CHROMATOGRAPHY 153 



togram, the acetone-water forms a film on the paper, and the petroleum 

 ether remains as a separate mobile liquid. Petroleum ether moves over 

 the paper more rapidly than the other solvent, carrying certain pigments 

 with it. Other pigments, more likely to remain in the acetone-water film, 

 move less rapidly. In the descriptions given earlier, no water was added 

 to the solvent mixture, but there nearly always is a small amount of 

 water adsorbed on the paper. Since chromatography of chloroplast pig- 

 ments can occur on dry paper with dry solvents, apparently adsorption is 

 also involved. 



Ion Exchange Chromatography: Materials which have the ability to 

 dissociate into positively and negatively charged ions can sometimes be 

 separated by virtue of the electrical charge. A few natural minerals and 

 a large number of synthetic resins bind ions on the surfaces of particles. 

 For example, a resin might consist of a substance containing a number 

 of acidic groups as part of the molecular structure. When the resin is in 

 water, these acidic groups ionize, leaving negatively charged spots on the 

 resin particle. Cations will be held at these spots, some more tightly than 

 others. If a solution containing a mixture of cations is poured over this 

 resin, these positively charged ions, such as Na"^, K+, or Ca++, will 

 displace hydrogen ions from the resin. In effect, the resin exchanges its 

 H+ ions for metallic ions. 



If a mixture of positively charged ions is allowed to flow continuously 

 over such a resin in a long column, some ions will be carried along with 

 the water faster than others. Ions which form a strong electrical bond 

 with the negatively charged radicals on the resin will move very slowly. 

 If the column is long enough, the various kinds of ions will emerge at 

 the end of the column one at a time. 



A fairly complex set of equilibria exists in an ion exchange column. 

 Imagine a synthetic resin which binds Ca+"'" ions more strongly than 

 Na""" ions and Na+ more strongly than H+ ions. If the resin exists 

 almost entirely in the acid form, that is, holding H+ ions, and a solu- 

 tion of Na+ ions is added, the Na"^ ions displace hydrogen ions from 

 the resin. Later, the addition of Ca++ will cause the removal of Na+ 

 and the binding of Ca+ + . The binding is not permanent, however, and 

 a large excess of hydrogen ions, as from HCl, could cause displacement 

 of the Ca++ ions and thus bring about the regeneration of the original 

 resin. 



Other resins, themselves positively charged, attract negatively charged 

 ions. A mixture of organic acids might be separated on such a resin. 



The theoretical treatment of the elution of materials from ion ex- 



