RAOULT, FRANçOIS MARIE

(b. Fournes, France, 10 May 1830; d. Grenoble, France, 1 April 1901)

chemistry.

Raoult early chose a scientific career. Despite family encouragement, he lacked the financial resources to complete his work at the University of Paris, although he presented a brief note on electrolytic transport and on electrical endosmosis to the Académie des Sciences. This note, which appeared in Comptes rendus … de l’Académie des sciences in 1853, was the first of more than 100 published papers.

In 1853 Raoult was employed as a teacher at the lycée in Reims, then moved to the College of St. Dié as régent de physique. At St. Dié he received the baccalauréat ès lettres and baccalauréat ès sciences, passed the licencié examination, and was appointed an agrégé de l’enseignement secondaire spécial. In 1862 he moved to the lycée at Sens, where, on his own, he carried out research on the electromotive force of voltaic cells; this research led in 1863 to the docteurès sciences physiques from the University of Paris.

In 1867 Raoult was called to the Faculté des Sciences of Grenoble as chargé du cours de chimie and in 1870 was elevated to the chair of chemistry. Raoult taught and carried out extensive research for thirty-one years at Grenoble, almost until the day he died. Beginning in 1899 he was elected dean of the Grenoble Faculté des Sciences for five consecutive terms and was instrumental in bringing about a major reorganization of the University of Grenoble in 1896.

In 1865 Raoult was appointed an officier d’Académie. In 1872 he became an officier de l’Instruction publique, and was awarded the Médaille des Sociétés Savantes. In 1883 he was presented the Prix International de Chimie La Caze, ten thousand francs, and was named correspondant de l’Institut de France, which awarded him its biennial prize in 1895. In 1892 he was awarded the Davy Medal of the Royal Society. He also became a foreign fellow of the Chemical Society of London in 1898, and of the Academy of St. Petersburg in 1899. In 1890 he was chosen chevalier of the Legion of Honor, elevated to officer in 1895, and honored by the high distinction of commander in 1900.

Raoult’s work may be divided into three stages: physical, chemical, and physicochemical. During the first stage, which lasted until Raoult moved to Grenoble, he discovered that the chemical heat of reaction of galvanic cells of the Daniell type was generally different from the heat equivalent of the electrical work done in these cells. He also used a voltameter, an instrument that measures the quantity of electricity by the amount of electrolysis, to study the heat evolved in voltaic cells. Raoult was one of the first to recognize that the electrical work done by voltaic cells was not equal to the heat evolved by the chemical reaction driving these cells. He showed that whenever the electrical work done in voltaic cells was less than the heat of all reactions, heat was evolved. Moreover, he stated that changes in concentration, oxidation, and in acid-base relationships, but not changes in aggregation such as dissolving, melting, or solidifying, were the sources of the electromotive force of voltaic cells.

These experiments did not excite much attention despite the fact that by 1870 Raoult had published twelve papers on his thermochemical and electrochemical work, almost all of which appeared in Comptes rendus and Annales de chimie. The theories through which his work could have been fruitful had not yet been evolved, and the appreciation of his pioneering work had to wait until the theoretical studies of Gibbs, Helmholtz, van’t Hoff, Arrhenius, and Nernst made its value apparent to all.

From 1870 to about 1882 Raoult turned his attention to studies on the effect of carbon dioxide on animal respiration, on the absorption of ammonia by ammonium nitrate, and on the rate of inversion of cane sugar under the influence of sunlight. Fourteen papers appeared during this stage, mainly in Comptes rendus. The papers show excellent and careful work but were relatively trivial and far less interesting than the pioneering investigations of his earlier years. He had begun, however, to study the vapor pressure and freezing points of salt solutions, and the freezing point of alcohol solutions. This interest was to lead him to the physicochemieal third phase in which he made his most significant contributions to chemistry.

Richard Watson in 1771 and Charles Blagden in 1788 had shown that the freezing point of salt solutions is lowered in proportion to the weight of salt dissolved. This work was extended in 1871 by Louis de Coppet, who calculated what he termed “atomic depressions” in compounds lowering the freezing point of water. In 1878 Raoult experimentally examined the lowering of the freezing point and of the vapor pressure of water caused by eighteen different salts. He found that the lowering of the vapor pressure caused by each of these salts was approximately proportional to the lowering of the freezing point. This work confirmed Guldberg’s theoretical, thermodynamic proof in 1870 of the proportionality of the lowering of the freezing point, the rise of the boiling point, and the lowering of vapor pressure that resulted from the dissolving of a homogeneous substance in water. In 1882 Raoult published a significant paper on his investigation of the lowering of the freezing point of a large number of aqueous solutions of organic compounds.

In a series of papers appearing in Comptes rendus from 1882 to 1884, Raoult described a method for finding the molecular weight of an organic compound by the determination of the lowering of the freezing point of water that resulted from dissolving that compound in water. He first found by experimentation the simplest formula of the compound and the coefficient of lowering of the freezing point. Then he calculated the “molecular depression” from the sum of the“atomic depressions” that he had previously elucidated. From the formula below, the molecular weight could be directly ascertained: M = T/a, where M is the molecular weight; T is the molecular depression; and a is the coefficient of lowering of the freezing point.

This method for determining molecular weight soon received wide application, although most chemists turned to the somewhat simpler but completely analogous elevation of the boiling point with the development of Beckmann’s thermometer for this purpose in 1888. Victor Meyer and Emanuele Paternò publicized Raoult’s method because chemists could ascertain with relative ease the molecular weight of organic compounds.

