Relevant component extraction-partial

Analysis of complex mixtures often requires separation and isolation of components, or classes of components. Examples in noninstrumental analysis include extraction, precipitation, and distillation. These procedures partition components between two phases based on differences in the components physical properties. In liquid-liquid extraction components are distributed between two immiscible liquids based on their similarity in polarity to the two liquids (i.e., like dissolves like ). In precipitation, the separation between solid and liquid phases depends on relative solubility in the liquid phase. In distillation the partition between the mixture liquid phase and its vapor (prior to recondensation of the separated vapor) is primarily governed by the relative vapor pressures of the components at different temperatures (i.e., differences in boiling points). When the relevant physical properties of the two components are very similar, their distribution between the phases at equilibrium will result in shght enrichment of each in one of the phases, rather than complete separation. To attain nearly complete separation the partition process must be repeated multiple times, and the partially separated fractions recombined and repartitioned multiple times in a carefully organized fashion. This is achieved in the laborious batch processes of countercurrent liquid—liquid extraction, fractional crystallization, and fractional distillation. The latter appears to operate continuously, as the vapors from a single equilibration chamber are drawn off and recondensed, but the equilibration in each of the chambers or plates of a fractional distillation tower represents a discrete equihbration at a characteristic temperature. 

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