Kohlrausch, Friedrich Wilhelm Georg

(b. Rinteln, Germany, 14 October 1840; d. Marburg, Germany, 17 January 1910)

chemistry, physics.

Kohlrausch is best known for his experiments on the electrical conductivity of solutions. The son of Rudolph Kohlrausch, he was educated at the Polytechnikum at Kassel and at the universities of Marburg, Erlangen, and Göttingen, receiving his doctor’s degree at Göttingen in 1863 under Wilhelm Weber. He then acted as assistant in the astronomical observatory at Göttingen and in the laboratory of the Physical Society at Frankfurt before being appointed extraordinary professor at the University of Göttingen (1866-1870).

Kohlrausch held the professorship of physics in the Polytechnikum at Zurich (1870-1871), at Darmstadt (1871-1875), and at the University of Würzburg (1875-1888), and then succeeded Kundt as director of the physical laboratory at Strasbourg. On the death of Helmholtz in 1894, he left Strasbourg to accept the appointment of director of the Physikalisch Technische Reichsanstalt at Charlottenburg. He was elected a member of the Academy of Sciences in Berlin in 1895 and was a member of scientific societies in many countries.

Kohlrausch’s contributions to physical science were characterized by a high degree of precision. They included research on the electrical conductivity of electrolytes, on elasticity (begun in 1866), on magnetic measurements (begun in 1869), and on the determination of the electrochemical equivalent of silver in 1886 with his brother Wilhelm.

When Kohlrausch began his research in conductivity of solutions, the structure of a solution was controversial. The determination of whether or not Ohm’s law applied to electrolytic solutions was confused by the question of polarization of the electrodes. When a direct current was forced through the electrolyte, ions gathered around the electrodes and partially neutralized the electric potential, decreasing the current; the effect produced inconsistent values for the conductivity of the solution being measured.

In 1868 Kohlrausch began to study the problem, developing the technique of using an alternating current rather than a direct current. In this way the decompositions which took place at the electrodes was reversed many times each second. The alteration of the solution was thus kept at a minimum while conductivity measurements were being made. At the same time the products of decomposition were not allowed to collect at the electrodes, and polarization was thus also reduced to a minimum. In a paper published in 1870 with W. A. Nippoldt, Kohlrausch showed that there was a maximum in the conductivity curve of sulfuric acid diluted with increasing amounts of water. He concluded that there was something fundamentally associated with the act of mixing itself that imparted conductivity to solutions. In a later paper, written with Otto Grotrian (1874), Kohlrausch showed that the conductivity of solutions increased with increasing temperature.

In 1876 Kohlrausch pointed out that, following the work of Hittorf on the migration of ions, the ions in very dilute solutions did not encounter appreciable resistance to their movement from other similar ions, and that the water in which the ions were dissolved provided the only friction serving to retard their motion. He concluded that “in a dilute solution every electrochemical element has a perfectly definite resistance pertaining to it, independent of the compound from which it is electrolyzed” (The Fundamental Laws of Electrolytic Conduction, p. 86). Thus the conductivity of electrochemically equivalent solutions of two electrolytes which have a component in common would vary inversely with the transference numbers of the common component. Kohlrausch was able to substantiate his conclusion by a comparison of the transference numbers measured by Hittorf with his own values for the conductivity of the same solutions.

The work produced by Kohlrausch on the conductivity of electrolytic solutions was important in leading to the eventual statement by Arrhenius postulating the electrolytic dissociation theory of solution structure.

Kohlrausch was also one of the first teachers to prepare an instructive work on physical laboratory methods, Leitfaden der praktischen Physik (1870). It was widely used and republished, being translated into four languages, including English.

BIBLIOGRAPHY

I. Original Works. Kohlrausch’s works have been collected and published under the title Gesammelte Abhandlungen, 2 vols. (Leipzig, 1910-1911). He summarized his contributions and their place in the field in other books: Das Leitvermögen der Elektrolyte, Methode, Resultate, und Anwendungen (Leipzig, 1898) and Die Energie oder Arbeit und die Anwendungen des elektrische Stromes (Leipzig, 1900).

The first eight eds. of his laboratory manual were entitled Leitfaden der praktischen Physik (1st ed., Leipzig, 1870), while the 9th through the 16th eds. were published under the title Lehrbuch der praktischen Physik (9th ed., Leipzig, 1901). The memoir in which Kohlrausch stated his final conclusions with respect to conductivity and ions was “Ueber das Leitungsvermogen der in Wasser gelosten Electrolyte in Zusammenhang mit der Wanderung ihrer Bestandtheile,” in Göttingen Nachrichten (1876), p. 213; it was republished in Harry Manly Goodwin, The Fundamental Laws of Electrolytic Conduction (New York-London, 1899), with memoirs of Faraday and Hittorf.

II. Secondary Literature. For discussions of Kohlrausch’s work on conductivity of solutions, see Wilhelm Ostwald, Elektrochemie, ihre Geschichte und Lehre (Leipzig, 1896), or Harry C. Jones, The Theory of Electrolytic Dissociation and Some of its Applications (New York, 1900).

Ollin J. Drennan