Question

What conditions must be true about a weak acid solution in order to use the Henderson‐Hasselbalch equation to find solution pH and in what region of a titration curve do these conditions exist?

Answer 1

Ans. HH equation:     pH = pKa + log ([A^-] / [AH])

            Where, [A^-] = concentration of conjugate base

                        [AH] = concentration of weak acid.

To find the pH of a weak acid solution using HH equation, there must be the simultaneous presence of the weak acid and its conjugate base in the solution. The pH of a solution containing only weak acid or only the conjugate base can’t be determined using HH equation.

# The titration curve of weak acid (CH₃COOH) vs NaOH is shown below. The HH equation is valid to calculate of the solution in the region-

            I. All the titration curve region BEFORE reaching the titration equivalence point. At the titration equivalence point, all the weak acid is completely consumed. So, absence of weak acid from solution won’t permit the use of HH equation.

            II. The HH equation can’t also be used at initial point when there is only the weak acid in solution, but no conjugate base.