﻿326 Prof. J. N, Mukherjee on 



only on the total number of ions (of both signs) fixed per 

 unit area of the surface. Kataphoretic and electro-endosmotic 

 experiments give us a quantitative idea of the relative ad- 

 sorption of both ions. 



The electric charge helps to peptize the adsorbent, and a 

 qualitative idea of the adsorption of ions can be formed 

 from peptization by electrolytes. An insoluble precipitate 

 formed by the union of two oppositely charged ions has a 

 marked tendency to adsorb its component ions. In many 

 cases the connexion between the adsorbed ion and the 

 electrical charge has been established. These instances have 

 been given in the earlier paper. The nature of the chemical 

 forces responsible for this adsorption has also been defined. 

 Instances of adsorption of ions as judged from peptization by 

 electrolytes are given below. 



Bancroft (Rep.' Brit. Assoc, p. 2 (1918)) remarks: — ;; It 

 seems to be a general rule that insoluble electrolytes adsorb 

 their own ions markedly, consequently a soluble salt having 

 one ion in common with a sparingly soluble electrolyte will 

 tend to peptize the latter. Freshly precipitated silver halides 

 are peptized by dilute silver nitrate or the corresponding- 

 potassium halide, the silver and the halide ions being ad- 

 sorbed strongly. Many oxides are peptized by their chlorides 

 and nitrates, forming so-called basic salts. Sulphides are 



peptized by hydrogen sulphide The peptization of 



hydrous oxides by caustic alkali can be considered as a case 

 of adsorption of a common ion or as the preferential adsorption 

 of hydroxy lion. Hydrous chromic oxide gives an apparently 

 clear green solution when treated with an excess of caustic 

 potash ; but the green oxide can be filtered out completely 

 by means of a collodion filter, a colourless solution passing 

 through."" 



" Hanztsch considers that hydrous beryllium oxide is 

 peptized by caustic alkali, copper oxide is peptized by con- 

 centrated alkali, and so is cobalt oxide. In ammoniacal 

 copper solutions part of the copper oxide is apparently colloidal 

 and part is dissolved. Freshly precipitated zinc oxide is 

 peptized by alkali, but the solution is very unstable " (cp. also 

 negative hydroxide sols — Freundlich and Leonhardt, Koll. 

 Chem. Beihefie, vii. p. 172 (1915)). 



At least in some of these cases the formation of new com- 

 plex anions is possible, and it is not definitely known to what 

 ion the peptization is due. Regarding the peptization of 

 stannic acid gel by small quantities of alkali, Zsigmondy 

 (Kolloidchemie, p. 122 et seq. (1920) ; also Varga, Koll. 

 Chem. BeiJiefte, xi. p. 26 (1919)) remarks: "Dieses kann 



