Bjerrum, Niels Janniksen

(b. Copenhagen, Denmark, 11 March 1879; d. Copenhagen, 30 September 1958)

chemistry, physics, history of science.

Bjerrum was the son of the well-known ophthalmologist and university professor Jannik Petersen Bjerrum and Anna Johansen, and the nephew of Kirstine Bjerrum Meyer, who edited the works of Oersted (1920) and wrote an important treatise on the history of the concept of temperature, Temperaturbegrebets udvikling (1909). He completed under S. M. Jørgensen with a dissertation entitled Studier over basiske kromiforbindelser; Bidrag till hydrolysens teori (Copenhagen, 1908). He had begun to lecture on elementary inorganic chemistry in 1907, and in 1914 he was made professor of chemistry at the Royal Veterinary and Agricultural College in Copenhagen, a post he held until his retirement in 1949. During the early years of his career he studied with Robert Luther in Leipzig (1905), Alfred Werner in Zurich (1907), Jean Perrin in Paris (1910), and Walther Nernst in Berlin ( 1911). In 1907 Bjerrum married Ellen Emilie Dreyer. Their son Jannik has been professor of chemistry at the University of Copenhagen since 1948. In many ways his work is an extension of his father’s.

In 1928 Bjerrum was awarded the Oersted Medal. He was a member of the Carlsberg Foundation, the Rask-Oersted Foundation, and the Committee of the Solvay Institute for Chemistry, as well as the academies of science of Denmark, Norway, Sweden, Vienna, and New York. His honorary memberships were in the chemical societies of Belgium, the Netherlands, Sweden, and Switzerland.

Noteworthy for the historian of science, primarily because it presents the point of view of a prominent theoretical and experimental physical chemist, is Bjerrum’s Fysik og kemi (in Det nittende aarhundrede, 18 [1925], 71–192). This volume gives his interpretation of various late nineteenth-century developments in atomic theory, thermodynamics, the electromagnetic theory of light, relativity and the physics of the ether, and the structure of the atom. In a lecture of 1922, “Kemiens udvikling i det 19. aarhundrede,” Bjerrum emphasized the historical importance if the “mathematization” of chemistry, the exploitation of the atomic theory for stereochemical considerations, and the coordination of science and technology, of theory and practice.

Bjerrum’s contributions to chemical physics, an outgrowth of his work with Nernst in Berlin, were made primarily in four papers (1911–1914). They deal with the application of the kinetic and quantum theories, and employ information obtained from absorption measurements in the infrared to elucidate the constitution and the optical and thermal properties of matter. His theoretical studies on specific heat as a function of temperature for gases represent advances over the specific heat studies that had been made for solids by Einstein, Nernst, and Lindemann. In this work Bjerrum succeeded in demonstrating the interdependence of specific heats and the spectrum as required by the quantum theory. The infrared absorption spectra of water vapor were further related, on the quantum hypothesis, to line broadening caused by molecular rotational frequencies that vary discontinuously and to radiating atoms that do not rotate—thus providing agreement with specific heat investigations which suggest that the rotational energy of atoms must be very small.

In a number of more general papers, such as “Nyere undersøgelser over atomernes bevaegelsermed saerligt henblik paa kvantehypotesen”(1915) and “Moderne atomlaere og kvanteteori” (1919), Bjerrum revealed an extraordinarily keen appreciation of the significance of the quantum theory for atomic-molecular problems in general.

Physical chemistry, theoretical as well as analytical, was Bjerrum’s lifelong interest and is the subject of the major part of his publications. As early as 1909 Bjerrum proposed a new form for the electrolytic dissociation theory. In 1916, at the sixteenth meeting of Scandinavian scientists in Kristiana (Oslo), he presented, in a most convincing form, his now-celebrated view on the dissociation of strong electrolytes—according to which some acids and hydroxyl compounds, and most salts, are almost completely dissociated into ions in the dissolved state. Arrhenius, who was chairman of the meeting, had won the Nobel Prize in chemistry in 1903 for his electrolytic theory of dissociation, and was not willing to accept this extension of his own theory. Bjerrum’s view, according to which the “anomalous” behavior of strong electrolytes should be interpreted in terms of interionic forces, was extended in a second paper in 1916 and then explored in depth in a series of studies (1920–1932) on electrolytic dissociation theory. These studies also dealt with the activity and distribution coefficients of ions, osmotic pressure, association of ions, the Debye-Huckel theory, and the solubility of gases.

The currently accepted method of introducing activity coefficients into the expression for the velocity of a chemical reaction was published in 1923 by the Danish chemist J. N. Brønsted in a paper entitled “Zur Theorie der chemischen Reaktionsgeschwindigkeit.” Bjerrum’s papers on chemical kinetics and the discussions that took place between the two men appeared in the Zeitschrift für Chemie of 1923–1925. Bjerrum’s view on how his own new theories regarding acids, bases, and salts developed within the context of the older views of Brønsted is treated admirably in a lecture delivered before the Danish Chemical Society in 1931 (“Syrer, salte og baser”).

