Practical investigations
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I am determining the alcohol content of wine. I have completed the distillation procedure and am now completing a titration in which you oxidise the wine sample with an excess of potassium dichromate(VI) followed by a back titration with iron(II) irons using ammonium iron(II) sulphate. An article by Derek Denby says you need to standardise the ammonium iron(II) sulphate by titrating it directly with the potassium dichromate(VI) - what does this mean and how to you determine an end point for this titration?
Igloo writes
Ammonium iron(II) sulphate can be obtained in a very high state of purity and is known as a primary standard. I have not read the article to which you refer, but I suspect that the suggestion is to determine the accurate concentration of the potassium dichromate(VI) by carrying out a titration with a standard solution of iron(II) ammonium sulphate, rather than the converse. If you go back to the React website and research the FAQs using "dichromate" as your search phrase, you will find an answer to your question regarding the choice of indicator - sodium diphenylamine-4-sulphonate.
Once you have determined the accurate concentration of the potassium dichromate(VI) solution you can then use it to determine the concentration of ethanol in your distilled wine sample. You add a known volume (in excess) of your dichromate(VI) solution to a known volume of your wine sample, in the presence of acid, and redox occurs. Once the reaction is over, you can determine the number of moles of dichromate(VI) ions which are in excess by titration with your solution of iron(II) ions, again using the same indicator as before. This technique is known as a back titration.
Once you have calculated the number of moles of dichromate in excess you can find out - by subtraction - how many moles must have been used up to oxidise the ethanol and, using the equation for the reaction between ethanol and dichromate(VI) ions, you can determine the number of moles of ethanol originally present.
Always carry out a risk assessment before starting practical work, and check with your teacher. In this case even more than usual, ensure that your teacher knows exactly what practical work is proposed, and do not start it until you have permission to do so. Never work unsupervised.
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.
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updated: 01 February 2006
