Indicator choice in a weak base–strong acid titration
A student standardises a solution of hydrochloric acid using 0.120 mol L−1 ammonia solution. Ammonia is a weak base: NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq). The student pipettes 25.0 mL of ammonia solution into a conical flask and titrates with hydrochloric acid of unknown concentration. The first trial gives an end-point at 19.8 mL; two accurate titres are 20.05 mL and 19.95 mL. The student uses phenolphthalein (pH range 8.3–10.0) and obtains a colour change. A teacher says the indicator is a poor choice and that the pH at equivalence is not 7. The student is asked to calculate the acid concentration, decide the indicator issue, and explain the equivalence pH.
1. Calculate the average titre and the concentration of the hydrochloric acid.
Read the worked answer
Use the accurate titres: 20.05 mL and 19.95 mL. Average titre = (20.05 + 19.95)/2 = 20.00 mL = 0.02000 L. n(NH3) = cV = 0.120 mol L−1 × 0.0250 L = 0.00300 mol. The reaction NH3 + HCl → NH4Cl is 1:1, so n(HCl) = 0.00300 mol. c(HCl) = n/V = 0.00300 mol/0.02000 L = 0.150 mol L−1.
2. Explain why the pH at equivalence is acidic, not neutral. Write an equation for the reaction responsible.
Read the worked answer
At equivalence the ammonia has been converted to ammonium chloride, so the flask contains NH4+, not NH3. NH4+ is the conjugate acid of a weak base and hydrolyses: NH4+(aq) + H2O(l) ⇌ NH3(aq) + H3O+(aq). This produces H3O+, so the solution is acidic (pH < 7). The equivalence pH is therefore not 7; it lies in the acidic region.
3. Evaluate phenolphthalein for this titration and suggest a better indicator.
Read the worked answer
Phenolphthalein changes colour over pH 8.3–10.0. In this titration the pH falls steeply near equivalence at about pH 5, so phenolphthalein would change colour before the equivalence point, giving a titre that is too low. A better indicator is methyl red (pH 4.4–6.2) or methyl orange (pH 3.1–4.4), because its colour change brackets the acidic equivalence pH. The indicator end-point should be as close as possible to the equivalence pH.