Bjerrum’s joint paper on Brownian motion with Jean Perrin, “L’agitation moléculaire dans les fluides visqueux” (1911), is noteworthy for having demonstrated that the equilibrium distribution of particles is independent of the viscosity of the fluid in the gravitational field.

The study of buffer mixtures and indicators, and the measurement of the hydrogen ion concentration of solutions, was an early and continuing interest. In a paper of 1905 Bjerrum introduced his well-known extrapolation method for the elimination of the diffusion potential. The theory upon which this study rests was developed further and improved by E. A. Guggenheim, who worked with Bjerrum for a number of years. In his study of the theory and the sources of error in acidimetric and alkalimetric titrations and of buffer solutions (Die Theorie der alkalimetrischen und azidimetrischen Titrierungen [Stuttgart, 1914]), Bjerrum showed how to determine the end point of a titration and how to estimate the error that accompanies the choice of pH values associated with the use of a particular indicator.

Bjerrum’ contributions to the theory of acids and bases include a novel method of using the experimentally determined strength constants of different acidic and basic groups in a molecule to establish the constitution and dissociation constants of ampholytes, particularly of amino acids. In a paper of 1923 he applied the notion of the different strength constants of polybasic acids to the determination of molecular distances. On the practical side, Bjerrum’s contributions to the needs of a country where agriculture is of paramount importance are seen in his papers devoted to the factors that determine the pH, and therefore the reaction, of the soil, the hardness of water, and the general application of physicochemical measurements to agricultural problems.

In his study of the coordination chemistry of inorganic complexes, Bjerrum went beyond the classical methods of analysis and synthesis of his teacher Jørgensen by emphasizing the importance of physicochemical principles. In 1906 Bjerrum published a comprehensive 120-page study on the chromic chlorides (Studier over kromiklorid) that revealed the existence and mode of isolation of the previously unknown chromium monochloropentaquo complex [Cr Cl (H²O)₅)⁺⁺. Two years later, in his doctoral dissertation on the theory of hydrolysis of chromium compounds, he investigated the formation and relation between “truly basic” and “latently basic” complexes. Bjerrum’ papers on the complex chromium and gold salts span a period of more than forty years.

Bjerrum also made substantial contributions to colloid theory, concentrating on the study of substances with high molecular weights, their colloidal properties (e.g., charge), and the preparation of collodion membranes with reproducible permeabilities (1924–1927).

The function of the thiocyanate group as a ligand in chromium and gold compounds was the subject of two significant papers of 1915 and 1918. In the second paper Bjerrum and Aage Kirschner proposed a sequence of reactions that explained the overall complex kinetics of the aqueous decomposition of dithiocyanogen—a compound the preparation of which from nonaqueous solution was first accomplished by Erik G. Söderbäeck in 1919. In 1949 Bjerrum investigated the gold chloride complexes.

During the last decade of his life, Bjerrum extended interest in problems of molecular structure to the study of ice. This work, summarized in Structure and properties of Ice (1951), treats the position of the hydrogen atoms and their zero-point entropy, changes in configuration, ionization and “molecular turns” and the proton-jump conductivity of ice and water.

On the frontier where physics and chemistry interact, Niels Bjerrum achieved world renown during the early decades of this century. He published about eighty scientific papers (not including translations) and ten books and monographs. Best known among the latter is his Laerbog i uorganisk kemi (1916–1917), which went through six editions and was translated into English, German, and Russian.

BIBLIOGRAPHY

Niels Bjerrum, Selected papers, ed. by friends and coworkers on the occasion of his seventieth birthday (Copenhagen, 1949), contains, besides the 27 selected papers trans. into English, J. A. Christians en, “A Survey of the Scientific Papers of Niels Bjerrum,” a foreword by Niels Bohr, and a bibliography of Bjerrum’ scientific publications, books, and papers (1903–1948), items 1–92; J. A. Christiansen, “Niels Berrum,” in Fysisk tidsskrift57 (1959), 24–36, appends a list of Bjerrum’ publications (items 93–103) that appeared after the publication of the papers listed in Selected Papers; Aksel Tovborg Jensen, “Niels Bjerrum” in Oversigt over det Kongelige Danske Videnskabernes Selskabs Virksomhed (1958/1959), 99–113, is an excellent short account of his life. See also Stig Veibel, “N. J. Bjerrrum” in Dansk biografisk leksikon III (1934), 183–185; and Kemien i Denmark, II, Dansk kemisk bibliografi 1880–1935 (Copenhagen, 1943), 61–67, which contains a list of Bjerrum’s 80 publications that appeared 1908–1935.

Erwin N. Hiebert