IGCSE Chemistry 0620: Preparing a Soluble Salt by Titration

NutriGrow Labs wants to produce pure, dry crystals of potassium nitrate for use as a laboratory-grade fertiliser standard. They have potassium hydroxide solution (a soluble alkali) and dilute nitric acid available.

(a) Explain why titration, rather than the excess-insoluble-base method, is the appropriate technique for preparing potassium nitrate here, and name a suitable indicator for carrying out the titration. [2]

(b) Describe how the technicians would use the results of the indicator titration to obtain a pure, dry sample of potassium nitrate crystals, including why the titration is repeated without the indicator present. [3]

(c) Write the balanced symbol equation, including state symbols, for the reaction between potassium hydroxide and nitric acid. [2]

Part (a): Choosing titration as the method

Potassium hydroxide is a soluble base (an alkali), not an insoluble solid. This rules out the excess-insoluble-base method, which relies on adding a solid in excess and filtering off whatever does not react. There is no undissolved solid here to filter.

Instead, titration is used: a measured volume of dilute nitric acid is added gradually, from a burette, to a measured volume of the potassium hydroxide solution, with a few drops of an indicator such as thymolphthalein added. The indicator’s colour change shows exactly when the alkali has been fully neutralised, so the volume of acid needed is known precisely.

Part (b): From titration result to pure, dry crystals

Carry out the titration described in (a), noting the indicator’s colour change and recording the exact volume of nitric acid needed to neutralise a known volume of the potassium hydroxide solution.
Repeat the reaction using the same measured volumes of acid and alkali, but this time without adding the indicator. This step is needed because indicators are coloured, organic substances that are not part of the desired product. If crystals were grown from the indicator-coloured mixture, the indicator would contaminate them.
This second mixture is a pure solution of potassium nitrate (and water) only.
Gently warm/evaporate the solution to drive off some water until it is hot and saturated.
Leave the saturated solution to cool slowly, so that potassium nitrate crystals form.
Filter off the crystals and dry them (e.g. between filter paper), giving pure, dry potassium nitrate crystals.

Part (c): Symbol equation for the neutralisation

Potassium hydroxide and nitric acid are both soluble, and their reaction is a simple acid + alkali neutralisation, forming a salt (potassium nitrate) and water:

$\text{KOH(aq)} + \text{HNO}_3\text{(aq)} \rightarrow \text{KNO}_3\text{(aq)} + \text{H}_2\text{O(l)}$

Counting atoms: 1 K, 1 O (from KOH) + 3 O (from $\text{HNO}_3$ ) = 4 O, 1 H (from KOH) + 1 H (from $\text{HNO}_3$ ) = 2 H, and 1 N, on the left; on the right, $\text{KNO}_3$ supplies 1 K, 1 N, 3 O, and $\text{H}_2\text{O}$ supplies 1 O and 2 H, a total of 4 O and 2 H. All atoms balance with no extra coefficients needed.

Final answers

(a) Titration is used because potassium hydroxide is a soluble alkali; a suitable indicator is thymolphthalein.
(b) Titrate with indicator to find the neutralising volume, repeat without indicator to avoid contamination, then evaporate to saturation, cool to crystallise, and filter and dry the crystals.
(c) $\text{KOH(aq)} + \text{HNO}_3\text{(aq)} \rightarrow \text{KNO}_3\text{(aq)} + \text{H}_2\text{O(l)}$