238 PRINCIPLES OF GENERAL PHYSIOLOGY 



Putting the four values together and converting them to the same temperature 

 (25), we find : 



By acid-alkali battery 1 -19x10-". 



By hydrolytic dissociation of solute 1-10 x 10-". 



By saponification of esters 1*20 x 10~ 7 . 



By electrical conductivity - 1 '05 x 1 0"". 



It is impossible to believe that values so near together could depend on accidental 

 impurities. 



It should be remembered also that Arrhenius (1889, p. 103), on this hypoth. -i^. 

 was enabled to predict the high temperature coefficient of its conductivity. 



On the other hand, Walden (1910) finds that water has no higher conductivity when 

 dissolved in prussic acid, contrary to binary electrolytes of the ordinary kind. It seems, 

 however, that there are anomalous conditions present, owing to chemical combination with 

 the solvent. 



Water, as Nernst points out, is capable of a second electrolytic dissociation, since 



OH'JO" + H- 



But the separation of the second hydrogen ion from such a dibasic acid always takes place 

 with great difficulty, so that the concentration of oxygen ions would probably be so small as 

 to escape detection. 



HYDROLYTIC DISSOCIATION OF SOLUTES 



There are a few more facts in connection with this question which require 

 mention. 



Denham (1908) has shown that the hydrogen electrode can be used with 

 good results in determining the degree of hydrolysis. The most interesting 

 facts, for our purposes, obtained in this way are that ammonium chloride is only 

 hydrolytically dissociated in water to a minute extent, namely, 0'018 per cent, 

 for a O'Ol molar solution at 25, while aniline hydrochloride 0'031 molar is 2'6 per 

 cent, dissociated, whence ammonium is about seventy thousand times as strong 

 a base as aniline. 



The hydrolytic dissociation of Indicators is of importance as showing that 

 their strength as acids or bases must not be too small ; otherwise the end point 

 is inaccurate. The rule is that weak bases and weak acids are not to be used 

 together ; that is, weak acid indicators are not to be used for titrating weak bases, 

 nor weak bases for titrating weak acids. For more details see Nernst's book (1911, 

 p. 535). 



The fact that hydrolysis can be reduced by the addition of excess of acid 

 or base, respectively, enables precipitations to be avoided where the product of 

 hydrolysis is insoluble. Thus acetic acid is added to mercuric acetate. Conversely, 

 by reducing the H' ion concentration in ferric chloride solutions by the addition 

 of sodium acetate, ferric hydroxide is precipitated. Or silicic acid may be pre- 

 cipitated from sodium silicate by addition of ammonium chloride. This kind of 

 action is obviously of much importance in the reactions of analytical chemistry 

 (Nernst, 1911, p. 548). 



Finally, the circumstance that, when the weak base or acid of a hydrolytically- 

 dissociated salt has a very small conductivity, it is found that addition of excess 

 of this component beyond a certain degree causes no further change in the molar 

 conductivity of the solute, as shown by Bredig (1894, 1, p. 214), enables the degree 

 of hydrolysis to be determined by an independent method. Such a case is that of 

 aniline salts. 



WATER AS CATALYST 



The phenomenon known as " catalysis " will come up for discussion in a later 

 chapter. It will suffice here to state that there are substances which produce 

 a great increase in the rate of reactions, although they themselves are not 

 constituents of the final system in equilibrium and, as a rule, reappear finally in 

 the same state as they were to begin with. 



Hydrogen ions constitute one of the most powerful of these catalysts and, 



