Dissociative Leaching for Simple Large Scale Purification

 

Dissociative leaching separations are a subset of dissociative extractions. Sometimes, the ionizable substances to be separated are both not significantly soluble in an aqueous medium. If the more reactive species (more acidic or more basic) forms the more water-soluble salt upon reacting with an insufficiency of salt former while the less reactive constituent remains insoluble in the water then excellent separations can be achieved simply by leaching the crude mixture with an aqueous solution of the less than stoichiometric reagent and filtering the residual solid from the aqueous liquid. The more reactive component is isolated from the aqueous phase and the less reactive material from the solid on the filter. 

For example, the separation of a mixture of o-chlorobenzoic acid and p-chlorobenzoic acid is carried out by suspending the solid mixture in an aqueous solution containing just enough sodium hydroxide to neutralize the ortho isomer. The separation factor was as high as 26. At 65 C the o-isomer was leached out completely from a mixture initially containing 40% o-isomer. Higher solubility coupled with a lower pKa of o-isomer is responsible for such excellent separation.