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23_Extraction

# 23_Extraction - For a solute Z in equilibrium exists...

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For a solute, Z, in equilibrium exists between an aqueous and organic solvent: Z (2) Z (1) At equilibrium, we have: K P = This assumes ideal behavior at low concentrations. It actually results in a ternary system. [ Z ] 1 [ Z ] 2 When dealing with aqueous species, the solute may exist in equilibrium with several other forms. Example - a weak acid organic phase 1 aqueous phase 2 HA HA H + + A - Due to competing equilibria, we define an alternate form of the partition coefficient: distribution ratio = D c = = Total Z represents the total of all equilibrium forms of species Z. This ratio is based on specific solution conditions such as pH. [ total Z ] 1 C 1 [ total Z ] 2 C 2 If K P = [HA] 1 / [HA] 2 and K a = [H + ] 2 [A - ] 2 / [HA] 2 then D c = = = [HA] 1 [HA] 2 + [A - ] 2 [HA] 1 [HA] 1 K a [HA] 1 K P K P [H + ] 2 + K P [H + ] 2 [H + ] 2 + K a In the case of a weak acid, D c is dependent on solution pH. D c = K P [H + ] 2 [H + ] 2 + K a pH log D c (1) (2) A plot of log D c vs log pH shows two regions. 1 - [H + ] >> K a , D c K P 2 - D c is pH dependent Its best to hold pH and other factors constant. ~

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The D c can be defined based on total equilibrium concentrations as: D c = where: 1 is the phase being extracted into 2 is the phase being extracted from All solution conditions are assumed constant. Total solute amounts are based on solution volume.
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23_Extraction - For a solute Z in equilibrium exists...

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