The question may be raised, why the first solvents mentioned should have the power to cause ionization, while the second series of solvents named do not have this power, or have it only to a very slight extent. Without attempting to enter into an elaborate discussion of this important question, it may be said that J. J. Thomson[101] and Nernst[102] suggested that the ionizing powers of solvents must be intimately connected with their dielectric behavior, and this view has now been well established. It may be said, in simple terms, that the so-called dielectric constant of a solvent determines the force with which electrical charges will attract and repel each other; the higher the dielectric coefficient of a medium, the smaller will be the attraction between opposite electrical charges, other conditions being the same. In solvents, then, of high dielectric powers, the coëxistence of oppositely charged particles must be more favored than in solvents of low dielectric powers. The dielectric constants of a number of solvents are given in the following table: [p063]

Hydrogen cyanide, HNC 95
Hydrogen peroxide, H2O293
Water, H2O81
Methyl (wood) alcohol, CH4O32
Ethyl (ordinary) alcohol, C2H6O22
Ammonia, H3N22
Chloroform, CHCl35
Ether, (C2H5)2O4
Benzene, C6H62

It is quite apparent that the good ionizing media have, as a matter of fact, the highest constants; those which cause ionization, at most minimally (e.g. benzene), the lowest.

Recent extended and exact investigations by Walden[103] have succeeded in bringing the ionizing power of solvents into definite quantitative relations to their dielectric constants, with the result that order has been brought out of a condition of chaos that, for a number of years, existed in this field, as the result of conclusions based on incomplete data. Conductivity being a function both of the proportion of dissociated electrolyte and of the mobility of the ions in a given solution, Walden determined, for a certain salt (an organic derivative of ammonium iodide, namely, tetraethyl ammonium iodide N(C2H5)4I), for all solvents used, not only the conductivities for finite dilutions but also, by extrapolation, the limiting values for infinite dilution. He was thus able to determine the degree of ionization of the salt. Some of his results are particularly interesting; for instance, a poorly conducting solution, such as that of the salt in glycol, a solvent resembling glycerine in general character, may contain the dissolved electrolyte in a highly ionized state, while in a much better conducting solution the degree of ionization may be much smaller—the low conductivity of the first solution being the result of a very high friction and of the slow motion of the ions, while the well-conducting solution might show a very high degree of mobility of the ions. The mobility changes with the nature of the solvent, and the limit, Λ∞, of the equivalent conductivity of the salt, as found by Walden, ranges from 8 in glycol, which is a thick, viscous oil like glycerine, to 200 in acetonitrile, a thin mobile solvent. In the one solution, an observed conductivity of 4 represents 50% ionization of the salt, in the other only 2%.

Now, for solutions of a given electrolyte—tetraethyl ammonium iodide was used—Walden[104] found the following exceedingly interesting relation between the ionizations in, and the dielectric constants of, various solvents:

e1 : ∛c1 = e2 : ∛c2 = a constant,

where e1 and e2 represent the dielectric constants of different solvents, and c1 and c2 represent the concentrations of the salt in the solvents when the salt is ionized to the same degree[105] in the two solutions.

The bearing of the relation is apparent from the data in the following [p064] table.[106] The upper half of the table gives the dielectric constants (column two) of the solvents named in column one; the concentrations which show identical degrees of ionization—47%—are given in the third column, and the last column gives the value of the relation e : ∛c. The lower half of the table presents the same kind of data, for the same salt, when its degree of ionization is 91%, in the different solutions examined. It is clear that the numbers in the third column of each part represent approximately constants.

All solutions, including aqueous solutions, are thus brought into one general relation.

Solvent.ece : ∛c
Methyl alcohol32.50.125 65
Ethyl alcohol21.70.020 80
Acetyl bromide16.20.010 75
Benzaldehyde16.90.016 78
Acetonitril35.80.100 77
Water80 0.00910383
Furfurol39.40.00125365
Nitromethane40 0.00125371
Acetonitril36 0.00100358
Methyl alcohol32.50.00050365