A-Level Chemistry Practical Manual Extract: Redox Titrations with Sodium Thiosulphate

This extract from an A-Level chemistry practical manual covers redox titrations using sodium thiosulphate. It explains how thiosulphate reacts with iodine and the difference between iodimetric and iodometric titrations. Short answers explain why iodine and sodium thiosulphate are poor primary standards, why iodine is dissolved in potassium iodide, and why thiosulphate breaks down in acid or air. It lists common reactions that liberate iodine and explains how to detect the end point with starch indicator. A worked calculation on chlorine in bleach is followed by practice questions in P525 Paper 1 and 2 style. A UNEB-style practical experiment covers standardising thiosulphate with potassium iodate and finding the percentage of a dichromate salt, with results tables.