The degree of ionisation of some binary electrolytes and the hydration of their ions
Authors: van Ruyven B. H. 
Published: September 2010

This study* does not claim to be more than an endeavour to penetrate along the experimental road into the nature of the solutions of some concentrated binary electrolytes.

Therefore we have to go back to Raoult, who included concentrated solutions too in his experimental investigations on the depression of the freezing point and the lowering of the vapour pressure.

For different reasons we shall follow Raoult in his method of indicating concentrations, which we shall express by the number of grammolecules of substance dissolved in 1000 grams (practically 1000 cm3) of water.

If e.g. 1 grammolecule of glucose is dissolved to give 1 I solution, its concentration is one and remains one even if the glucose molecules have bound water or not. But the amount of solvent („free water”︁) has diminished and thus the osmotic pressure etc. must alter.

With dilute solutions, for which van't Hoff's law and Debije‐Hückel's law hold good, that alteration may be exceedingly small, with concentrated solutions it must be important.

By means of Raoult's concentration we are able to use the same amount of “free water” for all determinations and so to avoid this difficulty.

In order to explain some discrepancies of Arrhenius' hypothesis, Bjerrum and his school imagine the electrolytes to be either weakly, or nearly completely ionised.

Bjerrum also made use of the ion‐hydration in his studies and wants it in concentrated solutions.

Accepting the hydration of the ions we intend to explore the region of some concentrated electrolytes by means of this hydration, adhering to the theory according to which a degree of ionisation of the electrolytes exists and by so doing we shall explain some discrepancies.

Debije c.s. too accept the hydration of the ions according to Kossel;'s theory: the smaller the charge of the ions and the greater their volume. the fewer molecules of water are bound by the ions.

This theory does not exclude the possibility of nonhydrated („naked”︁) ions.

Both the possibility and the mode of water‐binding by the ions have often been subjects of discussion.

Remy supposes the ions bind a fixed number of water molecules and he was the first who tried to determine absolute values by means of his diaphragm method.

In this study use will be made only of the conductivity of some 1‐1 electrolytes for the determination of the degree of ionisation of their aqueous solutions and of their lowerings of the vapour pressure.

If C represents the number of grammolecules of a solvent and c1 the number of grammolecules of the solute in the monomolecular state, then, according to Raouit's law

\documentclass{article}\pagestyle{empty}\begin{document}$$ \frac{{S - S_1 }} {S} = \frac{{c_1 }} {{C + c_1 }} $$\end{document} in which S equals the vapour pressure of the solvent and S1 the vapour pressure of the solution at the same temperature.

Raoult's law holds good for solutions of divergent concentrations and at different temperatures.

By means of these data we shall try to gain information upon the number and nature of the particles in the solutions of some 1‐1 electrolytes.