Titrations

Why measure reacting volumes?

An acid and an alkali neutralise one another. If you know a fixed volume of one solution, a titration lets you find how much of the other solution is needed to react with it. A suitable indicator signals when enough has been added. This is AQA Chemistry required practical 2: determining the reacting volumes of a strong acid and a strong alkali. For this titration work, the acids are hydrochloric, nitric or sulfuric acid. Higher Tier students also use the measured volumes and a known concentration to calculate the concentration of the other solution.

Measuring the volumes

Use a pipette to transfer a fixed volume of alkali, such as 25.0 cm³, into a conical flask. Add a few drops of a suitable indicator and place the flask on a white tile so its colour change is easier to see. Fill a burette with the acid and record its initial reading. The pipette gives an accurately measured starting volume; the burette can deliver acid in variable amounts and show how much has been added. The acid and alkali can be exchanged between the flask and burette: what matters is knowing which volume was fixed and which was delivered.

Run acid into the flask while swirling it. First do a rough trial to locate the approximate end point. Then repeat: add acid more slowly as you approach that volume, and drop by drop near the end, stopping when the indicator's colour change persists. A sharp-change indicator such as phenolphthalein or methyl orange makes the end point easier to judge than universal indicator's gradual range of colours. Read the bottom of the burette's meniscus at eye level each time. Subtract the initial reading from the final reading to obtain the titre, the volume delivered. Repeat carefully until titres agree closely, then find their mean without including the rough trial.

The colour change is an experimental sign that the reacting amounts have been reached; it is not a direct measurement of how many ions are present. Good volume measurements and a clear end point make the result more reliable.

Higher Tier: from volumes to concentration

Concentration in mol/dm³ tells you the number of moles of solute in 1 dm³ of solution. Since titration volumes are usually read in cm³, first use $1\,\mathrm{dm^3}=1000\,\mathrm{cm^3}$. Then calculate the moles in the solution whose concentration is known:

$n=cV$

Here $n$ is the amount in mol, $c$ is concentration in mol/dm³ and $V$ is volume in dm³. Use the balanced chemical equation to find the moles of the other reactant: the mole ratio is not always 1:1. Finally, divide those moles by that solution's volume in dm³ to obtain its concentration. To change a concentration in mol/dm³ to g/dm³, multiply by the substance's relative formula mass, $M_r$.

For example, suppose 20.0 cm³ of 0.100 mol/dm³ sulfuric acid exactly neutralises 25.0 cm³ of sodium hydroxide solution of unknown concentration. The equation is $H_2SO_4+2NaOH\rightarrow Na_2SO_4+2H_2O$: one mole of sulfuric acid reacts with two moles of sodium hydroxide. The acid volume is , so it contains of . It therefore reacts with of NaOH. The alkali volume is , giving . With , this is of NaOH. These are the concentrations of the alkali solution, not of the acid used from the .

Method

Pipette a fixed volume of strong alkali into a conical flask; add a suitable indicator. Record the initial burette reading, add strong acid while swirling, and slow to drops near the lasting colour change. Record the final reading. $\text{titre}=\text{final reading}-\text{initial reading}$. Repeat and average closely agreeing , excluding the rough trial. The specified acids are hydrochloric, nitric and sulfuric.