A student monitors the rate of the acid-catalysed iodination of propanone:
CH3COCH3(aq)+I2(aq)→H+(aq)CH3COCH2I(aq)+H+(aq)+I−(aq) \text{CH}_3\text{COCH}_3(\text{aq}) + \text{I}_2(\text{aq}) \xrightarrow{\text{H}^+(\text{aq})} \text{CH}_3\text{COCH}_2\text{I}(\text{aq}) + \text{H}^+(\text{aq}) + \text{I}^-(\text{aq}) CH3COCH3(aq)+I2(aq)H+(aq)CH3COCH2I(aq)+H+(aq)+I−(aq)The student withdraws 10.0 cm3 samples of the reaction mixture at regular time intervals. Each sample must be immediately quenched before titrating the remaining iodine against standard sodium thiosulfate solution.
Which of the following is the most suitable quenching agent to immediately stop this reaction?
Sodium hydrogencarbonate, to neutralise the H+\text{H}^+H+ catalyst
Sodium thiosulfate, to react with the remaining I2\text{I}_2I2
Starch solution, to bind and stabilise the iodine molecules
Ice-cold water, to cool and dilute the reaction mixture