C. Edmund Marshall 6j 



this combination of clay and cation. C is a cationic free energy curve. 

 At each point the free energy value is calculated from the actual activity 

 and the total concentration of the cation concerned. The formula may 

 be written 



( AF ) cat ion = RT ln Nation/* cation • 



The reciprocal of the cationic fraction active is therefore used in calcu- 

 lating (AF) Na , since a Na (measured) /V Na (total) is the fraction active 

 for sodium. 



Corresponding to the sharp changes in slope in curve B, we find 

 regions of rapidly changing free energy in curve C. The range, in mean 

 free bonding energy of the sodium, runs from about 500 to 800 calories 

 per mole ion. Similar curves are found for potassium and ammonium 

 although the extremes are not quite so far apart. As regards the other 

 three clays: beidellite shows many of the features of montmorillonite 

 but with a considerably smaller fraction active corresponding to greater 

 free energy values; illite gives somewhat simpler activity and free 

 energy curves and high free energy values; kaolinite resembles illite in 

 its curves, but the actual free energies are the least of any of the four 

 clays. In making comparisons of one clay mineral with another, it is 

 important to keep the total cation concentration approximately con- 

 stant, since for each clay the fraction active and, hence, the cationic free 

 energy varies considerably with clay concentration. A series of values 

 of the fraction active is given in Table III for definite stages in the 

 neutralization of the acid clays with bases. The table does not, of course, 

 display all the variations in the fraction active shown by the complete 

 curves. The following conclusions may be drawn. 



t. If the state of each cation at the inflexion of the pH curve is taken 

 as standard, then with decreasing degrees of saturation the fraction 

 active falls, the rate of decrease being rapid down to 80 per cent satura- 

 tion and slower between 80 per cent and 30 per cent. The bonding 

 energy in the latter range runs from 20-50 per cent higher than that at 

 the inflexion. 



2. The fact that the fraction active is considerably less than unity sug- 

 gests at first sight that we are dealing with weak electrolytes. However, 

 the ionization is not appreciably changed by adding soluble salts with 



