CHEMICAL AFFINITY. 427 



there they fail entirely, and we have to resort to 

 equilibrium methods for ascertaining the affinities of those 

 substances. There is one substance which is very con- 

 venient as a standard of comparison against which to 

 compare the strengths of both weak acids and weak bases, 

 namely, water. Weak bases give off comparatively few 

 hydroxyl ions OH, and weak acids few hydrogen ions H. 

 Now water in the pure state dissociates electrolytically to a 

 minute extent into these ions, H,0 = H + OH, so that it 

 can act both as a very weak acid and a very weak base, 

 according as it competes against another acid or another 

 base. The amount of the dissociation of water is extremely 

 slight, a calculation by Kohlrausch showing that a cubic 

 meter, i.e., about a ton, of water at 18 C. contains only 1 64 

 milligrams dissociated into hydrogen and hydroxyl ions. 



A base like diphenylamine is so weak that, although it 

 can form a hydrochloride, the salt, when treated with water, 

 has the hydrochloric acid washed entirely away, leaving the 

 free base behind. Here the water plays the part of a base 

 against the diphenylamine, and competes with it for the 

 hydrochloric acid. Aniline is a much stronger base than 

 diphenylamine, but still its hydrochloride, when dissolved 

 in water, is split up partially into aniline and hydrochloric 

 acid. By comparing the amounts of free hydrochloric acid 

 obtained from equivalent solutions of the hydrochlorides, 

 we can ascertain the relative strengths of the bases, for the 

 stronger the base is the less will its salt be decomposed by 

 a given quantity of water: The amount of free hydro- 

 chloric acid in the solutions cannot be estimated by any 

 ordinary analytical process, but can be determined indirectly 

 by ascertaining, for example, how rapidly the solution 

 inverts cane sugar, the rate depending on the amount of 

 free acid present. 



Similarly salts of weak acids may be decomposed by 

 water. The cleansing properties of washing soda and of 

 soap depend on the fact that these are sodium salts, whose 

 acids are so weak that there is partial, though slight, 

 decomposition into the acid and free caustic soda when the 

 salts are dissolved in water. The amount of soda present 



