ADSORPTION 53 



to one (mother is a common attribute of all long molecules capable of 

 forming a closely packed surface f hit. (See C'hap. XL, MeinlH-anes.) 



Adsorption. — II has been found that surl'aees of soHds and 

 htjuids exert an attraction for gases. It is very diineuit, for 

 instance, completely to reniovx air from the surface of a glass 

 container. Repeated evacuation is necessary. The amount of gas 

 so adsorbed varies with the nature of the surface, and of the gas, as 

 well as with the pressure, and inversely with the temperature, and 

 is a reversible process. That is, the processes of adsorption and 

 de-adsorption will proceed together till a condition of equilibrium 

 has been reached which will remain undisturbed as long as 

 the temperature and pressure of the system remain unaltered. 

 (Principle of Le Chatelier.) Charcoal, which has an enormous 

 surface area per unit volume, will adsorb large quantities of gases, 

 e.g., 1 c.c. of coconut charcoal at 0° C. will adsorb 2 c.c. of helium, 

 4 c.c. of hydrogen, 15 c.c. of nitrogen, 18 c.c. of oxygen, 75 c.c. of 

 ethylene, 171 c.c, of ammonia. Use is made of this property in the 

 preparation of high vacua, in the manufacture of " gas masks," and 

 for the removal of foul gases, etc. 



At a liquid-gas, liquid-liquid, or at a liquid-solid interface 

 adsorption readily occurs and may easily be demonstrated by the 

 use of coloured solutes. (Part II., Experiments 18/43 (c).) If 

 we increase the surface of a liquid by the introduction of a finely 

 divided gas, immiscible liquid, or solid, we may be able completely 

 to remove substances in solution. Here we find that the physical 

 and chemical nature of the adsorbing surface and of the adsorbed 

 material are of prime importance. Some adsorbents are almost 

 omniverous, e.g., charcoal in aqueous solutions, others, like kaolin 

 and ferric hydroxide, which have the greatest conceivable specific 

 surface (see p. 72), will only adsorb certain types of solute, due 

 to the sign of the electric charge which they develop in contact 

 with water. Most of them, like kaolin, become negative, but a 

 very few, like ferric hydroxide and haemoglobin, become positive. 

 The former adsorbent will, therefore, fix electro-positive dyes such 

 as the " basic " dye methylene blue and the latter electro-negative 

 dyes. 



Practically all dyes are salts of a coloured and a colourless ion. If the 

 organic base, united generally with hydrochloric acid is coloured the dye is 

 termed basic. A coloured organic acid, may unite with a colourless inorganic 

 base to give an acid dye. The former because of its hydrion is electro- positive, 

 the latter is electro-negative on account of its natrion. 



Chemical forces come into play in some adsorptions, for example, 

 the adsorption of that class of substances termed " capillary or 

 surface active " is determined not by the extent or the physical 



