XT 



36 MINERAL SALTS ABSORPTION IN PLANTS 



a =mKCn 

 where m = the amount of adsorbent 

 C= external concentration 

 A" and n are constants for particular solutes and adsorbents. 



When ajm is plotted against C, the familiar Freundlich adsorption 

 isotherm is obtained (Fig. 8a), and if log ajm is plotted against 

 log C a straight line results with slope = n, and intercept on the 

 ordinate = log K, in accordance with the equation : 



log a/m = \og K+n log C 



The Freundlich relationship is an empirical one which holds 

 fairly well at low values of C, but breaks down at high concentrations 

 where the adsorbent approaches saturation. 



Another way of expressing the relationship between adsorption 

 and concentration is by the Langmuir equation which has general 

 application in physical chemistry. In this case: 



a/m==(kiC)l{l +^2Q where k^, k^ are constants. 



At high values of C, unity can be neglected and the equation reduces 

 to: 



ajm = kjk2 



kjk2 is thus the saturation value for a given adsorbent. If experi- 

 mental data fit the Langmuir equation, a straight line is obtained 

 when the reciprocal of afm is plotted against the reciprocal of C. 

 Fig. 8b shows that adsorption of acetic acid on charcoal occurs 

 according to the Langmuir equation. 



It is sometimes convenient to distinguish between two kinds of 

 adsorption — mechanical and polar. Both depend on the operation 

 of electrical forces at the adsorbing surface, but they differ in the 

 strength of binding. Mechanical adsorption involves weak residual 

 and secondary valencies, and substances adsorbed in this way are 

 removable by washing in water. Polar adsorption on the other hand 

 depends on the formation of salts, e.g. between an alkali cation 

 (K"^) and a carboxyl group of an organic molecule, as follows : 



K+ + R.COO-^ R.COOK 



or between an anion (e.g. Cl~) and a basic group, e.g. 



Cl--|-R.NHt^R.NH3Cl 



