Practical investigations
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I am doing an investigation on the analysis of wine. I have looked at an article from Chemistry Review (Jan ’98) by Derek Denby and in it he talks about how to find out what the alcohol content of wine is. One method is to oxidise ethanol using an excess of potassium dichromate and then doing a back titration using ammonium iron(II) sulphate. I really need an indicator for this experiment or to know when the end point is for the indicator sodium diphenylamine sulphonate. I haven't had much luck with the internet so I thought you could help!
Igloo says
Potassium dichromate titrations are notoriously difficult to carry out because dichromate ions are orange, whilst the reduction product, containing chromium(III) ions, are green.
The usual indicator of choice is indeed sodium diphenylamine-4-sulphonate. This gives an intense violet colouration in the presence of any oxidising agent, in this case, the dichromate ions. When you carry out the back titration you will have a murky orange-green mixture due to the excess dichromate ions and the chromium(III) ions formed during the redox process with the ethanol. Once you add the indicator, the colour should turn violet, but on adding the ammonium iron(II) sulphate solution from a burette, there comes a stage where all the excess dichromate ions have been destroyed, so the violet coloration disappears to be replaced by a rich green colour. This is the end-point, but I cannot deny that it’s hard to distinguish between the violet and the green in this context. Good luck.
Incidentally have you considered the density approach described in our tutorial on the React website? This would give you an additional procedure to back up your results from the dichromate titration.
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updated: 03 March 2006
