Introduction to Titrations Revision Notes | GCSE Edexcel Higher Chemistry

1. What is a Titration and Why Do We Need It?

Picture this: you're given a bottle of hydrochloric acid, but the label has fallen off. You know it's HCl, but you don't know how concentrated it is. Is it strong enough to clean metal, or weak enough to add to food? This is where titrations save the day.

A titration is like a chemical detective story. You add a solution of known concentration (the standard solution) drop by drop to your mystery solution until they react completely. The moment this happens, an indicator changes colour - that's your clue that the reaction is finished.

2. The Titration Toolkit - Essential Equipment

Every titration needs five key pieces of equipment, and each has a specific job. Think of them as your chemical toolkit - miss one piece and the whole investigation falls apart.

The conical flask holds your unknown solution plus indicator. Why conical? The shape means you can swirl without splashing, and the narrow neck concentrates the colour change so you can spot the endpoint clearly.

3. Indicators - Your Chemical Traffic Lights

Indicators are like chemical traffic lights - they change colour to tell you when to stop adding solution. For strong acid-strong alkali titrations, you have two main choices, and picking the right one matters for accuracy.

Methyl orange works brilliantly for strong acid-strong alkali titrations. It's red in acid and yellow in alkali, with a sharp colour change at the endpoint. Phenolphthalein is colourless in acid and pink in alkali - also sharp and reliable.

Universal indicator might seem like the obvious choice, but it's actually terrible for titrations. Why? It changes through multiple colours gradually, making it impossible to spot the exact endpoint. You need a sharp, sudden change.

4. The Step-by-Step Method

Here's where theory meets practice. Follow this method exactly and you'll get reliable, accurate results every time. Miss a step and your results will be all over the place.

Worked example: Carrying Out an Acid-Alkali Titration

Find the concentration of sodium hydroxide solution using 0.100 mol/dm³ hydrochloric acid

1. Rinse burette with HCl, pipette with NaOH. Fill burette with 0.100 mol/dm³ HCl — Rinsing removes water that would dilute your solutions
2. Use pipette to add exactly 25.0 cm³ NaOH to conical flask. Add 2-3 drops methyl orange — Pipette gives accurate, fixed volume. Too much indicator affects the endpoint
3. Read burette to nearest 0.05 cm³. Record as initial reading — Always read at eye level to avoid parallax error
4. Add HCl drop by drop near endpoint, swirling constantly — Swirling mixes solutions. Slow addition near endpoint prevents overshoot
5. Stop when solution changes from red to yellow permanently — Permanent change means endpoint reached - no more reaction occurs
6. Read burette again. Calculate titre = final - initial reading — Titre is the volume of acid needed to neutralise the alkali

Answer: Repeat until you get concordant results (within 0.10 cm³ of each other)

5. Calculating Concentrations from Titration Results

The whole point of doing a titration is to find an unknown concentration. This is where your careful practical work pays off - you can now use your titre values to calculate exactly how concentrated your mystery solution is.

Worked example: Calculating Unknown Concentration

25.0 cm³ of NaOH solution was neutralised by 23.5 cm³ of 0.100 mol/dm³ HCl. Find the concentration of NaOH.

1. NaOH + HCl → NaCl + H₂O — 1:1 molar ratio - 1 mole NaOH reacts with 1 mole HCl
2. Moles = concentration × volume Moles HCl = 0.100 × (23.5/1000) = 0.00235 mol — Convert cm³ to dm³ by dividing by 1000
3. From equation: moles NaOH = moles HCl = 0.00235 mol — 1:1 ratio means equal moles react
4. Concentration = moles/volume [NaOH] = 0.00235/(25.0/1000) = 0.094 mol/dm³ — Convert volume to dm³, then divide moles by volume

Answer: Concentration of NaOH = 0.094 mol/dm³

Mark-winning tip: Always show the balanced equation first - it's usually worth a mark on its own. Then show each calculation step clearly with units throughout.

6. Exam Technique - Getting Full Marks

Titration questions are mark-rich in exams, but they're also where students lose clear and systematic marks through sloppy technique. Here's how to nail every mark available.

For 6-mark 'describe the method' questions, structure your answer like this: 1) Equipment setup and rinsing, 2) Measuring solutions accurately, 3) Adding indicator, 4) Carrying out the titration, 5) Recording results, 6) Repeating for reliability.

Common exam trap: Questions often ask why you rinse the burette with the acid solution. The answer is to remove water that would dilute the acid and make the concentration lower than expected. Don't say 'to clean it' - that's not specific enough for the mark.

A titre is the volume delivered from the burette, found by subtracting the initial burette reading from the final burette reading. Concordant titres are usually within 0.10 cm3 of each other. If one result lies outside that range, it should be treated as anomalous and left out of the mean. Students should also remember that either acid or alkali can be in the burette depending on the question setup.