Practical investigations
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I am investigating wines for my individual investigation, but am a bit unsure of the theory behind part of it. When analysing the ethanol content of the wine using the reaction of ethanol with dichromate ions, and then a back titration with iron(II) ions, I was wondering what the complex ion formed is. The indicator I will use is sodium diphenylamine sulphonate and phosphoric acid is used to provide hydrogen ions. The indicator is needed because the chromium ions reduced form a complex with the water I presume and the phosphate ions from the acid in a ligand exchange reaction. I was wondering if you knew precisely what this ion is likely to be, or whether it actually matters.
Igloo replies
An indicator is needed with dichromate titrations, because dichromate ions are orange whereas the chromium(III) ions formed in the redox process are green. The complete removal of dichromate ions by iron(II) ions would be difficult to detect since the green chromium(III) ions mask the disappearance of the orange dichromate(VI) ions at the end point. This is where sodium diphenyl-4-sulphonate comes in. It shows up intensely blue-violet in the presence of many oxidising agents (not just dichromate ions), and so the end point in your case is achieved when the violet coloration is replaced by green (not an easy end-point to detect!).
The indicator has an oxidized form that is a different colour from its reduced form. Potassium dichromate first oxidises diphenylamine to the effective indicator which is the colourless compound N,N'-diphenylbenzidine. This compound is reversibly oxidized to the intensely coloured diphenylbenzidine violet.
The phosphoric acid helps to sharpen the end point. It forms a complex with iron(III) ions which is colourless - unlike hydrated iron(III) ions which are yellow. This makes the colour change at the end-point clearer.
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A titration using ferrous ethyleneammonium sulfate in solution can be used with a ferroin indicator to a "salmon" endpoint in order to back calculate the amount of ethanol oxidized by dichromate in your analyte sample flask.
Andrew 12 May 2006
updated: 29 November 2004
