Available chlorine — the mass of Cl2 that can be liberated per unit mass of bleaching powder (calcium hypochlorite, CaOCl2) — is a critical quality-control parameter for disinfection and bleaching applications. This experiment employs the iodometric method: glacial acetic acid acidifies the bleaching powder solution, causing the hypochlorite to liberate Cl2 gas, which immediately oxidises excess potassium iodide (KI) to iodine (I2). The liberated I2 is then titrated against standard N/10 sodium thiosulphate (Na2S2O3, a secondary standard), with freshly prepared starch as indicator. The deep blue starch-iodine complex turns colourless at the end point as the last trace of I2 is consumed. The chapter carefully distinguishes this iodometric approach from iodimetric titration. The percentage available chlorine is calculated from the strength of Cl2 (g/L) relative to the known bleaching powder concentration (12 g/L). Safety precautions cover the release of toxic Cl2 gas during acidification, I2 staining, and proper waste disposal.