AQA Chemistry (8462) 4.4.2.5 · Required practical

Titrations

One drop of acid turns a flask of alkali from pink to colourless. How do you measure exactly how much acid that took, and show the number can be trusted?

Triple Science · Higher tier

Required practical · titration

Start here

One drop turns the whole flask.

A conical flask holds 25.00 cm³ of sodium hydroxide solution and a few drops of phenolphthalein, so it is pink. Acid runs in from a burette. Near the end, each splash makes a colourless patch that vanishes when the flask is swirled. Then, after one more drop, the pink is gone for good.

If you had to guess, has the indicator been used up, or has the alkali been neutralised?

Required practical 2 (Chemistry) · the AQA method

Pipette the alkali. Burette the acid. Stop on one drop.

AQA titrations use strong acids only (sulfuric, hydrochloric, nitric) and strong alkalis such as sodium hydroxide.

Use a pipette and pipette filler to put 25.00 cm³ of sodium hydroxide solution into a conical flask on a white tile.
Add a few drops of a suitable indicator, such as phenolphthalein or methyl orange.
Rinse the burette with the acid, then fill it through a funnel, below eye level. Remove the funnel. Record the initial reading to the nearest 0.05 cm³, at the bottom of the meniscus.
Do a rough titration first. Then repeat, swirling the flask and adding acid drop by drop near the end point, until one drop gives a permanent colour change.
Record the final reading. Titre = final reading − initial reading.
Repeat until at least two titres are concordant (within 0.10 cm³ of each other). Calculate the mean of the concordant titres.

Measured: The titre, in cm³.
Kept the same: 25.00 cm³ of alkali, the acid's concentration, the indicator and how many drops.
Risks: Wear eye protection: sodium hydroxide harms eyes. Never pipette by mouth. Indicators in ethanol are flammable: keep them away from flames.

Think again · filling the burette

“I rinsed the burette with water first, so it was really clean.”

Spot the flaw

Predict

Water left inside the burette mixes with the acid. What happens to the titre?

Choose the indicator

Identify

Each strip shows an indicator's colour as acid is added to the alkali. Which indicator cannot give a sharp end point?

Run the practical

Call the rough run first

Rough to do · 0 accurate titres

Commit first. The rough run goes in 1 cm³ or more at a time. Compared with the careful runs, its titre will be…

Errors and the titre

Titre too large, too small, or no effect?

Place each slip.

Tap a card, then tap the box it belongs in. Tap a placed card to take it back.

Equations · processing the data

Learn it · chemistry has no equation sheet

Titre = final burette reading − initial burette reading

Mean titre = sum of concordant titres ÷ number of concordant titres

Never the rough run, never an anomalous titre. Give the mean to 2 decimal places.

Higher

From titre to concentration

On the Higher paper the mean titre starts a calculation. Find the moles in the solution whose concentration you know, with n = c × V and V in dm³. Use the ratio in the balanced equation to get the moles of the other substance, then c = n ÷ V. For g/dm³, multiply the concentration in mol/dm³ by M_r.

n = c × V

moles (mol) = concentration (mol/dm³) × volume (dm³). cm³ ÷ 1000 = dm³.

g/dm³ = mol/dm³ × M_r

Command words in this lesson

Describe a method: Each step in order, with the apparatus named: pipette, indicator, burette, end point, repeats.
Identify: Pick out the one that fits, such as the anomalous titre or the unsuitable indicator.
Suggest: An improvement or a source of error, applied to this titration.
Calculate: Show the working. A mean titre is in cm³, to 2 decimal places.

Key note · AQA 4.4.2.5 · RP2 (chemistry only)

Titrations

Titration: exact volumes of acid + alkali that neutralise each other.
Burette (acid) + pipette (alkali 25 cm³) + indicator + conical flask.
Endpoint = permanent colour change from one drop.
Concordant results within 0.10 cm³.
Mean from concordant only.
Discard rough run.
Higher: n = c × V with V in dm³; use the mole ratio; c = n ÷ V; g/dm³ = mol/dm³ × Mr.

Photograph this card. Tomorrow, cover it and say each line first.

Previous and next

Connects to

Ask MrBadmus

Not sure which titres are concordant?

The burette simulation uses model solutions: each run has a set end point near 24.70 cm³, and one run is planted as anomalous, as real runs sometimes are. Acid goes in whole drops of 0.05 cm³, and the end point shows as one sharp change; in a real flask, judging the colour adds its own scatter.