Titration of Strong Acids and Alkalis

What a titration finds

A titration compares an unknown solution with a solution of known concentration. In this practical, a carefully measured volume of strong alkali is neutralised by a strong acid. Measuring the acid volume needed for complete reaction allows the acid concentration to be found.

“Strong” and “concentrated” do not mean the same thing. A strong acid ionises completely in water; its concentration tells us how many moles of acid are present in each cubic decimetre of solution. The purpose of this titration is to determine that concentration.

For hydrochloric acid and sodium hydroxide, the reaction is:

$\mathrm{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}$

At particle level, neutralisation involves:

$\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}$

The balanced equation shows that one mole of hydrochloric acid reacts with one mole of sodium hydroxide. This reacting ratio connects the measured volumes to the unknown concentration.

How the apparatus controls the volumes

A volumetric pipette measures one fixed volume of the known alkali and transfers it to a conical flask. A pipette filler is used rather than drawing liquid up by mouth. The flask's sloping sides make it possible to swirl the mixture without easily spilling it.

A burette is held vertically in a clamp and contains the acid of unknown concentration. Its scale allows the volume delivered to be found from two readings:

$\text{titre}=\text{final burette reading}-\text{initial burette reading}$

A few drops of a suitable pH indicator are added to the alkali. For example, phenolphthalein is pink in the alkali used in the supplied method and becomes colourless at the end point. A white tile makes a faint colour change easier to see.

Wear eye protection and follow the stated laboratory precautions because acids and alkalis can damage eyes or skin. Deal with splashes immediately using the procedure given by the teacher.

Finding the end point

First measure the known alkali with the pipette, transfer it to the conical flask and add only two or three drops of indicator. Record the initial burette reading at eye level, then place the flask on the white tile beneath the burette.

Run acid into the flask while swirling so that the solutions mix thoroughly. As neutralisation approaches, add the acid more slowly. The end point is the first permanent indicator colour change, not the darkest possible colour. Record the final burette reading and calculate the titre.

The first run is a rough titration. It shows approximately where the end point lies. For the next run, add acid fairly quickly until about $2$–$3\ \text{cm}^3$ before the rough end point, then add it drop by drop while swirling. This reduces the chance of passing the end point.

Repeat with fresh measured portions of alkali until concordant titres are obtained. Concordant values are close enough to show that the technique is repeatable; use the acceptance range specified for the experiment. Calculate a mean using the concordant accurate titres, not the rough titre or an anomalous value.

Why the method improves accuracy

The pipette gives the same accurately measured alkali volume each time. Initial and final burette readings reveal the actual volume delivered even if the burette did not begin at zero. Reading the scale at eye level reduces parallax error. Swirling prevents a local excess of acid from changing the indicator before the whole mixture has reacted, while dropwise addition near the end point prevents one large addition from overshooting it.

Repeating does not remove a consistent error in the apparatus or method, but concordant results reduce the influence of random variation in judging the colour change.

Turning the titre into a concentration

Concentration, amount and volume are related by:

$n=cV$

Here $n$ is amount in moles, $c$ is concentration in $\text{mol dm}^{-3}$ and $V$ is volume in $\text{dm}^3$. Convert a volume in to by dividing by 1000.

Use the known alkali concentration and pipetted volume to calculate its moles. Next use the coefficients in the balanced equation to find the moles of acid that reacted. Finally divide those acid moles by the mean acid titre in $\text{dm}^3$.

For the 1:1 reaction between hydrochloric acid and sodium hydroxide:

$c_{\mathrm{acid}}V_{\mathrm{acid}}=c_{\mathrm{alkali}}V_{\mathrm{alkali}}$

Therefore:

$c_{\mathrm{acid}}=\frac{c_{\mathrm{alkali}}V_{\mathrm{alkali}}}{V_{\mathrm{acid}}}$

This shortcut depends on the 1:1 reacting ratio. For a different balanced equation, the mole ratio must be applied explicitly. The calculated result means that each cubic decimetre of the unknown acid contains that many moles of acid.