 19th century, the science of chemistry was in its infancy Most historians of science mark the beginning of modem chemistry with the fall of the phlogiston theory late in the 18th century (1,2). Led by Antoine Lavoisier in France and others across Europe, the new science slowly took form. William Henry, a h whom the law was named, was from this generation of chemists. It would be misleading to claim that modern chemistry had a snecific vear of birth and eauallv unfair to credit a single i h v i d i a l with its establishment. Nevertheless, it is convenient to mark 1789 as the beeinnine of the modem science of chemistry. In this year Lavoisier published nuit6 d'El6mentaire de Chemie, the first textbook of mode m chemistry.
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The Acceptance of Henry's Law In December 1802, H e d s experimental observations were presented to the Royal Society of London. Henry studied the effect of pressure on the solubility of a few gases in water (3). Although his apparatus was crude and his materials probably impure, he came to a bold and general conclusion. He stated
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...water takes un...of eas condensed bv one. two or more additional atmoapheres, a quantity which, ordrnanly rornprrsscd, would bc equal to twice, thnce, &r. the volumc absorhed under the common pressure of the atmosphere. Although the language is a bit old-fashioned, this is a recognizable statement of Henry's law. The proportionality constant implied by this statement was called the Henry's law constant. It is interesting to note that, by his own admission, the law was not strictly obeyed. By frequent repetition of the experiments, I obtained resulta differing from the general principle above stated; but for all
practical purposes, I apprehend, the law has been announced with sutlident accuracy. Also, water was the only solvent that Henry examined. Today the law is applied in some form to virtually all solvents. Throughout the 19thcentury investigators tried to prove or disprove Henry's law. Many different solutes and solvents were examined. By late in the 19th century, it was generally accepted, although a few exceptions remained. In fact, many investigators incorporated the law into their experiments. Gas solubilities were measured a t near-atmospheric pressure and values were reported a t 1 atm aFter being corrected using Henry's law. This practice continued into the 20th century. The Nonconformists Henry was a contemporary of such famous British scientists as Joseph Priestley and John Dalton. Many of these scientists were nonconformists. They were Protestants, that is, not members of The Church of England. Most refused to pledge allegiance, and thus their freedoms were restricted. Because they were not allowed to enter the great schools of England, they founded their own schools. They taught science and trades, as well as their religion. The nonconformists applied the conviction of their religious beliefs to their work. Thus, they were hard workers.
Applied Science and Industrial Pursuits The nonconformists were, by and large, industrialists, not strictly scientists. Their scientific pursuits were usually applied rather than pure. For example, the nonconformist scientists studied chemistry and not astronomy. It was the nonconformists who drove the Industrial Revolution in Britain. The nobles and gentry gladly s m n dered these industrial jobs to the nonconformists. Many nonconformists became wealthv selline ., material .,eoods to the upper class. Not surpnsmgly, the nonconformists were from the industnal centers hke Blrmineham and Manchester and not London, the intellectual i d cultural center of England. William Henry Henry was fmm Manchester where his father, Thomas, was an apothecary. The elder Henry was a founding member of the Manchester Literary and Philosophical Society and was its president in 1807. The son was also a member (2). Although the nonconformists were allowed to become members of the Royal Society (later in his life William Henry was elected a Fellow) they were freer to discuss their own philosophy in the provincial societies. The younger Henry was trained as a physician, but practiced medicine for only a few years. He took over the chemical works that his father established for makine remedies. notably magnesia. William Henry was also a prolific
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writer. and he oublished several oooular chemistrv textThe elder Henry was a scientist of some repute as well. Thomas Henry discovered that a plant subjected to an environment of ~ u r carbon e dioxide would oerish. whereas in a mixture d i k e in carbon dioxide it thived better than in pure air (4). William Henry's son, William C. Henry, wrote a famous biography of John Dalton. Hence, the fathers and sons are oRen confused, especially William and William C. John Dalton
One of Henw's close friends was John Dalton. Dalton was a Quaker &hoolmaster. He too was a member of the Manchester Society, for a while serving as its president (2). He proposed his famous atomic the06 at abbut the same time that Henry presented his law. Dalton's principles were presented over a ten-year period beginning in 1802. Dalton's Law of Partial Pressures As a part of these theories Dalton proposed the law of partial pressure. As with many aspects of Dalton's theories, this law was widely criticized at first. Henry, who was initially opposed to this law, was among the first converts. Dalton presented a paper to the Manchester Society that would bewme famous (5).Although the paper was on the solubilities of gases, its fame arises because appended to this paper was Dalton's first table of relative weights (now called molar masses). However, in this paper Dalton showed that the solubility of individual components in a Dalgaseous mixture depended on their partial ton reasoned that the solubility of each component of a gaseous mixture was indeoendent of the other com~onentsin the gas. Henry concluded that Dalton was indeed correct. The solubility of a gaseous component in a mixture depended only on its own "partial" pressure (4). This is the form of Henry's law that is used most often today.
