Starch indicator added at the END (iodine titration) Watch
Here's the full question
''To determine the composition of a sample of brass, 1.5 g of the alloy was treated to give 250 cm^3 of a neutral solution of copper (II) and zinc nitrate.
Excess potassium iodide solution was added to 25cm^3 portions of this solution and the liberated iodine titrated with 0.1moldm^-3 sodium thiosulphate reaction.
2Cu (2+) + 4I (-) > 2CuI + I2
2S2O3 (2-) + I2 > 2I (-) + S4O6 (2-)
Explain why the indicator is not added until the reaction is nearly complete (1 mark)''
- Study Helper