Question

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.

Answer 1

Option E.

Step-by-step explanation:

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)

`D = (S_(or))/(S_(aq))` (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.

I hope it helps you!