Practical investigations
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I am doing the iodine clock experiment for my coursework and I understand the mechanism of how it works and why the thiosulphate is used but could you explain to me what actually is a clock reaction. I guess it has something to do with the delay in the colour being produced but is there a better explanation you could give me?
Igloo replies ...
I can’t do much better than you!
As you will know from your experiments, the sudden appearance of the blue-black colour in the mixture occurs somewhat dramatically, and this is why this experiment is often demonstrated in “magic” shows or chemistry demonstrations. In fact the concentrations of all the ingredients can be arranged in such a way that the appearance of the blue colour can be made to occur after, for example, exactly 20, 30, 40, 50 seconds in a series of different flasks, and these appearances are so precise that they match the times simultaneously recorded by a stopwatch.
The main problem (as I know from personal experience) is temperature control. It’s all very well practising a demonstration like this in a laboratory at say 25°C, but if the demonstration is then carried out in an unheated lecture theatre at 20°C, all the times lengthen alarmingly. Not such a good clock after all!
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 20 January 2006
