Base and Acid Extractions

The discussion of Section 5.2 focuses on partitioning one substance between two immiscible solvents. Now consider what happens if a mixture of two or more compounds is present in a given volume of solvent and an extraction using a solvent is performed. If the partition coefficient K of one component. A, is significantly greater than 1.0 and if those of other components are significantly less than 1.0, the majority of A will be in S, whereas most of the other compounds will remain in Sq. Physical separation of the two solvents will give a partial separation, and thus purification, of the solute A from the other components of the mixture. 

Solutes differing significantly in polarity should have very different coefficients K for partitioning between nonpolar and polar solvents. For example, consider the distribution of two organic compounds, the first neutral and nonpolar, and the second ionic and polar, between a nonpolar solvent and a polar solvent. If a solution of these compounds in the nonpolar solvent is shaken with the polar solvent, the neutral compound will preferentially partition into the nonpolar phase, whereas the polar constituent will preferentially partition into the polar phase. Separating the two phases effects a separation of the two solutes. 

two water-insoluble organic compounds, HA, which is acidic, and N, which is neutral, that are dissolved in an organic solvent may be separated by selectively extracting the acidic compound into a basic aqueous phase. After the aqueous and organic phases are separated, HA is recovered from the aqueous phase upon neutralization, and N is obtained by removing the organic solvent (Fig. 5.1). 

The choice of the base is determined by the acidity of the organic acid HA. This is defined in aqueous solution by in Equation 5.14 and is often expressed as pK (Eq. 5.15). 

The next step in determining what base is needed is to consider the equilibrium shown in Equation 5.16, in which HA is the organic acid, B is the base being used, and HB is the conjugate acid of this base. The equilibrium constant, for this 

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