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.
Reading it isn't running it.
Collect the apparatus, build the setup and take your own readings — as many times as it takes.