Practical investigations

Read our general notes on Risk Assessment

I am doing an investigation on wine where I plan to determine the ethanol content. I know I need to use back-titration with the excess potassium dichromate. What I do not understand is the theory of the back titration. What exactly happens in the reaction to cause the colour change at the end point? Would you be able to help me please?

Igloo writes ...
You do not give details of how the back titration is being carried out, so I shall assume that you are using the traditional method described below:
 
A known volume of wine distillate is refluxed with a known excess of acidified potassium dichromate(VI). In this process the ethanol is oxidised to ethanoic acid and some of the dichromate(VI) ions are reduced to chromium(III) ions:
 
3CH₃CH₂OH + 2Cr₂O₇^2- + 16H⁺ -> 3CH₃COOH + 11H₂O + 4Cr³⁺
 
The cooled mixture is then back-titrated with a standard solution of ammonium iron(II) sulphate, using sodium diphenylamine-4-sulphonate as indicator. This indicator gives an intense violet colouration in the presence of any oxidising agent, in this case, the dichromate ions.
 
6Fe²⁺ + Cr₂O₇^2- + 14H⁺ -> 6Fe³⁺ + 7H₂O + 2Cr³⁺
 
At the start of the titration the mixture will be a murky brown colour owing to the excess orange dichromate ions and the green chromium(III) ions formed during the redox process with the ethanol. Once the indicator is added, the mixture 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 colouration disappears and is replaced by a rich green colour (Cr³⁺). This is the end-point, but it’s not an easy one to detect since it is hard to distinguish between the violet and the green in the mixture.
 
I hope that the “colour change” to which you refer is the one I have mentioned above, and that you now have a fuller understanding of what is going on.
 
Incidentally have you considered the density approach described in our tutorial on Investigating wines (revised)? This would give you an additional procedure to back up your results from the back titration.
 
Always carry out a risk assessment and check with your teacher before starting any practical work.

Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website. For further guidance see our tutorial on Risk Assessment.

back to Practical investigations

---

Rate this page or react
Share your views on this page, 15 ratings so far , rated at

---

updated: 15 February 2006