Raoult soon turned to the anomalous results with salts in water, which had puzzled previous investigators. He classified the salts he used according to the valence of the radicals and found that the lowering of the freezing point could be accounted for by assigning certain numbers to these radicals. He demonstrated that the freezing point lowering obtained with these salts was consistent with the hypothesis that the salt radicals themselves acted as if they existed independently in the solution, and that certain radicals were more effective than others in lowering the freezing point of water. With the statement that “the neutral salts of mono and di-basic salts … act as if the electropositive and electronegative radicals of these salts when dissolved in water solution do not combine, but remain as simple mixtures” (Annales de Chimie, 20 [1890], 355), Raoult showed that he had come to accept much of Arrhenius’ work on ionization.

In 1892, using extremely dilute solutions, Raoult found that the molecular constant for organic compounds dissolved in water was 18.7, whereas for sodium and potassium chloride, it was 37.4 and 36.4 respectively. Raoult argued that this was a strong argument for Arrhenius’ hypothesis of electrolytic dissociation, which had been published in 1887.

During this time Raoult experimented with vapor pressure lowering in solutions. He confirmed the work of Adolph Wüllner and Lambert H. J. von Babo that the relative lowering of the vapor pressure, (f—f″)/f is independent of the temperature, where f—f″ is the lowering of the vapor pressure, and f is the vapor pressure in dilute solution. He also showed that the lowering of the vapor pressure of water by a nonvolatile solute is proportional to the concentration of the solute.

In a brilliant paper in 1886, Raoult derived an expression for the relative lowering of the vapor pressure of solutions in ether, which is still in use:

where P is the number of grams of solute in 100 grams of solvent; M is the molecular weight of solute; and K is the constant.

In 1887 Raoult made an extended study of a large number of solvents other than ether. He formulated the law: “One molecule of a non-saline substance dissolved in 100 molecules of any volatile liquid decreases the vapor pressure of this liquid by a nearly constant fraction, 0.0105” (Comptes rendus, 104 [1887], 1433).

Raoult’s law precisely coincided with an equation that van’t Hoff had derived thermodynamically in 1886. Raoult was deeply moved by this harmony in their work, and he declared that the “agreement between experiment and theory is therefore, on all points, as complete as one could desire in the case of these substances” (Comptes rendus, 105 [1887], 859).

Raoult was a pioneer in demonstrating that the lowering of the vapor pressure of solvents by solutes, the rise in the boiling point of the solvent by adding a solute, and the lowering of the freezing point of the solvent, depend only on the ratio between the number of solute molecules and solvent molecules.

Raoult was a careful and accurate experimenter. He built most of his own apparatus himself because he felt that he could rely only on his own craftsmanship to attain the precision he sought. He was not a theoretical chemist, and he rarely resorted to the kind of mathematical analysis introduced by the thermodynamic chemists of his time. He gloried instead in laborious and painstaking experiments through which he accumulated a mass of useful information on the freezing point and vapor pressure depressions of dozens of solvents and solutes under a variety of conditions. These data were heavily drawn on by van’t Hoff for his study of osmotic pressures and by Arrhenius, who explained Raoult’s anomalous results with salts that ionize in solution.

Without question, Raoult was a leading French experimental physical chemist in the nineteenth century. As Victor Meyer said in 1890,

What a change our conceptions will have to undergo if we have to accustom ourselves to regard a dilute solution of sodium chloride as one containing not undecomposed molecules of this salt, but separated atoms of sodium and chlorine.

We owe these revolutionizing innovations to the investigations of van’t Hoff, Arrhenius, Ostwald, Planck, and de Vries, but in regard to experimental research especially to the splendid work of Raoult, which during recent years has effected this mighty theoretical progress [Victor Meyer, “The Chemical Problems of Today,” trans. by L. H. Friedburg in Annual Report of the Board of Regents of the Smithsonian Institution, 1890 (Washington, D.C., 1891), p. 371].

BIBLIOGRAPHY

I. Original Works. Raoult wrote more than 100 papers, the majority of which are listed in Poggendorff, III (1898), 1089; IV (1904), 1212; and in Royal Society Catalogue of Scientific Papers, V, 97; VIII, 700; XI, 106; XVIII, 52–53. Among the most important are “Recherches sur les forces électromotrices et les quantités de chaleur dégagées dans les combinaisons chimiques,” in Annales de chimie, 4th ser., 2 (1864), 317–372; “Loi de congélation des solutions aqueuses des matières organiques,” in Comptes rendus hebdomadaires des séances de l, Académic des sciences, 94 (1882), 1517–1519; “Méthode universelle pour la dètermination des poids motèculaires,” in Annales de chimte, 6th ser.,8 (1886), 317–319; and “Sur les tensions de vapeur des dissolutions,” in Annales de chimie, 20 (1890), 297–371. He summarized his work in two small volumes: La tonométrie (Paris, 1900); and La cryoscopie (Paris, 1901).

II. Secondary Literature. There is no detailed study of Raoult’s life and work. The major sources are J. H. van’t Hoff, “Raoult Memorial Lecture,” in Journal of the Chemical Society, 81 (1902), 969–981; William Ramsay, “Prof. François Marie Raoult,” in Nature, 64 (1901), 17–18; Harry C. Jones, “François Marie Raoult,” in Science, n.s. 13 (1901), 881–883; and Frederick H. Getman, “François-Marie Raoult—Master Cryoscopist,” in Journal of Chemical Education, 13 (1936), 153–155. Excellent brief accounts of his scientific work are in M.M. Pattison Muir, A History of Chemical Theories and Laws (New York, 1907), 145–170; and J. R. Partington, A History of Chemistry, IV (London, 1964), 645–650.

Louis I. Kuslan