Acid–base titration: find an unknown concentration
Deliver standard NaOH drop by drop, catch the drop where the indicator turns, and map the whole titration curve with a pH meter.
Aim Determine the concentration of an unknown acetic acid solution and compare the indicator endpoint with the inflection of the pH curve.
Procedure
- Pick an unknown sample number. The flask holds 25.00 mL of sample; the burette holds 0.1000 M NaOH.
- Make a rough run first: add 1 mL at a time, recording a point after each, to locate the pH jump.
- Refill and repeat. About 2 mL before the jump, switch to single drops and record after each one.
- When the pink colour persists, you have reached the endpoint — read the burette.
- Compute the acetic acid concentration from the endpoint volume, then reveal the true value and your relative error.
What to notice
- At half-equivalence (half the endpoint volume) the pH equals the pKa of acetic acid (4.76).
- The equivalence-point pH is above 7: the acetate formed is a weak base.
- Near the endpoint each drop flashes pink where it lands, then fades as it mixes — the signal that you are close.
- Switch to methyl orange: it changes at pH 3–4, long before equivalence, and gives a large systematic error.
In a real lab
- NaOH absorbs CO₂ from air, so in a real lab it is first standardised against potassium hydrogen phthalate (KHP).
- Read the bottom of the meniscus at eye level and estimate to 0.01 mL; one drop is about 0.05 mL.
- A pH electrode must be calibrated with two standard buffers first; the simulated meter here is an ideal instrument.
- Wear eye protection: 0.1 M NaOH is corrosive to eyes.