SURFACE ACTION 



1334, 1896). It can also be deduced theoretically from the law of mass action, as 

 shown by Freundlich in the place referred to. Cases of combination between weak 

 acids and bases which do not result in precipitation are not comparable. 



When charcoal adsorbs either bromine, benzoic acid, aniline, or phenol, the 

 value of the exponent in the formula of Freundlich varies only between 0*5 and 

 0'2. It is difficult to believe that any process of a chemical nature can be in 

 question here. 



The amount of any particular adsorption compound formed depends on the concentration, 

 not the total mass of the adsorbed substance. Now, Brailsford Robertson (Roll. Zeita., 3, 

 p. 54) argues that this is also found in cases of reversible reactions like that of the formation 

 of acetic ester from ethyl alcohol and acetic acid. He forgets, however, that the ester formed, 

 although varying in amount with the concentration of the acid present, has always the same 

 composition, whereas an adsorption compound would contain more acetic acid the greater 

 the concentration of it in the mixture. 



Raehlmann (1906, p. 152) has described how the constituents of certain 

 adsorption compounds can be seen to be merely in close apposition. One of his 

 experiments is as follows- The 

 extract of a yellow wood, used 

 in dyeing, and known as fustic, 

 shows itself under the ultra- 

 microscope to be a suspension 

 of , minute particles too small 

 to be visible as separate dots. 

 By the addition of alum, these 

 " amicrons " can be caused to 

 aggregate together to form 

 larger ones, visible as such, and 

 of a greenish colour. Serum 

 albumin behaves similarly, and, 

 under the influence of alum, 

 forms yellow particles. The 

 dye, " Congo fast blue," even 

 without alum, consists of 

 visible particles of a red colour 

 by the reflected light of the 

 ultra-microscope. Taking each 

 separately, we have then green, 

 yellow, and red particles. 

 When the three solutions are 

 mixed, an adsorption com- 

 pound, which gives a green 



FIG. 34. ADSORPTION COMPOUND OF THREE CONSTITU- 

 ENTS, as seen by ultra-microscopic observation of 

 a mixture of fustic (white in the figure), Congo -blue 

 (grey), and albumin (black, outlined by white). 



The fustic particles actually were greenish in colour, those of the 

 Congo-blue were red, and albumin yellow. 



(After Raehlmann.} 



solution, is formed. This 

 solution, under the ultra-micioscope, is seen to consist of compound particles, each 

 containing three of the simpler ones, one each of the red, green, and yellow ones. 

 Fig. 34 is a diagrammatic representation of Eaehlmann's fig. 4, the original being 

 in colours. If albumin, Congo-blue, and fustic are mixed, without alum, the 

 particles do not run together. It appears that Congo-blue, and probably also the 

 other colloids, have a negative charge, which must be neutralised by the trivalent 

 aluminium ion before aggregation can occur. The meaning of this experiment will 

 be appreciated better after Chapter IV., On the Colloidal State," has been read. 



Let us take, as the next case for consideration, a solid in mass immersed in 

 a solution of some substance which lowers surface tension, and is, therefore, 

 deposited on the surface of the solid. Further, let us suppose that this adsorbed 

 substance is capable of entering into true chemical combination with the s 

 It is clear that this reaction can only proceed at the surface, and it will depei 

 upon the solubility of the products of the reaction whether the whole 

 solid finally enters into combination, or whether there is merely a layer ot to 

 products on its surface. In any case, it is plain that the reaction will not 

 the law of mass action in its usual form, since the rate of the reaction wil 

 depend, not on the mass of the solid, but on its surface. 



