Practical investigations

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I am trying to determine the alcohol content of ethanol solutions using the technique of an oxidation and back titration. I have oxidised the ethanol with an excess of potassium dichromate(VI) and am following this with a back titration. The method I have been given suggests using ammonium iron (II) sulphate solution and sodium diphenylamine-4-sulphonate made up in a 25% phosphoric acid solution as my indicator. However I had many problems determining an end point and am titrating using 0.1 mol dm^-3 sodium thiosulphate solution instead of the ammonium iron sulphate solution. This gives me a colour change to green which I am interpreting as my end point However I do not know the chemistry behind the reaction and am struggling with it. Please can you help me?
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Igloo writes ...
 
Sodium thiosulphate is a reducing agent like iron(II) ammonium sulphate.
 
Unfortunately I am not completely sure as to whether S₄O₆^2- ions are formed in this reaction or SO₄^2- ions instead. Generally speaking, stronger oxidising agents such as dichromate(VI) ions tend to oxidise S₂O₃^2- to SO₄^2- and the half reaction for this is:
 
S₂O₃^2-(aq) + 5H₂O(aq) ----> 2SO₄^2- (aq) + 10H⁺(aq) + 8e^-
 
The corresponding half equation for the dichromate(VI) reduction is:
 
Cr₂O₇^2-(aq) + 14H⁺(aq) + 6e^- ----> 2Cr³⁺(aq) + 7H₂O(l)
 
With 8 electrons released in the oxidation step and 6 gained in the reduction step, this is a horrible ratio for combining them together, but the complete equation will be:
 
3S₂O₃^2-(aq) + 4Cr₂O₇^2- (aq) + 26H⁺(aq) ----> 6SO₄^2-(aq) + 8Cr³⁺(aq) + 13H₂O(l)
 
The fact that your solution has turned green shows that the thiosulphate ions are reducing the dichromate(VI) ions successfully, but I have never before used sodium thiosulphate in this context.
 
My worry is that since thiosulphate ions are colourless – as are the sulphate ions formed – the end point is only reached when the orange dichromate(VI) ions have been totally replaced by green Cr³⁺ ions, and I cannot believe that this end-point is a sharp one, since the intermediate colour will be a murky mixture of orange and green. However, you seem to have found that it has worked, so good luck to you!
 
You can see that 4 moles of dichromate(VI) ions need 3 moles of thiosulphate ions in the reaction above, and you will of course need this 4:3 ratio for your calculation.

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updated: 13 April 2008