260 PLANT BIOCHEMISTRY 



1. Adsorbed ions are replaced by other ions in equivalent amounts. 

 Thus, if calcium replaces potassium, one calcium ion will replace 

 two potassium ions. 



2. Ions vary greatly in the ease with which they can be adsorbed or 

 replaced by other ions. The ease or efficiency with which one ion re- 

 places another ion is determined by its valence and its characteristic 

 activity. Divalent ions are more potent than monovalent ions, with 

 the exception of hydrogen ions. At equal ionic concentrations calcium 

 and magnesium have greater replacing power than sodium and potas- 

 sium. The activity of ions with the same valence also varies and seems 

 to depend on the relative size of the hydrated ion. The smaller the 

 hydra ted ion, the greater its activity. When the following hydrated 

 monovalent ions are placed in order of increasing size, we have Rb+, 

 NH4 + , Na + , Li + . The decrease in replacing power of these ions fol- 

 lows the same order. If we compare the activity of these ions with 

 the activity of some of the common divalent ions and with hydrogen 

 ion, the descending order of replacing ability is found to be H+ > Sr+ + 

 > Ba++ > Ca++ > Mg++ > Rb+ > K+ > NH4+ > Na+ > Li+. A 

 comparison of the ease of adsorption of two of the ions in this series 

 is shown in the work of Joseph and Oakley. They found that six 

 times as many calcium ions as sodium ions were adsorbed when 

 equimolar solutions of salts of these ions were allowed to percolate 

 through certain soils. 



3. Base-exchange reactions follow the law of mass action. An exam- 

 ple with which the reader is familiar is found in the operation of the 

 permutite water softener. Calcium and magnesium will replace 

 sodium from the permutite as hard water is passed through the sof- 

 tener. But the reaction can be reversed and the permutite regenerated 

 by forcing a saturated sodium chloride solution through it. 



To soften water: 



NaoO • AI2O3 • 2Si0.2 + CaSOi -^ CaO • AI2O3 • 2Si02 + Na2S04 

 To regenerate the permutite: 



CaO • AI2O3 • 2Si02 + 2NaCl -^ Na20 • AI2O3 • 2Si02 + CaCl. 



Another illustration of the operation of the law of mass action can 

 be found in base-exchange reactions. If a substance is present that 

 forms an insoluble compound with an adsorbed positive ion, such a 

 cation will be replaced. For example, in the presence of phosphate 

 and oxalate ions, calcium will be replaced by sodium and potassium, 

 for calcium phosphate and oxalate are insoluble but the sodium and 

 potassium compounds are soluble. 



