Medical brain notes 14

  INTRODUCTION Liquid-Liquid extraction  is a versatile and dependable separation technique wherein an aqueous   solution is usually brought into contact with another organic solvent, exclusively immiscible with the former, so as to affect a legitimate and actual transfer of either one or more solutes into the latter. The normal-feasible separations which can thus be achieved are found to be rather easy, fast, convenient and effective resonably. Invariably such separations may be performed by shaking the two liquids in a separatory funnel for a few minutes ; and may be extended either to large quantities of pharmaceutical substances or trace levels.   In the case of pharmaceutical chemicals that are mostly  ‘organic solutes’,  the liquid-liquid extraction system may very often make use of two immiscible organic solvents ( e.g.,  alcohol and ether) instead of the aqueous-organic type of extraction. On the contrary, the  ‘inorganic solutes’  no...

  THEORY The behavioural pattern of two immiscible solvents, say ‘ a ’ and ‘ b ’, is essentially nonideal with respect to one another. Now, if a third substance is made to dissolve in a two-phase mixture of the solvents ( i.e.,  ‘ a ’ and ‘ b ’), it may behave ideally in either phases provided its concentration in each individual phase is approximately small. Therefore, under these prevailing experimental parameters the ratio of the mole fractions of the solute in the two respective immiscible phases (‘ a ’ and ‘ b ’) is found to be a constant which is absolutely independent of the quantity of solute present. It is termed as the  Nernst Distribution Law  or the  Partition Law  and may be expressed as follows : where, [A] a  = Mole fraction of solute A in Phase ‘ a ’   [B] b  = Mole fraction of solute B in Phase ‘ b ’, and   K p  = A constant.   The constant (K p ) is also known as the  distribution coefficient  or the...

  FACTORS INFLUENCING SOLVENT EXTRACTION A number of cardinal factors exert a positive influence on the phenomenon of solvent extraction, namely :   ( a ) Effect of temperature and inert solutes,   ( b ) Effect of pH on extraction,   ( c ) Effect of ion-pair formation, and   ( d ) Effect of synergistic extraction.   These factors shall be discussed briefly below :   1. EFFECT OF TEMPERATURE AND INERT SOLUTES   The physical as well as chemical interactions of a solute is capable of changing its apparent partition coefficient between a pair of solvents. Therefore, it is absolutely necessary to take this into consideration while selecting an appropriate extraction-system. Craig and Craig* have advocated that the partition coefficients are normally not sensitive to temperature when the two solvents in question are more or less immiscible and also the concentrations are fairly low in both the phases. Thus, the effect of temperature on the partition coe...