122 STAINING. 



We take, then, a piece of the purest analytical filter paper, wash 

 it with distilled water to remove possible traces of acid, and place it 

 in a dilute solution of Congo red, freed from foreign salts. It is 

 scarcely stained at all. Add next a very small amount of a neutral 

 salt, say, sodium chloride. The paper is deeply stained. How are 

 these facts to be explained ? In view of the chemical inertness of 

 cellulose, it seems unlikely that a chemical combination occurs 

 between the dye and the paper under the influence of a neutral salt 

 at ordinary temperatures. Moreover, the same behaviour is shown 

 by such different substances as charcoal, silk, alumina, silica, and so 

 on. The process must be one of adsorption or deposition of the dye 

 on the surface by some means. In other words, it must be associated 

 with the decrease of surface energy of some kind. In the absence 

 of foreign electrolytes, adsorption may be due to decrease of surface 

 energy of the ordinary kind, shown as surface tension. This is 

 confirmed by the fact that the dye, in the absence of electrolytes, can 

 be washed out again by water. But since the degree of staining is 

 very small, there must be some influence at work restricting the 

 mechanical adsorption. There is, indeed, another property of the 

 boundary surfaces between phases which demands attention here. 

 This is the electrical charge, nearly always present. If we test 

 paper in water, we find that it has a negative charge. Similarly, by 

 appropriate means, we find that the dye itself has a negative charge. 

 Whether this is that of the coloured anion or of complex aggregates 

 of these ions with undissociated salt is not certain, but, according to 

 Freundlich, " acidic " dyes are adsorbed as a whole. In the present 

 case it is immaterial, because the material to be adsorbed has a 

 negative charge in either case. This being so, there are repellent 

 forces acting between the dye and the paper. Or, if we take the 

 point of view of energetics, the adsorption of electro-negative dye 

 would increase the negative charge on the paper, with an increase of 

 free energy, which is contrary to the Second Law of Thermo- 

 dynamics. Suppose, however, that we have also present the ions 

 into which a neutral salt dissociates. The cations, being positively 

 charged, are deposited on the surface of the paper, decreasing or 

 annulling its negative charge and reducing the free energy. There 

 is now little or no obstacle to the adsorption of the dye. From the 

 work of PERRIN we know that an ion may be adsorbed on an 

 oppositely charged surface to so great an extent that the charge on 

 this surface may actually be reversed in sign. This occurs, as it 

 appears, when surface energy of some kind other than electrical is 

 diminished by the presence of such ions, and is found mainly with 



