38 INTRODUCTION TO GENERAL PHYSIOLOGY 







opposite signs of coloured ions are reversed. This is the explanation 

 of the effect of electrolytes in certain histological staining reactions. 

 The phenomena in general are spoken of as " electrical adsorption" 

 a term which also includes the adsorption of the precipitating ion 

 in the ordinary process of precipitation of colloidal solutions by 

 electrolytes. 



Hydrolytic Dissociation 



There is a form of dissociation to be met with in salts of weak 

 acids or weak bases which is not of an electrical nature, and, to a 

 certain extent, antagonistic to electrolytic dissociation. This is 

 known as " hydrolysis," or, better, " hyclrolytic dissociation," because 

 it is brought about by interaction with the hydrogen and OH ions 

 of water. 



We have already seen that the distinction between weak and 

 strong acids or bases is that the former are only slightly dis- 

 sociated electrolytically, unless very strongly diluted. If we take 

 a salt of such a weak base, say ammonia, with a strong acid, such 

 as hydrochloric acid, or of a strong base, such as sodium hydroxide, 

 with a weak acid, such as acetic acid, or of a weak base with a weak 

 acid, such a salt as ammonium acetate, and dissolve in water, we 

 find that the solution is not neutral in reaction. There is evidence 

 of the presence of free acid and free base. The reaction will 

 be either acid or alkaline, according to which is the stronger. 

 Ammonium chloride is acid, sodium acetate is alkaline. Further 

 details of the process will be found in the larger works (p., 

 p. 196). 



Although the fact has sometimes to be reckoned with, it is not 

 to be supposed that it is usually of any great magnitude, unless 

 both the acid and the base are extremely weak ; and in such cases 

 the question naturally arises as to whether we are justified in 

 speaking of their being in combination at all. It is usually not 

 more than I to 2 per cent, and is sometimes absent when it might 

 have been expected to be present. 



The nature of the phenomenon may be realised somewhat in 

 the following way. If we take a. solution of sodium acetate and 

 suppose that electrolytic dissociation occurs in the usual way, it 

 would be almost completely dissociated into sodium ions and 

 acetic anions. Now, although the former can exist in high con- 

 centration in water, the latter cannot, since acetic acid is but little 

 dissociated. The acetic anion accordingly combines with hydrogen 

 ions from the water, forming undissociated acetic acid ; more 

 hydrogen ions are set free from the water until the normal 

 proportion of dissociated and undissociated acetic acid is present. 



