How do you do the titration required practical?

What does the titration practical investigate?

It finds the exact volumes of an acid and an alkali that react with each other. You add acid from a burette to a measured volume of alkali until an indicator shows the alkali has just been neutralised. You may also hear it called the neutralisation practical.

Titration is on separate (triple) Chemistry only. If you take Combined Science: Trilogy, titration is not on your course, so you do not need this practical.

AQA sticks to strong acids and strong alkalis, and the acids it names are sulfuric, hydrochloric and nitric. On the Higher tier you also use your results to work out the concentration of one of the solutions, in mol/dm³ and in g/dm³.

What equipment do you need?

The two pieces that make a titration accurate are the pipette, which measures the alkali, and the burette, which measures the acid. Both measure volumes far more accurately than a measuring cylinder. You also need an indicator, and a white tile to stand the conical flask on so the colour change is easy to see.

• A 50 cm³ burette, clamped upright in a stand
• A 25 cm³ pipette and a pipette filler
• A 250 cm³ conical flask and a white tile
• A small funnel for filling the burette
• Dilute sulfuric acid (hydrochloric or nitric acid also work)
• Sodium hydroxide solution
• An indicator: phenolphthalein or methyl orange
• Eye protection

What is the method?

Measure the alkali into a flask with the pipette, add indicator, then run acid in from the burette until the colour just changes. Do one quick rough titration first, then careful repeats until your results agree.

• Use the pipette and filler to transfer 25.0 cm³ of sodium hydroxide solution into the conical flask.
• Add a few drops of phenolphthalein. The solution turns pink.
• Stand the flask on the white tile under the burette.
• With the burette below eye level, fill it with acid using the funnel, then remove the funnel. Run a little acid through the tap so there is no air bubble below it.
• Read the burette at eye level, from the bottom of the meniscus (the curved surface of the liquid), and record the starting reading.
• Do a rough titration: add acid a few cm³ at a time, swirling, until the pink colour disappears. This tells you roughly where the end point is.
• Rinse the flask, pipette in a fresh 25.0 cm³ of alkali, add indicator and note the new starting reading. This time, run in acid quickly until you are about 2 cm³ short of the rough value, then add it drop by drop, swirling after each drop.
• Stop when one drop turns the solution colourless and it stays that way. Record the final reading: the titre is the final reading − the starting reading.
• Repeat until you have at least two concordant titres, which means titres within 0.10 cm³ of each other.

What are the variables?

A titration is a measurement rather than a test of how one thing affects another, so nothing is changed on purpose. The thing you measure is the titre, and the rest of the table is about keeping every run identical so the titres can be compared.

How do you work out and present the results?

Record your readings in a table with a column for each run (rough, 1, 2, 3) and rows for the final reading, the starting reading and the titre. Give every reading to two decimal places, ending in 0 or 5, because a burette can be read to the nearest 0.05 cm³.

The mean titre uses concordant results only, never the rough one. Say your titres were: rough 20.70 cm³, then 20.35, 20.05 and 19.95 cm³. The concordant pair is 20.05 and 19.95, which are within 0.10 cm³ of each other, so the mean titre is (20.05 + 19.95) ÷ 2 = 20.00 cm³.

On the Higher tier you then find the unknown concentration. Work out the moles in the solution you know, use the balanced equation to get the moles in the other, then divide by its volume in dm³. Multiplying by the relative formula mass (Mr) turns mol/dm³ into g/dm³. Worked example: 25.0 cm³ of 0.120 mol/dm³ sodium hydroxide needs a mean titre of 20.00 cm³ of sulfuric acid.

• Moles of NaOH = 0.120 × (25.0 ÷ 1000) = 0.00300 mol.
• The equation is H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O, so 1 mole of acid reacts with 2 moles of alkali. Moles of H₂SO₄ = 0.00300 ÷ 2 = 0.00150 mol.
• Concentration of H₂SO₄ = 0.00150 ÷ (20.00 ÷ 1000) = 0.0750 mol/dm³.
• In g/dm³: the Mr of H₂SO₄ is (2 × 1) + 32 + (4 × 16) = 98, so 0.0750 × 98 = 7.35 g/dm³.
• With hydrochloric or nitric acid the ratio is 1:1, so you skip the halving step.

How do you make it accurate and safe?

Accuracy here is all about the end point. Swirl the flask the whole time and go drop by drop near the end, so you do not overshoot. The white tile makes the colour change easier to spot. Read the burette at eye level, from the bottom of the meniscus, and check there is no air bubble in the tip.

Rinse the burette with a little of the acid and the pipette with a little of the alkali before you use them, so leftover water does not dilute them. The conical flask can be rinsed with distilled water, because extra water does not change the amount of alkali inside it.

Choose an indicator with a sharp colour change, such as phenolphthalein or methyl orange. Universal indicator changes gradually through many colours, so it has no clear end point.

For safety, wear eye protection: sodium hydroxide solution is especially harmful to eyes. Always use a pipette filler, never your mouth. Fill the burette below eye level so acid cannot splash into your face, and wipe up spills straight away.

What do exam questions ask about it?

Method questions ask why each piece of kit is used: why a pipette and not a measuring cylinder, why a white tile, why you swirl, and why the acid goes in drop by drop near the end. Each answer comes back to measuring the volume, or spotting the end point, more exactly. Results questions ask you to pick out concordant titres, work out the mean and explain why the rough titre is left out.

Higher papers add the concentration calculation, sometimes in two parts: mol/dm³, then g/dm³. Here is one to try:

"A student titrates 25.0 cm³ of potassium hydroxide solution with 0.200 mol/dm³ nitric acid. The mean titre is 22.50 cm³. Calculate the concentration of the potassium hydroxide solution in g/dm³. (4 marks)"

The ratio is 1:1 and the Mr of KOH is 56, so it comes to 0.180 mol/dm³, which is 10.1 g/dm³.

Common mistakes

Each of these is a quick check once you know to look for it.

• Including the rough titre in the mean. It only tells you roughly where the end point is.
• Averaging titres that do not agree. Use only results within 0.10 cm³ of each other.
• Forgetting to convert cm³ to dm³. Divide by 1000 before you work out moles or a concentration.
• Using a 1:1 ratio with sulfuric acid. One H₂SO₄ reacts with two NaOH, so halve the moles of alkali to get the moles of acid.
• Reading the top of the meniscus, or reading it from above or below. Get your eye level with the bottom of the curve.