152 CHROMATOGRAPHY 



distributes itself in the two solvents according to a definite relationship. 

 For example, consider a solute X which will dissolve in solvent A or in 

 solvent B. If solvents A and B, immiscible in each other, are placed in 

 a separatory funnel and subsequently a small amount of X is added, 

 part of the X will go into each solvent. An equilibrium will be reached 

 when a certain constant fraction is contained in each solvent. The ratio 

 of the concentrations in the two solvents is called the partition coeffi- 

 cient: 



_ cone, in solvent A 

 cone, in solvent B 



Suppose that this ratio is large, meaning that most of the solute goes 

 into solvent A. Given a solution of X in solvent B, the material X could 

 be transferred to solvent A by successive additions of fresh solvent A. 

 Several examples of this behavior are illustrated by transferring pigments 

 from acetone to petroleum ether and then washing out the acetone with 

 water. 



In another instance, imagine that X distributes itself 5 per cent in 

 water and 95 per cent in petroleum ether. Another material, Y, reaches 

 its equilibrium when 90 per cent is in water and 10 per cent in petroleum 

 ether. Given an aqueous solution containing both X and Y, the two 

 solutes could be separated from each other by mixing the solution with 

 petroleum ether. At equilibrium, the water would contain only 5 per 

 cent of the X but 90 per cent of the Y. If this water layer is drawn off 

 and mixed with fresh petroleum ether a new equilibrium will be estab- 

 lished. The water then will contain only 0.05 X 0.05 = 0.0025 of the 

 X and 0.90 X 0.90 = 0.81 of the Y. After another step or two, X vir- 

 tually disappears from the water, but only a little of the Y is lost. If the 

 components in a mixture differ greatly in their partition in the two 

 solvents, this is a practical method of separation. 



If the components of a mixture differ only slightly in their concentra- 

 tions in two immiscible solvents, or if there are several different solutes 

 in the mixture, this method becomes impractical. If one of the solvents 

 is bound on the surfaces of a solid material, however, and the other 

 solvent is allowed to flow over it, even relatively similar solutes may 

 separate from each other. In most of the systems of paper chromatog- 

 raphy one of the individual solvents in the mixture is likely to adhere as 

 a film on the paper, while the other solvent flows over this film. The 

 paper merely serves as a base on which the natural distribution or 

 partition can occur. In a petroleum ether plus acetone-water chroma- 



