514 JOHN JOHNSTON AND PAUL NIGGLI 
through its effect on the solubility relations of the substances 
involved in the reaction; secondarily and much less generally, 
through its effect on the hydrolytic dissociation of salts. 
Salt hydrolysis —It is a matter of common knowledge that the 
aqueous solutions of many salts are not neutral; thus for instance 
sodium carbonate, sulphide, and silicate are alkaline; the chlorides, 
nitrates, and sulphates of most polyvalent bases are acid. This 
behavior is attributed to a process termed salt hydrolysis, which 
may be represented by the reaction 
BA+H,0sBOH+HA 
and occurs (to an appreciable extent) only when the acid, or the 
base (or both),? is weak.? 
It is impossible to enter here into the theory of hydrolysis, or to 
derive from the law of mass action the relations which determine 
its extent. Suffice it to say that under constant external conditions 
the extent of hydrolysis is perfectly definite, and depends upon the 
magnitude of the dissociation constant of the weak acid’ or the 
weak base; or rather on the relation between this dissociation 
constant and that of water; so that the process of hydrolysis may 
be regarded as a competition for the strong base (or acid) between 
the weak acid (or base) and water (which may exercise either basic 
or acidic functions). Thus in decinormal solution at ordinary tem- 
peratures: sodium chloride is practically not hydrolyzed, sodium 
acetate is hydrolyzed to an extent which is just appreciable (0.01 
per cent), while sodium sulphide or silicate is hydrolyzed to some- 
where about 90 per cent. With rise of temperature the dissocia- 
tion constant of water increases faster than that of the acids and 
bases; consequently the extent of hydrolysis increases markedly 
with rise in temperature. 
« This case is of less practical importance, hence its consideration is omitted here. 
2 The strength of an acid (or base), in the sense in which it is used here, is measured 
by its extent of ionization. Thus, in decinormal solution at 25°, hydrochloric acid is 
ionized to about 85 per cent; acetic acid to about 1 per cent; hydrogen sulphide to 
about o.1 per cent; corresponding to this the respective dissociation constants of 
these acids are 1 (approximately), 1.8X10—-5andg9X10—8. The dissociation constant 
of water at 25° is 1.2X1071!4. 
3 Jn salts which yield an alkaline solution. 
4JIn salts which yield an acid solution. 
