ELECTROLYTES AND THEIR ACTION 197 



but the evidence that such is the case is sufficiently strong to warrant us in making 

 use of the phenomenon in the explanation of many facts, an explanation which it 

 gives in a simple and reasonable way. The actual evidence itself will be given in 

 the following chapter of this book. 



A salt of a weak acid with a strong or weak base, or of a weak base with a strong or weak 

 acid, that is, any salt of which one or both components is a weak one, is hydrolytically 

 dissociated to a certain extent in water. There are present in the solution free acid and free 

 base. In this connection the designation "strong" and "weak" should be understood. 

 in a somewhat relative sense. For example, ammonium hydroxide behaves as a weak base 

 towards the strong acid, hydrochloric, but as a fairly strong base towards the very weak acid, 

 leucine. I refer to this point here on account of the fact that salts of weak acids with weak 

 bases are not so highly dissociated hydrolytically as might have been expected. The question 

 will be discussed below. 



In order to understand the process a little more detail is desirable. Remember- 

 ing that the dissociation constant of an electrolyte expresses the proportion in 

 which the non-dissociated part is capable of existing in the presence of its ions, 

 let us see in the first place what happens when a strong acid, such as hydrochloric, 

 is added to a solution of a salt of a weak acid, say to sodium acetate. Both of 

 these are highly dissociated electrolytically, but when mixed, opportunity is given 

 for the formation of two other electrolytes, sodium chloride and acetic acid, the 

 former of which is highly dissociated, but the latter very feebly so. The low 

 dissociation constant of acetic acid means that acetic ions and hydrogen ions 

 can exist together only to a very small extent. Hence, in our mixture, they 

 unite almost completely to form acetic acid, the result being that the hydrogen 

 ions of the hydrochloric acid very nearly disappear. For practical purposes 

 the reaction may be expressed thus : 



H- + Cl' + Na- -i- CH 3 COO' = CH 3 COOH + Cl' + Na-. 



.Further, owing to the great affinity of H* for OH' ions, the minutest quantity 

 only of either can exist in the presence of the other. Hence, the neutralisation 

 of a strong acid by a strong base may be represented by an equation similar to 

 that above : 



H- + Cl' + Na- + OH' = H 2 O + Cl' + Na-. 



Now water contains the small concentration of both H- and OH' ions which 

 can exist together. Applying the law of mass action to this equilibrium, 

 we have: 



where C H ., C HO ', and C H ., are the concentrations of the H- ions, the OH' ions 

 and the water respectively. Since the latter is always very large in relation to 

 the others, it may be taken as invariable, so that the product C H . x C OH / is constant 

 in any aqueous solution. It is numerically equal to 1-2 x 10~ 14 . 



Water, then, is both a very weak acid and a very weak base ; that is, it is 

 what we shall learn later to call an "amphoteric electrolyte." When a neutral 

 salt AB (using A' for the anion and B* for the cation) is dissolved in water, there 

 is the possibility of the formation of two new compounds with the ions of water, 

 viz., HA and BOH. How far this will occur depends on the strength of the acid 

 and the base. Suppose we take NaCl, the quantities of HC1 and of NaOH will be 

 very small, because of their great dissociation, and approximately equal quantities 

 of H' and OH' will be removed from the water for the purpose, being replaced 

 by a slight further dissociation to keep C H . x C OH - equal to l'2xlO 14 . Again, 

 suppose that we take borax instead of sodium chloride. Here HA is a very 

 weak acid, while BOH is a strong base. We have now in solution A', B", H', 

 and OH' ions, and HA and BOH will be formed as before. But, since HA 

 is very slightly dissociated, while BOH is highly dissociated, there will be excess 

 of OH' ions. As before, a little water will dissociate, but only to preserve the 

 equilibrium C H . x C OH , equal to 1-2 x 10' 14 , and this cannot get rid of the OH' ion., 

 so that the solution will have an alkaline reaction. The case where the acid is 

 strong and the base weak may be treated in a similar way, and the result will be 



