Book Reading Update - Fontana History of Chemistry Chapter 10

This chapter gave an account of the development of the ionic theory, primarily attributed to Arrhenius but, much like everything other theory or law in science, evolved into what it is today based on the work of many others.  The development and refinement and drama surrounding the ionic theory was the core of the emergence of the field of physical chemistry as the author contends.

When chemists first started referring to the field of physical chemistry , it was initially the study of the physical properties of chemical substances, in large part inspired by studies of optical activities observed for stereoisomers.  The author gave some examples of findings that would have fallen under the "physical chemistry" rubric such studies on correlations between boiling points and molecular size and complexity, refraction, polarimetry, specific heats and atomic weight, what became known as Hess' law, etc.

On the development of the ionic theory and Arrhenius' subsequent formulation of it had a big help from Raoult and van't Hoff.  The author began the chapter with a short story on how Arrhenius' dissertation thesis on the dissociation of particles in solution was not well-received.  His ideas, however, were supported by subsequent experiments carried out on properties of solutions.  Raoult carried out studies on the effects of particles on the freezing points of solvents and observed a relationship between the size of the FP depression and the number of solute particles relative to solvent particles. Van’t Hoff observed an analogy between osmotic pressure and gas laws as expressed by the mathematical relationship PV=kT.  With Arrhenius’ insight on dissociation, he was able to determine the physical significance of the proportional constant needed to make the equation work for acids, bases, and salts, as a measure of the dissociation of the solute particles.  Later experiments showed a good agreement between i values measured (van't Hoff in particular)  in the context of different physical processes supporting Arrhenius’ interpretation of this quantity as a measure of dissociation of the ions.  Essentially this series of events tied together point to the idea that the behavior of solutes in solutions was illuminated by how the properties of the solvent and solution have changed and the number of particles is the unifying concept (we now teach this as part of the section on colligative properties).  Arrhenius formally formulated his theory of ionic dissociation based on strong experimental support for particle dissociation in solutions (and contributions by others that made use of mathematical and qualitative analogies between particles in solutions and gas particles following gas laws).

In 1887, Ostwald formulated a dilution law after applying the electrolytic theory to acids, confirming Arrhenius’ finding that at infinite dilution the molecular conductivities of all acids were the same.  Ostwald used the ionic dissociation theory and applied it to acids and bases and was able to show that the hydrogen ion concentration in water is 10-7 M and established the hydrogen ion concentration as a measure of the relative strength of an acid using the equilibrium dissociation formula found for water.   Sorensen was to formulate the pH scale later on to remove the negative index.

The ionic theory met social ("discomfort" from organic chemists about this emerging field not tightly beholden to laboratory studies and political friction between Fance and Germany) and scientific objections.  A prominent one was brought up by Kahlenberg based on experiments he and his students conducted which showed non-correlation between dielectric constant measurements and dissociation of ions in solvents other than water, suggesting that solute-solvent interactions should be taken into account and that Arrhenius' ionic theory is defective.  Lewis and other prominent physical chemists of the time defended Arrhenius' ionic theory citing that "‘Perfection is rare in the science of chemistry. Our scientific theories do not spring full- armed from the brow of the creator. They are subject to slow and gradual growth, and we must candidly admit that the ionic theory in its growth has reached the ‘awkward age’. Instead, however, of judging it according to the standard of perfection, let us simply ask what it has accomplished, and what it may accomplish in scientific service."

By 1923, ionic theory was 'essentially complete" after refinements and corrections based on a mathematical model developed by Milner and simplified by Debye and Huckel were made based on solute ions – solvent interactions and “statistical interference of neighboring ions on ionic mobilities”  In 1927, Onsager added refinements to the theory for nonaqueous systems.