Practical investigations
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I am carrying out the investigation on wines to see how much ethanol there is in a variety of different wines. I am currently carrying out a back titration with iron(II) ions. I have read that I need to standardise the potassium dichromate solution with the ammonium iron(II) sulphate, as ammonium iron(II) sulphate is a primary standard. However, when carrying out this, I have been unable to find an end point. I was wondering if I needed an indicator and what the end point of this titration is.
Igloo writes ...
Dichromate titrations are awkward since dichromate(VI) ions are orange, and the chromium(III) ions formed are green, so the end-point is almost impossible to detect without the use of an external indicator.
The conventional one to use is sodium diphenylamine-4-sulphonate. This turns deep blue in the presence of an excess of oxidising agent, so, with the dichromate in the burette, and the iron(II) ion sample (plus a few drops of the indicator) in the flask, carry out the titration until the mixture suddenly turns deep blue.
In practice this is not an easy titration since the blue is still masked by orange, but it does work!
Try using Google and “sodium diphenylamine sulphonate” as your search phrase and you’ll find a fuller answer I gave on this topic to a student a few months ago. When I accessed Google today it was the second entry in the list under “Nuffield Advanced Chemistry”.
Risk assessment
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updated: 21 January 2008
