Core Practical: Acid-Alkali Titration

$\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}$

A suitable indicator makes the end point visible. With phenolphthalein in the sodium hydroxide, the solution starts pink. As acid is added and mixed in, the pink colour eventually disappears. The first lasting change to colourless is the end point for this procedure.

Set up the apparatus

Use a pipette and pipette filler to transfer exactly $25\ \mathrm{cm^3}$ of sodium hydroxide solution into a conical flask. Add a few drops of phenolphthalein and put the flask on a white tile. The pipette measures a fixed volume accurately; the white background makes a slight colour change easier to judge.

Clamp a burette vertically above the flask and fill it with hydrochloric acid. Run a little acid through the tap so that the tip fills and contains no air bubbles. Remove the filling funnel before taking a reading. Read the bottom of the curved liquid surface, or meniscus, at eye level, and record the initial burette reading. It does not have to be $0.00\ \mathrm{cm^3}$.

Find the end point reliably

First carry out a rough titration. Let acid from the burette into the flask while swirling it, and note approximately when the pink colour disappears. Swirling mixes each addition with the alkali so that a local splash of acid does not give a misleading colour change.

Prepare a fresh $25\ \mathrm{cm^3}$ portion of alkali with indicator and record a new initial burette reading. Use the rough result as a guide: add acid more cautiously as you approach that volume, then drop by drop while swirling. Stop at the first lasting loss of pink and record the final reading. The titre is the volume delivered:

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

Repeat with fresh measured portions until you have closely agreeing, or concordant, titres. In the method shown here, titres within $0.10\ \mathrm{cm^3}$ are treated as concordant. Record each run separately, with columns for the rough run and subsequent accurate runs, and rows for the initial reading, final reading and titre, all in $\mathrm{cm^3}$. Find the mean using the concordant accurate titres, not the rough one.

Work safely

Wear eye protection and fill the burette below eye level to reduce the risk of splashes. Avoid contact with the acid and alkali, wash any splashes from skin promptly, and handle the glass pipette and burette carefully. A pipette filler must be used: never draw solution into a pipette by mouth.