Chemistry Sim · Titration
Add sodium hydroxide drop by drop to hydrochloric acid with phenolphthalein. Stop at the first permanent pale pink — that is your endpoint.
Flask: 25.0 mL of HCl (unknown concentration) + phenolphthalein. Burette: 0.100 mol/L NaOH.
1 drop = 0.05 mL. Go slowly near 25 mL — one drop can take you past the endpoint.
Add NaOH from the burette. Watch the flask for the first permanent pale pink.
At the endpoint, moles of NaOH = moles of HCl (1:1 reaction).
The moment when the moles of added base exactly equal the moles of acid in the flask: HCl + NaOH → NaCl + H₂O. Before it, the flask stays colorless because every OH⁻ is consumed. Just past it, excess OH⁻ turns phenolphthalein pink.
The endpoint is what you see — the indicator's color change. The equivalence point is the exact chemical balance, and the two are rarely perfectly identical. Good technique picks an indicator whose endpoint falls as close as possible to the equivalence point, which is why phenolphthalein suits a strong acid–strong base titration like this one.
Far from the endpoint, a millilitre barely changes the flask. Near it, a single drop (0.05 mL) can swing the mixture from acidic to basic. Real lab technique: swirl constantly and add dropwise once the pink starts lingering.