The Pink Endpoint That Came Too Soon
Students standardize a sodium hydroxide solution with potassium hydrogen phthalate and obtain 0.0987 M NaOH. They then titrate 25.00 mL of an unknown acetic acid solution with phenolphthalein. Group A records an initial buret reading of 0.40 mL and a final reading of 22.15 mL. The endpoint is a pale pink that fades after about 20 seconds, so the student adds one more drop and records 22.20 mL. Group B uses a pH meter and finds the steepest part of the curve at 22.05 mL, pH 8.7. A third group used methyl orange and stopped at pH 4.0, obtaining 21.90 mL. The teacher must decide which data to treat as valid, how to explain the indicator choice, and how to handle the group that used the wrong indicator without simply giving them the answer.
1. Calculate the acetic acid concentration from Group A's accepted volume and Group B's volume. Show how the endpoint difference affects the result.
Read the worked answer
For Group A, the first endpoint volume is 22.15 − 0.40 = 21.75 mL; after the extra drop, 22.20 − 0.40 = 21.80 mL. Using the stabilized endpoint, moles NaOH = 0.0987 mol/L × 0.02180 L = 0.002152 mol. Because acetic acid reacts 1:1 with NaOH, [acid] = 0.002152 mol / 0.02500 L = 0.0861 M. Using 21.75 mL gives 0.0859 M. Group B gives 0.0987 × 0.02205 = 0.002176 mol, so [acid] = 0.0870 M. The difference between 21.80 mL and 22.05 mL is 0.25 mL, about a 1.1% difference in concentration. The methyl orange volume of 21.90 mL would calculate to about 0.0865 M, but it is not a valid equivalence volume because the indicator changes far before the equivalence point. Group B's pH-based value is the most defensible, with Group A's stabilized reading close enough to support the same result within expected technique error.
2. Explain why phenolphthalein is appropriate and methyl orange is not for this weak acid-strong base titration, using the pH at equivalence.
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At the equivalence point, all acetic acid has been converted to acetate ion, which hydrolyzes: CH3COO− + H2O ⇌ CH3COOH + OH−. The resulting solution is basic; for typical concentrations, the equivalence pH is about 8.7, as Group B observed. Phenolphthalein changes from colorless to pink over roughly pH 8.2–10.0, so its transition range brackets the equivalence pH. Methyl orange changes over roughly pH 3.1–4.4, which is in the buffer region before equivalence. At pH 4.0, a significant fraction of acetic acid remains unneutralized, so stopping there underestimates the NaOH volume and the acid concentration. The indicator must have a transition range that overlaps the steep part of the titration curve at equivalence. This is not a matter of color preference; it is a matter of matching the indicator's pKa range to the chemistry of the equivalence point.
3. Draft the feedback you would give the group that used methyl orange and the group with the fading endpoint. Include what they should record and why.
Read the worked answer
To the methyl orange group: do not simply say 'wrong indicator.' Ask them to overlay their indicator range on their pH curve or a provided curve and mark where pH 4.0 falls relative to the equivalence volume. Have them calculate the fraction of acid remaining at pH 4.0 using the Henderson–Hasselbalch relationship or a simple partition diagram. Then ask why a basic equivalence point requires an indicator that changes in the basic region. To the fading-endpoint group: explain that a pale pink that persists for about 30 seconds is the endpoint; a drop that produces only a transient flash is not. They should record the buret reading when the stable pale pink appears and note the drop size so the class can discuss uncertainty. Require both groups to report the final volume, the indicator used, the observed color change, and a sentence justifying whether their endpoint represents equivalence. This turns the error into evidence-based feedback.