Which species in a phosphate buffer contributes to acidity when lowering the pH?

• A. Disodium phosphate
• B. Monosodium phosphate
• C. Sodium hydroxide
• D. Phosphoric acid

Answer: (B)

In a phosphate buffer system, the components consist of weak acid and its conjugate base. The species that contributes to acidity when lowering the pH is monosodium phosphate. This compound is the salt of a weak acid (H2PO4-) and contributes to the buffer's ability to resist pH changes.

When the pH is lowered, there is a higher concentration of hydrogen ions (H+), and monosodium phosphate can release more H+ ions to respond to this change by further contributing to the acidic condition. The equilibria involved allow monosodium phosphate to act as a weak acid, dissociating to release H+ ions under conditions of higher acidity, thus contributing to the overall acidity of the solution.

Disodium phosphate is more basic in nature, and sodium hydroxide is a strong base that would instead raise the pH rather than lower it. Phosphoric acid, while an acid, is not the main species in a typical phosphate buffer system that actively adjusts to changes in pH when the pH is lowered; rather, it is more stable at a lower pH. Therefore, monosodium phosphate is the correct answer, as it effectively participates in maintaining and adjusting the pH in response to increased acidity.