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.

BY THE END YOU SHOULD BE ABLE TO

• 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

1. 1

   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.

2. 2

   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).

3. 3

   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.

4. 4

   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.

5. 5

   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.

6. 6

   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.

7. 7

   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.

8. 8

   Calculate

   CH₃COOH + NaOH → CH₃COONa + H₂O is 1:1. Moles NaOH = 0.1 × (titre/1000). Moles acid = same. Concentration of acid = moles ÷ 0.025 dm³.

Time at the bench

~60 min

School stage

Form 3–4 · Year 12

Examined under

IGCSE · A-Level · 8-4-4

Apparatus to collect

10 items