Weak-base anion exchange resins behave much like their weak-acid counterparts; they do not remove anions above what pH level?

• A. pH 4
• B. pH 6
• C. pH 8
• D. pH 10

Answer: (B)

Weak-base anion exchange resins are designed to exchange their hydroxyl ions for anions in solution, but their effectiveness is influenced heavily by the pH of that solution. As the pH increases, the protonation of the amine groups in the resin decreases, leading to a reduction in the resin's ability to effectively capture and hold onto anions.

At around pH 6, the weak-base anion exchange resin reaches a threshold where the resin becomes less favorable for binding anions since the resin's functional groups are less able to compete with the higher concentration of hydroxide ions in the solution. This reaction results in diminished exchange capacity, meaning the resin will no longer effectively remove anions from the water.

Therefore, anion exchange resins start losing efficiency at pH levels above this point, particularly at pH 6 and beyond, making it the correct answer in this context.