Which method will effectively lower the pH of a phosphate buffer solution from 7.6 to 7.2?

• A. Add enough 1 M Na2HPO4
• B. Add 1 M NaOH
• C. Alter the ratio of monosodium/disodium phosphate
• D. Add distilled, deionized water

Answer: (C)

To effectively lower the pH of a phosphate buffer solution from 7.6 to 7.2, altering the ratio of monosodium phosphate (NaH2PO4) to disodium phosphate (Na2HPO4) is the most appropriate method. Phosphate buffers consist of a weak acid (NaH2PO4) and its conjugate base (Na2HPO4).

In a buffer solution, the pH is determined by the ratio of the concentrations of the acid and its conjugate base, which follows the Henderson-Hasselbalch equation:

[ \text{pH} = \text{pKa} + \log\left(\frac{[\text{base}]}{[\text{acid}]}\right) ]

To lower the pH, the concentration of the weak acid (NaH2PO4) must be increased, or the concentration of the conjugate base (Na2HPO4) needs to be decreased. By adjusting this ratio, you can effectively lower the pH of the solution.

When considering the other options, adding 1 M Na2HPO4 would actually increase the concentration of the conjugate base, potentially raising the pH rather than lowering