Titrations

A titration finds the exact volume of an acid needed to neutralise a known volume of an alkali (or the other way round). From this, you can work out an unknown concentration.

• A pipette measures a fixed volume of one solution accurately, e.g. 25.0 cm³.
• A burette adds the other solution gradually and measures the volume added.
• An indicator shows the end point, when the acid and alkali have exactly neutralised each other.

The volume added from the burette is the titre.

1. Use a pipette and pipette filler to put 25.0 cm³ of sodium hydroxide solution into a conical flask, on a white tile.
2. Add a few drops of indicator, e.g. phenolphthalein (pink in the alkali).
3. Fill a burette with acid. Read the start volume at the bottom of the meniscus, at eye level.
4. Add acid while swirling the flask. Near the end point, add it drop by drop.
5. Stop when the colour changes permanently (pink → colourless). Record the final volume; titre = final − start.
6. Repeat until you get concordant titres (within 0.10 cm³ of each other), and use their mean.

Hazard: acids and alkalis are irritants — wear eye protection.

Better accuracy: do a quick rough titration first, then add acid drop by drop near the end point.

Example: 25.0 cm³ of sodium hydroxide is neutralised by 20.0 cm³ of 0.100 mol/dm³ sulfuric acid. What is the concentration of the sodium hydroxide?

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

1. Convert volumes to dm³: acid 0.0200 dm³, alkali 0.0250 dm³.
2. Moles of acid = concentration × volume = 0.100 × 0.0200 = 0.00200 mol
3. Ratio NaOH : H₂SO₄ = 2 : 1, so moles of NaOH = 2 × 0.00200 = 0.00400 mol
4. Concentration of NaOH = moles ÷ volume = 0.00400 ÷ 0.0250 = 0.160 mol/dm³

In g/dm³: 0.160 × 40 (Mᵣ of NaOH) = 6.4 g/dm³