Phase Equilibrium J. Willard Gibbs
In the area of phase-behavior calculations, Henry's model came well before any rivals. It was not until 1875 that J. Willard Gibbs, an American physicist, eloquently formulated the mathematical theory behind phase equilibrium (6). The paper On the Heterogeneous Equilibria of Substances is a scientific landmark. In it, Gibbs applies the principles of thermodynamics to obtain solutions to problems in chemistry. Thus, Gibbs was the fxst to unite the two previously separate sciences. More than 80 years after Henry published his law, Franqois-Marie Raoult, a French chemist, published his findings on the vapor pressures of solutions (7),which we now call Raoult's law. So closely related are the laws of Henry and Raoult, in their modem sense, that they are usually taught at the same time to students of physical chemistry. However, their publications are separated by more than three-quarters of a century from a historic point of view. Gilbert Lewis
At the turn of the 20th century, Gilbert Lewis,an American chemist, ~ostulateda new thermodynamic auantitythe fugacity (8,. The useofthe fugacity iubstantLally eased the application of Gibbs' principles to physical problems. Modern Cornerstones
The concepts of Henry, Raoult, Gibbs, and Lewis remain the corner stones of modern phase-equilibrium calculations. In some form, these theories continue to be used today. We now have better models and computation devices that make these calculations more practical, but the underlying theory is essentially unchanged.
Joseph Priestley
Summary
Another of Henry's contemporarieswas Joseph Priestley, who, more than any other individual, represented what it was like to be a scientist in the nonconformist society. Priestley lived in Birmingham where he was a Unitarian minister as well as a scientist. Priestley was a prolific investigator, but he is perhaps most famous for his discovery of oxygen, although credit for this discovery should be shared with othem. In Birmineham. the orovincial societv was the Lunar Society, so-called be'cause they met monthly. Priestley was a prominent member. Priestley was also a supporter of the French and American revolutions, positions that were counter to mainstream Britons. On the second anniversary of the "Storming of the Bastille" (ironically the same year of Lavoisier's death), a mob ransacked Priestley's house, and hence his lab, in search of incriminating documents. None was found. Neither Priestley nor his family were at home at the time. Eventually he fled to London, where he sought wmfort from his fellow members of the Roval Societv. but thev gave him little support. Eventually, 6e went toukerica a"t the invitation of Heniamin Franklin. Priestlev refused many offers of acadedc and scientificpositions."
Although this brief discussion places Henry's law in a historical context, it was be no means a thorough history. Unfortunately, many important events and personalities have been omitted. However, from this discussion we can see the place of Henry's law in the history of the science of chemistry. Specifically, Henry's law was proposed at the beginning of the era of modern chemistry, which is surprisingly early, and the law remains very popular today. From the first and second laws of thermodvnamics and their conseauences. Henry's law can be th&retically extended tocondiiions be: yond the reach of Henry's simple apparatus (91.
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Literature Cited 1. Schner, C. 3. ThEuolution ofPhysleolSciemx-M~siIdam ff B~rIieiotl i m t o t h P m m ( ; Universih.Plesaaf Amuica: Lanham,MD, 1960; Chapters 9and 11. 2. Mason, S. FAHistaryofSeisnee: M a d l a n : NewYmk, 1962; Chapfer26. 3. H e n ~W, R o d Soe LandonPhil. h s 1803,93,2943; 1805,93,27P276. 4. Partington, 3. R.A H i s f o p of Chamistry; M a d a n : Iandon, 1962; Val. 3, pp 690692. 773-715.823-826.
7. Raoult,F.-M. ~ ~ & t . R o n d1887,104,1430-1433. 8. Le-, G. N. Roc Am. A d M SSCi. 19W,36(9). 146-168: 1 8 0 1 . 3 7 ( 3 ) . 4 ~ . 9. C m U , 3 . 3 C h .E q .Pmg 1801,84(9),48-52.
