C. Edmund Marshall 



73 



Three sets of measurements were carried out on each clay system. 

 By titrating the acid clays with potassium hydroxide and with calcium 

 hydroxide and determining potassium and calcium activities, two bases 

 of comparison were established. Then mixed potassium-calcium sys- 



20 



40 60 



Percentage K 



80 



100 



Figure 4. Comparison of potassium ion activities and 

 mean free energy values in potassium-hydrogen and potas- 

 sium-calcium clay systems (2.5 per cent Putnam clay). 



terns were prepared (generally at a total saturation with bases of 100 

 per cent as judged by the inflexion on the pH curve), and the calcium 

 and potassium ion activities were measured. Thus, complete informa- 

 tion on the energy relationships of (a) hydrogen in presence of potas- 

 sium 01 calcium, (b) potassium in presence of hydrogen or calcium, 



