Neutralisation and Titration

What happens during neutralisation?

An acid forms hydrogen ions, $\mathrm{H^+}$, when it dissolves in water. An alkali is a soluble base: its aqueous solution contains hydroxide ions, $\mathrm{OH^-}$. ‘Aqueous’ simply means dissolved in water.

Common laboratory acids are hydrochloric acid, $\mathrm{HCl}$; nitric acid, $\mathrm{HNO_3}$; and sulfuric acid, $\mathrm{H_2SO_4}$. Common alkalis include sodium hydroxide, ; potassium hydroxide, ; and calcium hydroxide, .

When an acid reacts with an alkali, neutralisation produces a salt and water:

$$\text{acid} + \text{alkali} \rightarrow \text{salt} + \text{water}$$

For example, hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water:

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

The same essential change happens whichever of these acids and alkalis is used: hydrogen ions react with hydroxide ions to form water. This gives the general ionic equation:

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

The other ions remain in solution and make up the dissolved salt. In the hydrochloric acid–sodium hydroxide example, these are sodium and chloride ions.

How a titration measures neutralisation

A titration measures the volume of one solution needed to react exactly with a measured volume of another. If one solution has a known concentration, the reacting volumes and balanced equation can be used to determine the concentration of the other. This makes titration a useful form of quantitative chemical analysis.

Both acid and alkali solutions can be colourless, so their appearance alone does not show when enough acid has been added. A suitable gives a sharp colour change near the point when is just reached. The observed colour change is called the .

In the practical described here, hydrochloric acid is added to sodium hydroxide containing a few drops of phenolphthalein. Phenolphthalein is pink in alkaline solution and colourless in acidic solution. The endpoint is reached when the pink colour just disappears and does not return after swirling.

Setting up accurate measurements

Use a 25 cm³ volumetric pipette with a pipette filler, a 50 cm³ burette held vertically in a clamp and stand, a conical flask, a white tile, a funnel and a beaker. The pipette measures a fixed volume accurately; the burette measures the variable volume delivered and its tap allows controlled, dropwise addition.

Before starting, wear safety goggles. Avoid acid and alkali splashes on skin or in eyes, and immediately wash skin splashes with plenty of water. Fill the burette below eye level to reduce the risk of splashing your face. Always use a pipette filler, never your mouth. Handle glassware carefully and report broken glass to the teacher rather than picking it up by hand.

Rinse the pipette with a little sodium hydroxide solution and the burette with a little hydrochloric acid before filling them. This prevents residual water from diluting the solutions being measured. The conical flask can be rinsed with deionised water: any water left there does not change the amount of sodium hydroxide transferred by the pipette.

1. Use the pipette and filler to transfer exactly 25 cm³ of sodium hydroxide solution into the conical flask. Add a few drops of phenolphthalein and place the flask on the white tile.
2. Close the burette tap and fill the burette with hydrochloric acid of known concentration. Run some acid into a beaker so that the tip fills and any air bubbles are removed. Otherwise, part of the apparent delivered volume might fill the tip instead of reaching the flask.
3. Refill if necessary, remove the funnel and record the initial reading. Removing the funnel prevents extra drops entering after the reading has been taken.
4. Read the bottom of the curved liquid surface—the meniscus—at eye level. Looking from above or below gives a parallax error. For a typical school burette marked every 0.1 cm³, estimate readings to the nearest 0.05 cm³.

Finding the endpoint without overshooting

First carry out a rough titration. Add acid while swirling the flask, stopping when the pink colour disappears. Record the final burette reading. The volume delivered is the titre:

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

The rough titre locates the approximate endpoint, allowing later titrations to be performed more carefully near it.

Rinse the flask with deionised water and prepare a fresh 25 cm³ portion of sodium hydroxide with a few drops of indicator. Record a new initial burette reading. Add acid fairly quickly at first, but slow down before reaching the rough titre volume. Near the endpoint, add acid one drop at a time, swirling after each addition.

Swirling ensures that the added acid reacts with the alkali throughout the flask rather than producing a local colour change. The white tile makes the disappearance of the pink colour easier to judge. Stop at the first lasting disappearance of pink and record the final reading. Continuing to add acid would overshoot the endpoint and give a titre that is too large.

Repeat with fresh portions until accurate titres agree closely. In this practical, a useful concordance target is agreement within 0.10 cm³. Calculate a mean using only the concordant accurate titres, excluding the rough result.

Judging the quality of titration data

Record initial readings, final readings and calculated titres separately, with units in the headings. For example, suppose a student obtains the following readings, all in cm³:

Titrations 1 and 2 differ by only 0.05 cm³, so they meet the chosen concordance target. Titration 3 does not agree with them. It should be investigated and another careful titration performed; its larger value could result from overshooting, but the numbers alone do not prove the cause. The rough result is not included in the mean.

Precision concerns how closely repeated titres agree. Repeatability means the same person can obtain similar results with the same method and equipment. Concordant titres support both, and further agreeing results give greater confidence that the agreement is not accidental.

Accuracy concerns closeness to the true value. Closely agreeing titres can still all be wrong: repeatedly overshooting the endpoint or consistently misreading the meniscus can affect every result. Repeating the experiment does not automatically remove such systematic errors.

A valid titration must measure the volume needed to neutralise the intended amount of alkali. A suitable indicator, an accurately measured fresh portion, thorough mixing and an appropriate stopping point all help achieve this. When evaluating results, consider both the agreement of the data and whether the method could have biased them.

Neutralisation

• Acid + alkali → salt + water.