Practical investigations
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In my A2 investigation I am trying to find the volume of free sulphur dioxide in wine, through a titration with iodine. I understand how to make up the iodine solution but I am having trouble with the quantities involved when standardising the iodine solution. I know that I need to titrate the iodine against sodium thiosulphate, but what is the concentration of sodium thiosulphate needed? And which should go in the burette during the titration? Also, when titrating the iodine against the wine, which should go in the burette and which should be in the conical flask?
Igloo writes ...
When carrying out titrations it makes sense to use solutions of similar concentrations in the flask and burette, so that the titre required is neither incredibly small, e.g. less than 1.0 cm3, or greater than one “burette-full”.
If you know that your iodine solution has a concentration of, let’s say, about 0.10 mol dm-3, then make up the sodium thiosulphate solution to be exactly 0.10 mol dm-3. In this case, however, since one mole of iodine oxidises two moles of thiosulphate ions, you could instead make the sodium thiosulphate solution to be exactly 0.20 mol dm-3. In this way, the reacting volumes should be in a 1:1 ratio. In other words, 10.0 cm3 of the iodine solution, for example, should need a similar volume of the sodium thiosulphate solution to reach the end-point.
As to which solution goes where, in theory it really doesn’t matter. If there is any convention it is that the solution whose concentration you are trying to find goes in the flask (via a pipette) and the standard solution goes in the burette. In your case, this would mean that you run the thiosulphate solution into the iodine solution contained in the flask. For the second titration, according to what I’ve just written, the wine should go in the flask and the iodine in the burette. However, this means that iodine will be in the flask for the first titration and in the burette for the second, which isn’t very efficient.
So, in summary, the best arrangement is to have sodium thiosulphate solution for the first titration and wine for the second titration in a flask, with the iodine solution in a burette throughout.
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Risk assessment
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updated: 12 November 2007
