SURFACES 21 



Adsorption. 



With colloids in particular there occurs a third possibility of dis- 

 tribution, wherein the surface comes into play rather than the total 

 mass of the dispersed phase. The condition of distribution which 

 we are about to describe is called adsorption. Suppose we suspend 

 in water a substance which we may assume does not dissolve or 

 undergo chemical combination, e.g., pure carbon. We know that 

 bone black may to a greater or less extent decolorize dye solutions; 

 that it is used to bleach dark sugar juices and to decolorize the dark 

 solutions of the organic chemist. When suspension of powdered char- 

 coal is added to bromin water, we observe the following: If we add 

 very little bromin to the water the latter will become completely 

 decolorized; if we add more, a considerable part is taken up by the 

 charcoal but the water becomes brownish. With further addition 

 of bromin the water is colored more intensely and the charcoal takes 

 up proportionately less bromin. This process is reversible and the 

 distribution of bromin between charcoal and water follows a cer- 

 tain law. We cannot as in the case of a solvent, however, speak 

 of the concentration of the dispersed phase. In experiments, it has 

 become customary to insert the specific gravity of the dispersed 

 phase, and this custom is, as a rule, justified. Let us, for example, 

 designate by x the amount in millimols of bromin that is adsorbed 

 from a solution by m grams of charcoal, and by c, the concentration 

 of the bromin in the water after adsorption. If we deal with sub- 

 stances of unknown molecular weight, x indicates the weight in milli- 

 grams and c the weight which is present in 1 cc. of water after 

 determining the adsorption balance. Empirically we arrive at the 

 equation 



/ 



(adsorbed) 



c n (free) 

 in which the exponent - is always < 1. Inspection shows that if 



Tl 



/v* 



n = 3 and k = 20, the equation is satisfied when in the char- 



m 



coal = 200 and c (water) = 1000. 

 If we dissolve but very little bromin, then the equation is satisfied 



when, e.g.,%- = 20 and c = 1. In the first instance only 1/5 of the 

 in/ 



bromin is adsorbed by the charcoal, but in very great dilutions 20 

 times as much goes to the adsorbent. 



