Capillary Electrophoresis and its Basic Principles in Historical Retrospect. Part 4. Svante Arrhenius´ Electrolyte Dissociation. From 56 Theses (1884) to Theory (1887)
Authors: Kenndler Ernst 
Published: September 2022

Since the main interest of Svante Arrhenius, a student at Uppsala University, was the electrical conductivity of highly dilute electrolyte solutions, which had not yet been determined at the beginning of the 1880s, he decided to determine experimentally the molecular conductivities of aqueous solutions of about fifty electrolytes and their dependence on the dilution. In his dissertation, which he began in the winter of 1882/1883, he summarized his results and considerations in 56 "theses". He observed that strong acids had a high molecular conductivity, which increased only slightly with increasing dilution. Weak acids, in contrast, had low molecular conductivities, but these increased abruptly above a certain dilution. Arrhenius' innovative hypothesis was that electrolyte molecules are composed from two parts, "an active (electrolytic) and an inactive (non-electrolytic) part," with the proportion of the active part increasing with increasing dilution at the expense of the inactive part. Moreover, the electrically active part, which conducts electricity, was also the chemically active part. Arrhenius introduced the activity coefficient, later quoted as the degree of dissociation, which indicated the proportion of active molecules to the sum of active and inactive molecules. He tentatively related activity coefficient to molecular conductivity. He assumed that the higher the activity coefficients of different acids at the same equivalent concentrations, the stronger they are. Arrhenius tested his hypothesis taking the heat of neutralization of acids with a strong base measured by Thomsen and Berthelot. Strong acids developed the highest neutralization heats, i.e., the activation heat of water, since they consisted entirely of active H+ and OH- ions, which combined to inactive H2O. Weak acids developed correspondingly less. The established parallelism between the molecular conductivities of acids and their heats of neutralization was the first proof of Arrhenius' hypothesis. He relied on thermochemistry and completed his dissertation. He presented his dissertation in June 1883 and published it in 1884 to obtain his doctorate. At that time, Wilhelm Ostwald was investigating the affinities of acids to bases, i.e. the intensity of the effects of acids on the rates of reactions they cause. He took the rate constants as a measure of the relative strength of the acids. After receiving Arrhenius' thesis, he measured the acid´s molecular conductivities and found a remarkable proportionality to the reaction rate constants of the hydrolysis of methyl acetate and the inversion of cane sugar caused by them. This was the second proof of Arrhenius' hypothesis, based on the results of chemical kinetics. A memoir presented in 1885 by J. H. van 't Hoff on the analogy between the osmotic pressure of a highly dilute solution separated from the pure solvent by a semipermeable membrane and the pressure of an ideal gas containing the same number of particles as the solution led to probably the most convincing proof of the Arrhenius hypothesis. This analogy corresponded to Avogadro's well-known law, which is PV=RT. He found that the pressure for non-conductors such as glucose followed this law, but was higher for electrolytes. This deviation was accounted for by the van 't Hoff factor i, which indicates into how many particles the solute - at least partially - has dissociated, so that the modified law is PV=iRT. The factor i could be deduced from Raoult's freezing point depression, and could also be calculated using Arrhenius' degree of dissociation α. The degree of dissociation, in turn, was determined from the ratio of the conductivity of a dilute electrolyte solution and that under limiting conditions. The agreement found between the factors i determined by the two independent methods was the third proof of the Arrhenius hypothesis. There was a fourth proof, namely the additivity of physical properties. With these four nonelectrical and independent proofs, the 56 theses of Arrhenius' dissertation became the groundbreaking theory of dissociation of substances dissolved in water, which he published in 1887. In 1903 the Nobel Prize in Chemistry was awarded to him "in recognition of the extraordinary services he has rendered to the advancement of chemistry by his electrolytic theory of dissociation”.