WATER, ITS PROPERTIES AND FUNCTIONS 237 



The reason will be apparent from the following table given by Bousfield and 

 Lowry (1910, p. 21) : 



It will be noted that the proportion of water of hyd ration to total water 

 diminishes rapidly with the concentration and that it is only in the high 

 concentrations that it would be detectable, owing to the very small amount 

 of water taken up at the lower concentrations. Even in 0-2 molar concentration 

 97 per cent, of the water is free. 



Osmotic pressure, on the kinetic theory, being dependent on the energy 

 of movement of the molecules of the solute, it is clear that a certain degree of 

 polymerisation of the solvent, by which the total number of its molecules is 

 decreased, will not have any obvious effect on the osmotic pressure of the 

 solute. 



ELECTROLYTIC DISSOCIATION OF WATER 



The more carefully water is purified, the less is its power of conducting an 

 electric current ; so that the conclusion must be made that it is, at the most, 

 only very slightly dissociated into ions. H- and OH', therefore, can only exist 

 beside one another in the merest traces. We have seen above the importance 

 of this fact in the process of neutralising a base with an acid, and how, in 

 consequence, the heat of neutralisation of a strong acid by a strong base is 

 the same, whatever the acid or base used. 



Now, since the conductivity of ordinary distilled water is readily shown to be 

 due to impurities, it would seem that the view taken by Armstrong, that if water 

 were sufficiently pure it would be a non-conductor, is a justifiable one. It is 

 obvious that the argument cannot be disproved by direct measurements of the 

 conductivity of purified water. At the same time, the results of Kohlrausch and 

 Heydweiller (1894) distinctly point to a limit, beyond which further purification 

 has no effect. These experiments give a concentration of l'05x!0~ 7 gram-ions 

 per litre at 25, or - 78 x 10~ 7 at 18. This gives a value for the product of ionic 

 concentrations (H) (OH') of M x lO" 14 at 25. 



The substantial correctness of the view, moreover, is shown by the fact that 

 other independent methods give values almost identical with this. 



1. The addition of large quantities of a strong alkali to water will render 

 infinitesimal the concentration of any free hydrogen ions arising from the 

 dissociation of an acid present as impurity ; so that, if the presence of any such 

 ions can be detected, they must arise from the water itself. This can be done by 

 taking the electromotive force of a battery of acid and alkali by the method of 

 Nernst described above (page 191). The value of the concentration in hydrogen 

 ions found in this way was 0'8 x 10~" at 19. 



2. We have seen how satisfactorily the hydrolytic dissociation of certain salts 

 in water is explained by the existence of H* and OH' ions in water. This is, in 

 itself, evidence for the truth of the hypothesis, but the numerical "value of the 

 dissociation of water can be calculated from the degree of hydrolysis of a solute 

 and 0-68 x 10~" has been found in this way. 



3. In the chapter on " Catalysis " we shall see how acids cause an increased rate 

 of hydrolysis of esters in water and how alkalies cause an increased rate of 

 saponification. So that, if the rates of these reactions in pure water be determined, 

 we have another means of arriving at the concentration of hydrogen or hydroxyl ions 

 in water. Taking methyl acetate, Wijs (1893) found a value of 1-2 x 10~ 7 at 25. 



