An analytical chemist is preparing a highly pure calibration standard of solid potassium sulfate (K2SO4K_2SO_4K2SO4). The chemist first titrates 25.0 cm3 of potassium hydroxide solution with dilute sulfuric acid, using phenolphthalein as an indicator, to determine the exact volume of acid required for neutralisation.
To obtain the pure, dry sample of potassium sulfate, the chemist repeats the reaction by mixing the exact volumes of acid and alkali determined from the titration, but this time without adding the phenolphthalein indicator. The chemist then evaporates the water from this second mixture to crystallise the salt.
Explain why the chemist must prepare this second mixture without the indicator, rather than evaporating the mixture from the initial titration, to obtain a pure sample of solid potassium sulfate.
276 exam-style questions on Edexcel GCSE Chemistry 4.1 Acids, covering 4.1.1 Acids, alkalis and the pH scale, 4.1.2 Hydrogen ion concentration and pH, 4.1.3 Dilute and concentrated, weak and strong acids, 4.1.4 Bases, alkalis and the reactions of acids, 4.1.5 Chemical tests for hydrogen and carbon dioxide, 4.1.6 Neutralisation as a reaction of acids with bases, 4.1.7 Preparing soluble salts, and 4.1.8 Solubility rules and preparing insoluble salts. Each one has a worked solution and a mark scheme showing where the marks go.