Rates of reaction - kinetics (A2)
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I am investigating the effect of reactant concentration and temperature of the rate of reaction between potassium iodide and potassium peroxodisulphate. I cannot find any information on the colorimetric technique for determining the rate of reaction. Please could you help me! Thank you in advance
Ulex replies ...
The conventional way of doing this is to add a known quantity of sodium thiosulphate to the reaction mixture, together with some starch solution. This reacts with the iodine produced until all the sodium thiosulphate is used up. When this happens the mixture suddenly goes from colourless to dark blue. The reciprocal of the time taken for this to happen is proportional to the rate of the reaction. This type of method is called a ‘clock’ method.
Alternatively, you could use a colorimetric method using the colour of the iodine produced. To do this you would have to construct a calibration curve for your colorimeter using a range of known concentrations of iodine solution (in KI solution).
I have some reservations about this in that the reaction is a very slow one. In fact, it is often used as a way of demonstrating homogeneous catalysis by transition metal ions – even when catalysed it is slow enough for the rate to be measured quite easily.
You have got a very interesting idea here – I’d be very interested to know how you get on. Please reply via the React website.
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updated: 14 December 2006
