C. Edmund Marshall 65 



C). These membranes were used in the determination of the activities 

 of K+ (12), NH 4 + {is), and Na+ (75) when these cations were added 

 as hydroxides to carefully purified acid clays. Thus, complete titration 

 curves, involving both the hydrogen ion and the added cation, were 

 obtained for the four clay minerals: montmorillonite, beidellite, illite, 

 and kaolinite. 



Divalent cations were then considered, calcium being the first inves- 

 tigated. Accurate procedures were established for the determination of 

 calcium and magnesium activities (10, 14)', these were later extended 

 to barium (j). Results comparable to those given by the monovalent 

 cations have now been obtained for montmorillonite, beidellite, illite, 

 and kaolinite using magnesium, calcium, and barium (_?). 



In this way very interesting evidence on the ionization of the clays 

 was afforded and, as mentioned previously, its extension from homi- 

 onic clay suspensions to homionic soils offers no difficulties. 



Attention was then directed to a more difficult problem, namely, the 

 simultaneous determination of monovalent and divalent cations present 

 together upon the clay. Calcium and potassium were investigated first. 

 By means of high temperature selective membranes, the potassium 

 activities in such mixtures were readily and accurately determined. The 

 calcium estimation was subject to a considerably greater error but suffi- 

 ciently reliable information was afforded to give an over-all picture. 

 These potassium-calcium relationships have now been established for 

 the following clay minerals: two montmorillonites, a beidellite, two 

 illites, a kaolinite, a halloysite, and attapulgite (1,7). In a similar way 

 potassium-magnesium relationships are under investigation. Work is 

 also under way on similar evidence for the different fractions of soil 

 organic matter. 



In reviewing the information already available, it has been apparent 

 for some time that the complex nature of the complete titration curves 

 of the clay minerals indicates that they hold single cations with a con- 

 siderable range of bonding energies. It has proved possible to utilize 

 the activity measurements to calculate these bonding energies. Thus, 

 in addition to the pH curve and the cationic activity curve, we may 

 derive also a bonding energy curve for each cation upon each clay. 

 Where two cations are concerned we may express their mutual effects 



