Capillary Electrophoresis (CE) and its Basic Principles in Historical Retrospect. Part 3. 1840s –1900ca. The First CE of Ions in 1861. Transference Numbers, Migration Velocity, Conductivity, Mobility.

Abstract

Since electrophoresis is a physical phenomenon – it is the movement of dispersed charged particles relative to a liquid under the influence of a spatially uniform electric field - its history is not limited to its use as a separation method. The history of capillary electrophoresis in particular, i.e. electrophoresis in capillary-sized open tubes, therefore does not begin in the 1960s, as is commonly assumed, but already a century earlier, if one refers to its principles

Capillary electrophoresis of ions was first performed by the French physicist Edmond Becquerel in 1861, about the same year as that of colloidal particles. Becquerel owns therefore the priority. It was subsequently performed on three other occasions in the Long Nineteenth Century, by Wilhelm Beetz in 1865, by Wilhelm Ostwald and Walther Nernst in 1889, and by Friedrich Kohlrausch and Adolf Heydweiller in 1895. All of these experiments were carried out in the context of research on conductivity and ion migration.

Based on the theories of Grotthuß, Davy, and Faraday, it was believed until the 1840s that both the anions and the cations of a dissolved strong electrolyte - to which this review refers - travel at the same speed in an electric field.), migrate at the same velocity or speed in an electric field, but experimental observations in the mid-1840s cast doubt on this view. Wilhelm Hittorf was the first to show that these ions could migrate at different speeds, still consistent with Faraday´s laws. He was able to prove his hypothesis with experimental data and determined the migration velocities of the two types of ions in an electrolyte relative to the sum of their velocities, which he termed "Überführungszahlen" (transference or transport numbers). However, they did not initially yield the absolute velocities of the ions. This was achieved later by F. Kohlrausch, who devoted four decades of his research life, namely from the end of 1860 to about 1910, to the study of the conductivity of electrolyte solutions and the migration of ions. He discovered in 1879, that ions move independently from each other in solution (1st Kohlrausch law).

It is remarkable that until the late 1880s it was generally believed that free ions do not exist in solutions in the absence of an external electrical force, but that ions were always tightly bound to their counterions. This belief dated back to Grotthuß in 1805. Although Rudolf Clausius hypothesized in 1857 that free ions are actually present in solutions as result of their thermal motion, this did not find further resonance. It is also remarkable that during this whole period under consideration no attempt was ever made to separate ions with the same charge, although their different migration properties were already known.

Continuing his research, Kohlrausch found empirically in 1900 that at extremely low concentrations the molar conductivity of ions, i.e. the conductivity related to their concentration, is a function of the square root of their concentration and approaches a certain limit at infinite dilution (2nd Kohlrausch law). As a precursor to this law, he derived in 1885 for larger concentration ranges the little-known relationship of molar conductivity as a function of the cubic root of concentration. He calculated the migration velocities of ions from their conductivities and characterized the migration behavior by their mobility, which is a central property in electrophoresis.

Kohlrausch was certainly a formative investigator of the electrophoretic properties of ions, but his work focused mainly on strong electrolytes. This review covers the research results in this field in the period from 1840 to about 1910, but also reports on the historical background at this time and the personal background of some researchers who, despite important contributions, have been unjustly forgotten, as well as on researchers who were active outside the scientific community. Mention is made, for example, of Gustav Theodor Fechner, who was the first to prove the fact, indispensable for electrophoresis, that Ohm´s law also applies to electrolyte solutions. However, in contrast to the generally applied results of his investigations, he himself was rather ignored by later researchers.

The conductivities and electrophoretic properties of weak electrolytes, which were known to Kohlrausch and his contemporaries but hardly explicable to them, at least until 1884, are not discussed in detail in this review. In that year, Svante Arrhenius published his groundbreaking dissociation theory. This theory and the resulting consequences for the whole subject of electrolyte solutions require, however, a separate historical retrospect.