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Chemistry 062012.2

Acid–base titrations

Titration procedure, indicators, and concordant results.

Review these first

Learning objectives

What you need to be able to do

Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.

  • 12.2.1Describe how to carry out an acid–base titration and select an appropriate indicator.

6 minute read

Acid–base titrations

A titration answers one question very precisely: exactly what volume of one solution is needed to exactly react with a known volume of another. The whole procedure exists to nail down that volume as accurately as possible.

The procedure, stage by stage

  1. Use a pipette to measure a precise, fixed volume (e.g. 25.0 cm³) of one solution into a conical flask, and add a few drops of indicator.
  2. Fill a burette with the other solution and record the initial reading.
  3. Add the burette solution to the flask, swirling constantly, until the indicator just changes colour (the end point) — record the final reading. The difference between final and initial readings is the titre.
  4. Do a quick, less careful rough titration first to find an approximate volume, so the accurate repeats can be run faster (adding quickly until near the expected end point, then drop by drop).
  5. Repeat the titration until you get concordant results — titres within 0.10 cm³ of each other — and calculate the mean of the concordant titres only (discard any outliers).

Choosing an indicator

The indicator needs a colour change that happens sharply at the neutralisation point. For a strong acid–strong alkali titration, most indicators (phenolphthalein, methyl orange) work well because the pH change at the end point is very sharp and spans their colour-change range. Methyl orange is red in acid and yellow in alkali; phenolphthalein is colourless in acid and pink in alkali.

Think of it like this

A titration is like carefully adding salt to a pot of soup by the pinch instead of pouring it all in at once — you add roughly (the rough titration) to get close, then add drop by drop once you're near the answer, watching for the exact moment it "tastes right" (the colour change).

Worked examples

Method, step by step

In a titration, three titres were recorded: 24.60 cm³, 24.55 cm³, 27.80 cm³. Identify which titres are concordant and calculate the mean titre to use.

  1. 1Concordant results are within 0.10 cm³ of each other.
  2. 224.60 and 24.55 differ by 0.05 cm³ — concordant.
  3. 327.80 differs from the others by far more than 0.10 cm³ — it is an outlier and should be discarded.
  4. 4Mean of concordant results: (24.60 + 24.55) / 2 = 24.575, rounded to 24.58 cm³.

24.60 cm³ and 24.55 cm³ are concordant; 27.80 cm³ is discarded. Mean titre = 24.58 cm³ (2 d.p.).

Common misconceptions

  • Adding the titrant quickly all the way to the colour change on every attempt — the accurate repeats should be added drop by drop once near the volume found in the rough titration, otherwise the end point is easily overshot.
  • Averaging all titre results including outliers — only concordant results (within 0.10 cm³ of each other) should be averaged; a titre far from the others should be discarded, not included.

In the exam

  • A "describe the method" question is marked stage by stage: pipette + indicator, fill burette + initial reading, add while swirling until colour change + final reading, repeat for concordant results and average — missing any one stage costs a mark.
  • "Swirling" the flask while adding from the burette is a specific, frequently-forgotten mark point — it ensures the solutions mix evenly so the colour change is seen as soon as it happens.