Chemistry 3.16, 3.18 - Titration and pure salts

Why a soluble reactant needs titration

Hydrochloric acid and sodium hydroxide both form solutions. Their reaction makes sodium chloride and water:

$\ce{HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)}$

An excess of either reactant would remain dissolved and pass through filter paper. To obtain only salt and water, first use a titration to measure the volumes that react. Then mix those measured volumes again without indicator. This avoids both an excess soluble reactant and contamination by indicator.

The important distinction is the reactant's solubility, not the salt's: a soluble salt can be made from an insoluble base by adding excess and filtering, but a soluble alkali needs measured reacting proportions.

Titration: finding the reacting volumes

When both reactants are soluble, there is no visible solid excess to remove. A titration measures the volume of one solution needed to react exactly with a fixed volume of another. A suitable indicator gives a sharp colour change at the endpoint, which closely estimates the reacting proportion for this acid-alkali pair. The endpoint is not automatically pH 7: it depends on the indicator used.

For a hydrochloric acid and sodium hydroxide example:

The quantity deliberately changed during one run is the cumulative volume delivered from the burette. The observed response is the indicator colour at the endpoint. Keep the pipetted alkali volume and both solution concentrations fixed, use the same small number of indicator drops, and use the same endpoint judgement. Repeats assess precision.

Dilute acid and alkali can harm eyes by splashing, so wear eye protection and fill the burette below eye level. Use a pipette filler, never the mouth. Many indicator solutions contain flammable alcohol: use only a few drops and keep them away from flames; the later heating stage uses the indicator-free preparation solution.

Method

1. Wear eye protection. Clamp the burette vertically. Rinse it with a little of the acid, run acid through the jet to remove air, fill it below eye level and remove the filling funnel.
2. Read the bottom of the meniscus at eye level and record the initial burette reading. The scale increases downwards; the reading does not have to start at zero.
3. Use a pipette filler and volumetric pipette to place a fixed volume, for example $25.0\,\mathrm{cm}^{3}$, of sodium hydroxide solution in a conical flask. Add two or three drops of a suitable indicator and place the flask on a white tile.
4. Run acid from the burette while swirling the flask. Use a first, rough run to locate the endpoint and record both burette readings.
5. Repeat with fresh alkali. Add acid quickly at first, then one drop at a time near the rough endpoint while swirling. Stop at the first permanent endpoint colour and record the final reading.
6. Repeat until at least two titres are concordant. In the examples here, concordant titres differ by no more than $0.20\,\mathrm{cm}^{3}$; use the criterion supplied in a question or practical instruction.

Worked example: titre and concordant mean

For run 2, titre = $24.85 - 0.40 = 24.45\,\mathrm{cm}^{3}$.

Runs 1 and 2 are concordant because they differ by only $0.05\,\mathrm{cm}^{3}$. Run 3 differs from them by at least $0.30\,\mathrm{cm}^{3}$, so it is not used. The rough result is also excluded.

mean titre = $\frac{24.50+24.45}{2}=24.475\,\mathrm{cm}^{3}\approx24.48\,\mathrm{cm}^{3}$ to two decimal places.

The mean is close to the rough endpoint but slightly lower, which is plausible because a rough run commonly overshoots before the colour change is noticed.

The endpoint is judged by eye and a single extra drop changes the measured titre, so it is a source of uncertainty. A white tile, constant indicator amount, swirling, dropwise addition near the endpoint and concordant repeats reduce this uncertainty. Reading the meniscus at eye level reduces parallax error; rinsing the burette and pipette with their working solutions avoids changing those solutions' concentrations with leftover water.