Titration practical

In the titration practical, you add acid from a burette to a measured volume of alkali until an indicator changes colour. The volume of acid you needed, the titre, tells you the reacting volumes, and at Higher tier you use it to work out the concentration of the acid. It is required practical 2 in AQA's GCSE Chemistry (it is not in Combined Science Trilogy), and making a standard solution and titrating it is required practical 1 in AQA's A-level Chemistry.

Aim

AQA's specification states the aim as: determination of the reacting volumes of solutions of a strong acid and a strong alkali by titration, and (Higher tier only) determination of the concentration of one of the solutions in mol/dm³ and g/dm³ from the reacting volumes and the known concentration of the other. AQA's handbook splits it by tier:

• Foundation tier: find the volume of dilute sulfuric acid that neutralises 25 cm³ of sodium hydroxide solution.
• Higher tier: do the same with 0.1 mol/dm³ sodium hydroxide, then calculate the concentration of the sulfuric acid.

The specification only asks for titrations of strong acids (sulfuric, hydrochloric and nitric acid) with strong alkalis. Our guide to neutralisation covers the reaction itself.

Equipment

AQA's handbook lists:

• a 25 cm³ volumetric pipette and a pipette filler;
• a 50 cm³ burette, a small funnel, and a clamp and clamp stand;
• a 250 cm³ conical flask and a white tile;
• dilute sulfuric acid and sodium hydroxide solution;
• phenolphthalein indicator.

Method

These steps follow the student sheet in AQA's handbook.

1. Use the pipette and pipette filler to put exactly 25 cm³ of sodium hydroxide solution into the conical flask.
2. Stand the flask on a white tile.
3. Clamp the burette upright in the clamp stand, with just enough room underneath for the flask and tile.
4. Close the tap. Use the small funnel to fill the burette with dilute sulfuric acid. Before it is full, open the tap briefly over a small beaker so acid fills the tap, then close it and fill to the 0.00 cm³ line. Remove the funnel.
5. Add 5 to 10 drops of phenolphthalein to the flask and swirl. The solution turns pink.
6. Open the tap so 10 cm³ of acid flows in slowly, swirling all the time. Then add the acid drop by drop until a single drop turns the solution permanently colourless, and shut the tap at once.
7. Read the burette and record the volume of acid added to 2 decimal places.
8. Repeat twice more, and work out the mean.

AQA's handbook notes that to keep the sheet simple it leaves out some traditional technique, which teachers may add. Each one makes the result more accurate:

• Rinse the glassware. Rinse the burette with a little of the acid and the pipette with a little of the alkali, so leftover water does not dilute them.
• Read at eye level. Read the bottom of the meniscus with your eye level with it.
• Do a rough titration first. It gives you the approximate titre. In the accurate runs you can then add acid quickly to about 1 cm³ short of it, then go drop by drop.
• Let the pipette drain. Let it empty under gravity and touch the tip on the side of the flask. Do not blow out the last drop.

The handbook also suggests a version without a burette, adding the acid from a measuring cylinder with a dropping pipette. It is less accurate, but gives reasonable results.

Variables

A titration is a measurement rather than a test of how one variable affects another, so it has no independent variable in the usual sense.

Results table

AQA's handbook gives these results from its technician, for 25 cm³ of 0.1 mol/dm³ sodium hydroxide:

The rough titration is left out of the mean because it is an outlier. If you do not refill the burette to 0.00 cm³ each time, add columns for the start and finish readings and subtract them to get each titre.

How to calculate the concentration (Higher tier)

Use the mean titre and the balanced equation:

Two moles of sodium hydroxide react with one mole of sulfuric acid, so the acid has half as many moles as the alkali. Here are AQA's technician results worked through in four steps.

Step 1: moles of sodium hydroxide

Step 2: moles of sulfuric acid

Step 3: concentration in mol/dm³

Divide by the mean titre in dm³: 15.45 cm³ is 0.01545 dm³.

Step 4: concentration in g/dm³

M_r(H₂SO₄) = 2 × 1 + 32 + 4 × 16 = 98, so 0.0809 × 98 = 7.93 g/dm³.

The technician made the acid up to 0.08 mol/dm³, so this result is about 1% high. Our guide to titration calculations has more worked examples, including hydrochloric acid and vinegar.

Here is a quick check.

Concordant titres and uncertainty (A level)

AQA's A-level handbook says one titre is never enough. You repeat until at least two titres are concordant, which means within an agreed range, often 0.10 cm³, and average those. Its example: a rough titre of 23.85 cm³, then 23.20, 23.45 and 23.25 cm³. Titres 23.20 and 23.25 are concordant, so the mean is 23.23 cm³. By that standard, the GCSE technician's 15.55 and 15.35 cm³ are 0.20 cm³ apart, so an A-level student would do another run.

The handbook breaks the uncertainty in one titre into three parts:

For a titre of 23.23 cm³, that is a percentage uncertainty of 0.15 ÷ 23.23 × 100 = 0.65%. A bigger titre gives a smaller percentage uncertainty.

Making a standard solution

A-level required practical 1 starts by making a standard solution: dissolve a weighed solid in distilled water in a beaker, pour it into a volumetric flask with the beaker's rinsings, make it up to the line, then stopper and invert to mix. You then titrate it as above.

Choosing an indicator

AQA's method uses phenolphthalein, which is pink in the alkali and turns colourless at the end point. Methyl orange also works for a strong acid with a strong alkali: it is yellow in alkali and turns orange at the end point. Universal indicator is not used because it changes colour gradually, so there is no sharp end point. Our guide to the methyl orange colour change explains how to choose an indicator for weak acids and bases.

Try it in the Titration Lab

The Titration Lab runs a virtual titration and plots the pH as you go. Set it up like AQA's practical: base in the flask, 25 cm³ of 0.1 mol/dm³ sodium hydroxide, 0.08 mol/dm³ sulfuric acid in the burette and phenolphthalein. Add the acid drop by drop near 15 cm³ and watch for the colour change and the steep jump in the pH curve.

Try it

Titration Lab

Run a virtual titration and plot the pH curve as you go

Open the simulatorGo to its page

Risks

Errors and improvements

• Overshooting the end point. Adding acid too fast near the end point gives a titre that is too big. Use the rough titration to know when to slow down, and add drops near the end.
• An air bubble in the tap. If the tip is not filled before the start, the bubble's volume is read as acid. Fill the tap first, as step 4 does.
• Reading the burette wrongly. Take the reading from the bottom of the meniscus, with your eye level with it, and record it to 2 decimal places.
• Not swirling. Unmixed acid can give a false colour change.
• Wet glassware. Leftover water in the burette or pipette makes the solution more dilute. Water in the conical flask does not matter, because the number of moles of alkali in it is fixed by the pipette.

Common mistakes

• Forgetting the 1 to 2 ratio. With sulfuric acid and sodium hydroxide, halve the moles of NaOH to get the moles of acid.
• Using cm³ in c = n ÷ V. Divide the volume by 1000 to get dm³ first.
• Including the rough titre in the mean. Leave it out.
• Recording 15.4 instead of 15.40. Burette readings go to 2 decimal places.
• Saying the indicator turns clear. Clear means see-through; phenolphthalein turns colourless.

Practise chemistry in a short timed paper

GCSE Chemistry Higher and A-level Chemistry: Year 1 each give you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course, so titrations and required practicals turn up regularly but not in every paper.

Read next

Chemistry · Acids and basesTitration calculator: concentration, titre and pH curveWork out an unknown concentration from a titration, step by step, then check it in a free virtual titration lab that plots the pH curve as you go.