16 THE COLLOIDAL STATE [ch. 



particles in a colloidal solution all have the same charge, then they will 

 tend to repulse one another mutually. It is found that the particles 

 are charged, but the origin of the charge is not always clear. Sometimes 

 if the substance in colloidal state is capable of electrolytic dissociation,^ 

 the charge may arise in this way. Substances, however, as already 

 mentioned, which are not dissociated may also bear a charge, and most 

 frequently it is a negative one. It follows, then, that when an electrolyte 

 is added to a colloidal solution, the charges on the colloidal particles are 

 neutralized by the oppositely charged ions of the electrolyte, and they 

 coalesce together and are precipitated. 



As regards their behaviour to electrolytes the two classes, suspensoid& 

 and emulsoids, are very different. The suspensoids are very sensitive to- 

 traces of electrolytes, and, as they usually have a negative charge, it is 

 the cation of the electrolyte which is the active ion ; and of such, less of 

 a divalent ion, than of a monovalent ion, is needed for precipitation and 

 still less of a trivalent ion. 



The emulsoids are far less sensitive to electrolytes in solution than 

 the suspensoids ; in fact, electrolytes, such as neutral alkali salts, must 

 be added in very large quantities to emulsoids before precipitation takes- 

 place. Also, as a rule, whereas the precipitation of suspensoids is irre- 

 versible, that of emulsoids is reversible, that is, they pass into solution 

 again on addition of water. In the case of an emulsoid in neutral solu- 

 tion this form of precipitation, unlike that of the suspensoids, may be 

 regarded as consisting of two processes. First, a process analogous to 

 that of " salting out " of soaps, esters, etc., in organic chemistry, which is, 

 in effect, a withdrawal of water from one phase into another. Secondly, 

 the precipitation is also affected to some extent by the valency of the 

 ions of the salt used in precipitation. 



When, however, a neutral solution of such an emulsoid as protein is 

 made either slightly acid or alkaline, its behaviour towards neutral salts 

 becomes altered. The precipitating power of salts in acid or alkaline 

 medium is now in accordance with that on suspensoids. In alkaline 

 solution the coagulating power of a salt depends on the valency of the 

 cation ; in an acid solution it depends on the valency of the anion. 



The behaviour of proteins in acid and alkaline media is undoubtedly 

 due to the fact that they are built up of amino-acids containing both 

 amino and carboxyl groups. Such molecules may behave either as an 

 acid or a base with the formation of salts. These are subject to electro- 

 lytic dissociation and hence acquire an electric charge. Such substances 

 have been termed " amphoteric electrolytes " (see p. 134). 



