ELECTRICAL AND CHEMICAL ADSORPTION 29 



This is due to a greater attraction for the solute particles by the non-water 

 phase of the interface than the water molecules themselves exert. 



Specially treated ("activated") charcoal is one of the best adsorbents 

 known both for gases and for solutes. Many important applications are made 

 of this property of charcoal in chemistr}^ and technology. Gas masks for pro- 

 tection against poison gases in warfare owe their efficiency to the adsorptive 

 capacity of charcoal. Colored or other soluble impurities are often removed 

 from liquids by passing them through layers of charcoal. Solids such as 

 charcoal which are powerful adsorbents usually possess an enormous surface 

 area in proportion to their mass, which accounts for their effectiveness as 

 adsorbing agents. The surface of i g. of charcoal has been variously esti- 

 mated at from about 50 to about 600 square meters. Much of this surface 

 is in the form of internal submicroscopic capillaries. 



Electrical and Chemical Adsorption. — Adsorption in which the funda- 

 mental controlling forces are the cohesive and adhesive forces between mole- 

 cules is called Jiiechanical adsorption in order to distinguish it from certain 

 more complex kinds of adsorption phenomena. Of these the most important 

 is electrical adsorption. 



Most surfaces in contact with water bear an electrical charge. The pres- 

 ence of such charges markedly influences the adsorption of ions or other 

 charged particles. Cellulose, in common with many other substances, acquires 

 a negative charge when immersed in water. If a strip of cellulose filter paper 

 be arranged so that its lower end dips in a solution of eosin (a red dye) it 

 will be observed that the eosin will rise through the filter paper almost as 

 rapidly as the water from the solution rises by capillarity. If another strip 

 of filter paper be similarly arranged to dip in a solution of methylene blue 

 (a blue dye) the water will rise up the filter paper as rapidly as it does from 

 the eosin solution, but the dye will scarcely rise at all. The difference in 

 behavior of these two dyes is due principally to the electrical charge which 

 their colored ions carry. The colored ions of eosin are negatively charged; 

 those of methylene blue positively charged. The negatively charged particles 

 of eosin are repelled from the negatively charged surface of the cellulose 

 fibers. The eosin particles are therefore driven towards the center of the 

 capillary columns of water in between the fibers, and move up the filter paper 

 almost as rapidly as the water. As soon as the positively charged particles of 

 methylene blue come in contact with the negative charges of the cellulose, 

 however, they are held to the surface of the cellulose by the forces of electrical 

 attraction. In other words they are electrically adsorbed. While the water 

 rises by capillarity at about the same rate as it does from the eosin solution, 

 the rise of methylene blue is relatively very slow. 



