Titrations

Section 1 of 7What a titration does

Purpose

• Purpose A titration finds exactly what volume of one solution reacts with a fixed volume of another, usually an acid and an alkali.
• It is used to find an unknown concentration or to make a pure soluble salt.

Indicators

• Indicators An indicator shows the end-point, the moment of neutralisation.
• Phenolphthalein is pink in alkali and colourless in acid;
  • methyl orange is yellow in alkali, red in acid and orange at neutral.

Acids you may meet

• Acids you may meet Titrations are limited to the strong acids hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃) with alkalis.

Section 2 of 7Method for a titration

1

Use a pipette to transfer a fixed volume of alkali, commonly 25.0 cm³, into a conical flask and add a few drops of indicator.

2

Fill a burette with the acid and record the starting volume at eye level, reading the bottom of the meniscus.

3

Add the acid to the flask, swirling constantly, and slow to dropwise as the colour starts to change.

4

Stop when the indicator changes permanently and record the final burette reading.

5

Subtract to find the titre, then repeat until two or three results agree within about 0.10 cm³ and take the mean of those.

Section 3 of 7Preparing a pure soluble salt

1Find the volume

• Find the volume Carry out the titration once with indicator to find the volume of acid that exactly neutralises the alkali.

2Repeat without indicator

• Repeat without indicator Mix the same volumes again but with no indicator, so the salt is not contaminated.

3Evaporate

• Evaporate Heat the solution in an evaporating basin until crystals begin to form, then stop heating.

4Crystallise and dry

• Crystallise and dry Leave to cool, remove the crystals, and dry them between filter paper.

Section 4 of 7Comparing concentrations

Using volumes only

• Using volumes only For a 1:1 reaction, the solution needing the larger volume is the more dilute one.
• For example, if 30.0 cm³ of 0.2 mol/dm³ acid is neutralised by 15.0 cm³ of alkali, the alkali is twice as concentrated, at 0.4 mol/dm³.

Section 5 of 7Titration calculations (HT only)

1

Write the balanced equation, for example Ba(OH)₂ + 2HNO₃ → Ba(NO₃)₂ + 2H₂O.

2

Convert the known volume to dm³ by dividing by 1000 and find moles = concentration × volume: 0.15 × 0.02500 = 3.75 × 10^-3 mol of Ba(OH)₂.

3

Use the ratio from the equation: 1 mol of Ba(OH)₂ reacts with 2 mol HNO₃, so moles of HNO₃ = 7.5 × 10^-3 mol.

4

Concentration = moles ÷ volume in dm³ = 7.5 × 10^-3 ÷ 0.01280 = 0.59 mol/dm³ (2 d.p.).

5

To find a volume instead, rearrange: volume = moles ÷ concentration, then multiply by 1000. For 25.00 cm³ of 0.80 mol/dm³ KOH, moles = 0.020, so 0.50 mol/dm³ HNO₃ needs 0.040 dm³ = 40 cm³.

Section 6 of 7Key terms

Term	Meaning
Titration	A method of finding the volume of one solution that reacts exactly with a measured volume of another.
Burette	A long graduated tube with a tap, used to add a solution in measured amounts.
Pipette	Apparatus that measures one fixed volume of liquid accurately.
End-point	The point at which the indicator changes colour, showing neutralisation is complete.
Titre	The volume of solution added from the burette.
Concordant results	Repeat results that agree closely.

Section 7 of 7Exam technique

1

Adding indicator when making the salt crystals — the final mixture must be indicator-free.

2

Forgetting to convert cm³ to dm³ before using concentration × volume.

3

Ignoring the ratio in the equation, for example treating 2HNO₃ as 1:1 with Ba(OH)₂ (HT only).

4

Averaging a rough first titration with the accurate ones — use only concordant results.

5

Saying the solution needing the larger volume is more concentrated; it is more dilute.