Titration: ethanoic acid vs sodium hydroxide
The classic weak acid–strong base titration. Standardise 0.1 M ethanoic acid against 0.1 M NaOH using phenolphthalein, and see why the endpoint lands above pH 7.
WHAT YOU'LL LEARN
- ✓Reading a burette to ±0.05 cm³ and getting concordant titres
- ✓Why phenolphthalein is the right indicator here — and methyl orange is not
- ✓Why the equivalence point of a weak acid + strong base sits above pH 7 (the ethanoate ion hydrolyses)
- ✓Calculating concentration from titre values
ON YOUR BENCH
- □50 cm³ burette + stand and clamp
- □25 cm³ pipette + filler
- □250 cm³ conical flask
- □White tile
- □0.1 M sodium hydroxide (in burette)
- □~0.1 M ethanoic acid (to standardise)
- □Phenolphthalein indicator
- □Distilled water wash bottle
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Rinse everything with the right liquid
Rinse the burette with a little NaOH solution (not water — water left inside dilutes it), and the pipette with the ethanoic acid. Rinse the conical flask with distilled water only; extra water in the flask doesn't change the moles of acid.
- 02
Fill the burette
Clamp the burette vertically and fill with 0.1 M NaOH using a funnel. Remove the funnel, open the tap briefly to fill the jet below the tap, then read the bottom of the meniscus at eye level. Record the initial reading to 2 decimal places (e.g. 0.00 or 1.25 cm³).
- 03
Pipette the acid
Pipette exactly 25.0 cm³ of ethanoic acid into the conical flask. Touch the pipette tip against the flask wall to release the last drop — but never blow it out; pipettes are calibrated to retain that drop.
- 04
Add the indicator
Add 2–3 drops of phenolphthalein. The solution stays colourless (phenolphthalein is colourless in acid). Place the flask on a white tile under the burette so the colour change is easy to see.
- 05
Rough titration
Run in NaOH about 1 cm³ at a time, swirling constantly, until one drop turns the whole flask permanently pale pink. Record the volume — this rough titre tells you roughly where the endpoint is so your accurate runs can be fast then careful.
- 06
Accurate titrations
Repeat with fresh 25.0 cm³ portions of acid. Run in NaOH quickly to about 2 cm³ before your rough titre, then add drop by drop, swirling after each drop. Near the endpoint, a pink flash appears and fades — stop the instant one drop leaves a permanent pale pink.
- 07
Get concordant results
Repeat until you have two titres within 0.10 cm³ of each other. Average only the concordant titres — the rough titre never counts.
- 08
Calculate
CH₃COOH + NaOH → CH₃COONa + H₂O is 1:1. Moles NaOH = 0.1 × (titre/1000). Moles acid = same. Concentration of acid = moles ÷ 0.025 dm³.
What you should see
- →A sharp colourless → pale pink endpoint. If it goes deep magenta you overshot — record it, and creep up more slowly next run.
- →Titres around 24–25 cm³ if both solutions are close to 0.1 M.
- →The equivalence point sits near pH 8.7, not 7: the ethanoate ion left behind is a weak base and hydrolyses (CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻). That's exactly why phenolphthalein (range 8.3–10.0) works and methyl orange (3.1–4.4) would change colour far too early.
- →Expect your calculated concentration within ~2% of 0.100 M. Bigger errors usually trace back to a rinsing mistake or reading the meniscus from an angle.
THE pH CURVE YOU'RE TRACING
| Rough | Run 1 | Run 2 | Run 3 | |
|---|---|---|---|---|
| Final reading /cm³ | 25.90 | 24.65 | 24.55 | 24.60 |
| Initial reading /cm³ | 0.00 | 0.00 | 0.00 | 0.05 |
| Titre /cm³ | 25.90 | 24.65 | 24.55 | 24.55 |
| Concordant? | — | ✗ | ✓ | ✓ |
⚠ BEFORE YOU START
- •Wear goggles throughout — 0.1 M NaOH is an irritant and splashes sting.
- •Fill the burette below eye level, never on the stand above your head.
- •Report and rinse any spills immediately; NaOH feels soapy on skin because it's reacting with it.
Try it, then run it for real.
Practise the whole thing on the virtual bench, then book real lab time and do it with your own hands.