4.12
Extraction: Effects of pH
Consider a neutral form of an amine, B, with a partition coefficient, K, in a liquid mixture containing organic and aqueous phases. The pH of the aque…
Consider a weak base, B, with a partition coefficient, K, between the organic and aqueous phases.
While organic bases are usually more soluble in the organic phase, changing the pH can convert them to their water-soluble salt form.
Here, the conjugate acid, BH+, has the acid dissociation constant Ka.
If BH+ is soluble only in the aqueous phase, then the distribution coefficient, D, can be calculated from the total concentration of B.
Substituting for K and Ka and rearranging gives the fraction of neutral base in the aqueous phase.
The pH-dependent distribution ratio implies that a base can be extracted into the aqueous phase as its conjugate acid within a suitable pH range.
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Q1: How does pH affect the extraction of weak bases in liquid-liquid extraction?
pH controls whether a weak base exists in its neutral or charged conjugate acid form. At low pH, the base converts to its water-soluble conjugate acid, BH+, which preferentially dissolves in the aqueous phase. This pH-dependent shift in the distribution coefficient, D, allows bases to be selectively extracted into the aqueous phase by adjusting pH to favor the charged form.
Q2: What is the relationship between partition coefficient and distribution coefficient?
The partition coefficient, K, describes how a single neutral species distributes between organic and aqueous phases. The distribution coefficient, D, extends this concept by accounting for all forms of a solute, including its conjugate acid or base. D is calculated from K and the acid dissociation constant, Ka, making it pH-dependent and more useful for predicting extraction behavior of ionizable compounds.
Q3: Why is the conjugate acid form of an amine more soluble in the aqueous phase?
The conjugate acid, BH+, is charged and polar, making it highly soluble in the polar aqueous phase. In contrast, the neutral base form is less polar and preferentially dissolves in the organic phase. This difference in solubility between charged and neutral forms is the basis for pH-controlled extraction, allowing separation based on ionization state.
Q4: How can you determine the fraction of neutral base in the aqueous phase?
The fraction of neutral base in the aqueous phase is determined by substituting the partition coefficient, K, and acid dissociation constant, Ka, into the distribution coefficient equation and solving algebraically. This calculation reveals how pH influences the equilibrium between the neutral and charged forms, enabling prediction of extraction efficiency at different pH values.
Q5: What does a log D versus pH plot reveal about base extraction?
A log D versus pH plot shows how the distribution coefficient changes with pH. The plot demonstrates that at low pH, log D decreases as the base converts to its charged conjugate acid and shifts into the aqueous phase. This graphical representation helps identify the optimal pH range for extracting a base into the aqueous phase as its conjugate acid.
Q6: How does the acid dissociation constant affect extraction of a weak base?
The acid dissociation constant, Ka, determines the pH at which the conjugate acid, BH+, ionizes. A lower Ka value means the conjugate acid is stronger and remains protonated at higher pH values, extending the pH range for aqueous extraction. Ka directly influences the distribution coefficient calculation, making it essential for predicting extraction behavior.
Q7: Can charged acids be extracted into the organic phase by adjusting pH?
Yes, charged acids can be extracted into the organic phase at sufficiently high pH. At elevated pH, the acid loses its proton and converts to its neutral conjugate base, which is more soluble in the organic phase. This pH-dependent extraction mechanism mirrors that of bases but operates in the opposite pH direction.