66 PHYSICAL CHEMISTRY 



for adsorption and surface tension. The equation is not quite 

 correct, however, for concentrated solutions. As the concentra- 

 tion of the solution is increased the surface becomes saturated 

 and will not adsorb any more. 



Non-electrolytes that reduce the surface tension compete for 

 a place at the surface. The one which is the more effective in 

 reducing the surface tension will displace the other from the 

 surface. The effect of two or more substances on surface tension 

 is additive if the concentration curve is taken into consideration. 

 The same rules hold for adsorption. 



Electrolytes that reduce surface tension are adsorbed. If the 

 surface be electrically polarized, however, it affects the adsorp- 

 tion of electrolytes but not of non-electrolytes. When this is 

 the case, electrolytes and non-electrolytes that reduce the tension 

 do not displace one another on the surface, but act independently. 



The adsorbability of anions increases in the order: S0 4 <C1 

 <Br, NO a <I, and of cations Na <K <Rb <Cs and Ca <Sr 

 <Ba. The ions of heavy metals are adsorbed more than those 

 of lighter metals ; the nobler the metal the more this is true. 



Neutral salts, such as NaCl, that do not lower the tension of 

 the water-air surface are nevertheless adsorbed by charcoal, 

 paper, etc. The reason for this difference between the adsorp- 

 tion by the air and the solid surface is probably not electrical 

 polarization, since that could not favor the adsorption of both 

 ions of a salt, but is probably due to molecular attraction of the 

 salt by the solid (adhesion). The adsorption of a substance by 

 the air surface is due to the repulsion by the water molecules, 

 but the adsorption by a solid surface might be due either to the 

 repulsion by the water or attraction by the solid (McClendon, 

 1913 b). 



The electric charge of the solid, however, does influence the 

 adsorption of ions. Clay, aluminum hydroxide and ferric hy- 

 droxide are usually electropositive, whereas kaolin, mastic, 

 silicic acid, sulphur, charcoal, glass, silk, cotton, wool, starch 

 grains and many other substances are negative. Acid dyes are 

 in general adsorbed by clay but not at all by kaolin, the converse 

 being true of basic dyes. 



The electric charge of solid particles may be due to (first) 

 electrolytic solution tension in the case of particles of metal, or 



