Acid–base titrations (New)

Step-by-step.

1. RINSE the pipette with the alkali (so any residue inside doesn't dilute the volume you take). Rinse the burette with the acid.

2. PIPETTE a known volume of alkali (e.g. $25.0\,\text{cm}^{3}$) into a CLEAN conical flask. Add a few drops of INDICATOR.

3. FILL the burette with acid above the $0$ mark; tap to remove air bubbles; release acid until the level is at $0$ (or some recorded starting volume). Record initial reading.

4. TITRATE: SLOWLY release acid from the burette into the conical flask, swirling continuously to mix. Watch for the indicator colour change.

5. STOP at the end-point (first permanent colour change). Record final burette reading.

6. TITRE = final − initial reading.

7. REPEAT the titration. Get at least three CONCORDANT titres (within $0.10\,\text{cm}^{3}$ of each other). Average the concordant titres for the final value.

Why repeat? First titration is rough (you find the approximate end-point). Second and third are precise — drop-by-drop near the end-point.

Cambridge tip. Cambridge often awards marks for: pipette use, indicator addition, swirling during titration, drop-by-drop near end-point, repeating.

Indicators change colour over a range of pH.

Indicator	Acidic	Neutral	Alkaline	pH range of change
Methyl orange	Red	Orange	Yellow	$\sim 3-4$
Phenolphthalein	Colourless	(still colourless)	Pink	$\sim 8-10$
Universal indicator	Red → green → purple	over pH 1-14		wide

Choosing for titration. Cambridge is broadly:

• Strong acid + strong base: any indicator works (large pH jump at end-point covers all ranges).
• Strong acid + weak base (e.g. HCl + NH₃): use METHYL ORANGE (end-point in acidic range).
• Weak acid + strong base (e.g. ethanoic acid + NaOH): use PHENOLPHTHALEIN (end-point in alkaline range).
• Weak acid + weak base: titration not reliable (no sharp colour change).

Universal indicator is NOT used in titrations — colour change is gradual, not sharp. Used for measuring unknown pH only.

End-point colour change examples.

• Phenolphthalein in NaOH (pink) → as HCl is added, becomes colourless at end-point.
• Methyl orange in NaOH (yellow) → red at end-point.

Cambridge tip. Cambridge often asks "which indicator is suitable?" Answer with the indicator AND the colour change. Don't just say "phenolphthalein" — say "phenolphthalein turns from pink to colourless".

Standard sequence.

1. Write the balanced equation.
2. Work out moles of the KNOWN reagent: $n = c \times V$ (V in dm³).
3. Use the mole RATIO from the equation to find moles of the OTHER reagent.
4. Use the volume of the OTHER reagent to find its concentration: $c = n/V$.

Worked. $25.0\,\text{cm}^{3}$ of NaOH(aq) is neutralised by $20.0\,\text{cm}^{3}$ of $0.10\,\text{mol/dm}^{3}$ HCl. Find concentration of NaOH.

Step 1. Equation: $\text{NaOH} + \text{HCl} \to \text{NaCl} + \text{H}_{2}\text{O}$.

Step 2. Moles of HCl: $n = c V = 0.10 \times (20.0/1000) = 0.002\,\text{mol}$.

Step 3. Mole ratio NaOH : HCl = $1 : 1$. So moles of NaOH = $0.002\,\text{mol}$.

Step 4. Concentration of NaOH: $c = n/V = 0.002 / (25.0/1000) = 0.080\,\text{mol/dm}^{3}$.

Worked (different ratio). $25.0\,\text{cm}^{3}$ of NaOH(aq) neutralised by $15.0\,\text{cm}^{3}$ of $0.10\,\text{mol/dm}^{3}$ H₂SO₄.

Step 1. $\text{H}_{2}\text{SO}_{4} + 2\text{NaOH} \to \text{Na}_{2}\text{SO}_{4} + 2\text{H}_{2}\text{O}$.

Step 2. Moles H₂SO₄: $0.10 \times 0.0150 = 0.00150\,\text{mol}$.

Step 3. Ratio H₂SO₄ : NaOH = $1 : 2$. Moles of NaOH: $2 \times 0.00150 = 0.00300\,\text{mol}$.

Step 4. Concentration of NaOH: $0.00300 / 0.0250 = 0.120\,\text{mol/dm}^{3}$.

Cambridge tip. Watch out for stoichiometric ratios (especially 1:2 with H₂SO₄ or H₃PO₄). Always start with the balanced equation.