Analytical Chem istry - DePauw University

Chapter 7 Collecting and Preparing Samples335organic phaseHA orgK D+ H 2 O H 3 O + + A –HA aqK aFigure 7.28 Scheme for a liquid–liquid extraction of the weak acid, HA. Although the weak acid issoluble in both phases, its conjugate weak base, A – , is soluble only in the aqueous phase. The K a reaction,which is called a secondary equilibrium reaction, affects the extraction efficiency because itcontrols the relative abundance of HA in solution.ionic species are not soluble. In this case the partition coefficient and thedistribution ratio areHAorgK D= [ ][ HA ]aq[ HA ] [ HA ]org totalorgD = =[ HA ] [ HA ] + [ Aaq totalaq− ]aq7.287.29Because the position of an acid–base equilibrium depends on the pH, thedistribution ratio is pH-dependent. To derive an equation for D that showsthis dependency, we begin with the acid dissociation constant for HA.[ HO ][ A ]+ −3 aq aqK a=[ HA ]Solving equation 7.30 for the concentration of A –aq[ HA ]− a aq[ A ] = K ×aq+[ HO ]and substituting into equation 7.29 givesD =3 aq[ HA ]orgK+ × [ HA ]a aq[ HA ]aq+[ HO ]aqueous phase3 aq7.30Factoring [HA aq ] from the denominator, replacing [HA org ]/[HA aq ] withK D (equation 7.28), and simplifying leaves us with a relationship betweenthe distribution ratio, D, and the pH of the aqueous solution.