An application to electrolytes of the hydrate theory of solutions / by T. Martin Lowry.

  • Martin Lowry
Date:
[1905]
Catalogue details

Licence: In copyright

Credit: An application to electrolytes of the hydrate theory of solutions / by T. Martin Lowry. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    siderable complexity do not exist”* in dilute solutions. It is, however, note- worthy that in many cases the molecular depression begins to increase at concentrations as low as N/io to N/5. At these low concentrations the greater part of the salt must be in the ionised condition, and the experimental data are fully in accord with the view that combination with the solvent is characteristic of the ion even more than of the molecule, f VII. The theory of electrolytic dissociation owes much of its acceptability to the readiness with which it lends itself to exact mathematical treatment. The whole theory can indeed be summed up in the single equation— IK = specific conductivity ] m = concentration K — m a [u + 7’) 1 a __ coefftcjent Gf ionisation [u and v = ionic mobilities since this equation includes (1) the conception that only a part a of the solute is directly active in electrolysis and (2) the conception that each ion migrates independently and possesses a specific ionic mobility u or v. In the case of dilute solutions the constants in this equation can be determined, but the values for concentrated solutions are at present unknown.| The hydrate theory, on the other hand, has suffered because of the extreme difficulty of giving quantitative expression to phenomena that cannot readily be observed except in concentrated solutions. The essential constants of the hydrate theory are, however, quite as simple as those of Arrhenius’s theory. For any given solution the chief constants are—(1) The total hydration (or, more briefly, the hydration), H, of the solution, which expresses the total number of molecules of water present per molecule of salt. (2) The coefficient of combination, (3, which expresses the fraction of the total quantity of water that is actually combined with the salt to form hydrates. (3) The product of these two quantities will give the average composition of the hydrates in solution; these may be termed the average molecular hydration, or, more briefly, the “ molecular hydration,” of the solution and may be represented by the symbol h. Thus h— j3 H may be regarded as the funda- mental equation of the hydrate theory, and whilst the empirical composition of a salt solution may be expressed as NaCl + H H20, the average composi- tion of the hydrates will be NaCl + h H20, and the proportion of combined water /z//L Only two attempts appear as yet to have been made to determine the molecular hydration of dissolved salts. One of these is described by Jones and Getman in the paper quoted above. Assuming that the abnormally great molecular depressions of the freezing-point of salt solutions of moderate concentration were due to a combination of solvent and solute whereby the quantity of solvent water was reduced, they were able to com- pute the amount of solvent water remaining in the solution by comparing the observed molecular depression with that calculated from the coefficient of ionisation of the solution. No correction was made for the influence on the ionic mobility of the changing viscosity of the solution, and the results can therefore only be regarded as approximately correct. But in the absence of more reliable data the figures deduced for the weight of solvent water in * Ibid., p. 355. f The interpretation of the experimental data is discussed in the following section. I For an attempt to determine the values of the constants in less dilute solutions, see Bousfield and Lowry, Phil. Trans., 1905, 204, 253-322.