Question

If an acetate buffer solution was prepared using method 1 and it was supposed to have a pH of 4.7 but it actually came out to 4.9, what could be done to adjust the pH back down to 4.7 without completely remaking the solution and without altering the total amount of conjugate particles (HA + A‐)?

Method 1 Measure out 15.0 mL of 0.10 M HC2H3O2 and 15.0 mL of 0.10 M NaC2H3O2. Measure and record the pH of the individual solutions. Combine the solutions together, mix well, and measure and record the pH of the resulting buffer solution.

Answer 1

If base is added to a buffer, the weak acid will give up its H⁺ in order to transform the base (OH^-) into water (H₂O) and the conjugate base:

HA + OH^- → A^-+ H₂O.

Since the added OH^- is consumed by this reaction, the pH will change only slightly.

why the acid or base is not added in excess amount ...?

Because a buffer solution resists changes in pH upon the addition of a small amount of strong acid or strong base. ... (Sometimes a solution that is technically a buffer does NOT resist changes in pH. This occurs when so much acid or base are added to the buffer that they become the excess reactant.)

On addition of strong acid to a buffer, the weak base will react with the H⁺ from the strong acid to form the weak acid HA: H⁺ + A^- → HA. The H⁺ gets absorbed by the A^- instead of reacting with water to form H₃O⁺(H⁺), so the pH changes only slightly.

On addition of strong base to a buffer, the weak acid will give up its H⁺ in order to transform the base (OH^-) into water (H₂O) and the conjugate base: HA + OH^- → A^- + H₂O. Since the added OH^- is consumed by this reaction, the pH will change only slightly.