The partition coefficient is modified when the pH of the aqueous phase is adjusted or when a chelating agent is added. The partition coefficient is renamed the distribution coefficient. What is the impact on the extraction of a weak acid into an organic solvent as the pH of the aqueous phase increases? A) As the pH increases the value of D decreases and the percentage weak acid left in the aqueous phase decreases. B) As the pH increases the value of D increases and the percentage weak acid left in the aqueous phase increases. C) As the pH increases, the value of D decreases and the percentage weak acid left in the aqueous phase decreases. D) As the pH increases the value of D increases and the percentage weak acid left in the aqueous phase decreases. E) As the pH increases the value of D decreases and the percentage weak acid left in the aqueous phase increases.

The partition coefficient (D) relates the solute partitioning between two phases, in which the solute extraction from an aqueous phase into an organic phase is represented by:

S(aq) ⇄ S(or)

[tex] D = \frac{S_{or}}{S_{aq}} [/tex] (1)

When the pH of an aqueous phase increases by the use of an alkaline solution like NaOH, the solubility of a weak acid, for example, acetic acid, increases in the aqueous phase by the following reaction:

CH₃COOH + NaOH ⇄ CH₃COO⁻Na⁺ + H₂O (2)

Therefore, as the pH of the aqueous phase increase, the percent of the weak acid in the aqueous phase increases as a result of acid neutralization (equation 2).

The increase of the weak acid percentage in the aqueous solution produces a decrease in the partition coefficient, since the inverse relationship between the coefficient partition and the solubility in the aqueous phase (equation 1).

So, the correct answer is option E: as the pH increases the value of D decreases and the percentage weak acid left in the aqueous phase increases.

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