46 BIOLOGICAL CHEMISTRY 



HYDROLYTIC DISSOCIATION 



Salts formed from a weak base or a weak acid are dissociated 

 in solution. The dissociation is governed by the equations, 



v-/ H -l_ X (-'acid === -K-Cundlssociated acid 

 Cbase + X COH == -K-Wndissociated base 



where CH.J, C acld ~~, C base + etc., stand for the corresponding 

 concentrations and K is the equilibrium constant. 



All salts are highly ionised, therefore a solution of a salt 

 containing a weak base must show a decrease in the con- 

 centration of hydroxyl ions or an increase in the concentration 

 of undissociated base ; as a rule both these take place to 

 maintain the equilibrium condition. Therefore the solution 

 of a salt of a weak base with a strong acid is acid in reaction, 

 e.g. aniline hydrochloride. Conversely the solution of a salt 

 of a weak acid with a strong base has an alkaline reaction, e.g. 

 sodium carbonate. 



Removal of the products of dissociation, for instance by 

 dialysis, may lead to a complete dissociation of the salt. 



To summarise we find that diffusion, osmotic pressure and 

 vapour pressure all depend upon the concentration of the 

 substances in solution. In the case of non-electrolytes the 

 effective concentration usually corresponds to the number of 

 molecules in solution but in the case of electrolytes, owing to 

 electrolytic dissociation, the increased number of particles in 

 solution increases the effect on the vapour pressure, etc. 



Chemical reactions are affected by the number of particles 

 of reacting substances in solution and these effects can be 

 expressed by simple mathematical equations. 



Electrolytes cause electrical phenomena which can be 

 measured and the measurements used to calculate the number 

 of the various ions in the solution. 



These generalisations will be applied frequently in later 

 chapters of this book. 



Volume changes due to variations in vapour pressure may 

 produce some forms of movement and may thus be one of 

 the means by which chemical energy can be converted into 

 movement. 



GENERAL REFERENCES. 



A. FINDLAY : Osmotic Pressure. Longmans, Green & Co., 1913. 



W. C. McC. LEWIS : A System of Physical Chemistry. Longmans, Green & 



Co., 1916. 

 J. W. MELLOR : Chemical Statics and Dynamics. Longmans, Green & Co., 



1909. 

 E. B. R. PRIDEAUX : The Theory and Use of Indicators. Constable & Co., 



1917. 



