Practical investigations
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I am to plan and design an experiment to compare the alcohol content of two different types of wine. I have two questions involving method:
1. Do I simply do a fractional distillation, and the wine that produces the greater low fraction distillate will have greater alcohol content?
2. Or do I use the back titration with Cr2O72- with ammonium iron(II) sulphate with each alcohol distillate from the wines and compare the values?
Igloo writes ..
1. It’s not the volume of the distillates which matters, but the concentration of the ethanol in these. I assume that you intend to carry out density measurements on your distillates. The usual procedure is to carry out a distillation until it is believed that virtually all the alcohol has been driven off, and then to make up the volume of distillate to the original volume of wine used. This effectively means that the density measurements which follow are being carried out on the wine with all ingredients removed except water and alcohol. Thus official density measurements of alcohol-water mixtures can easily be converted into alcohol concentrations, and are not complicated by the presence of other solutes. See our tutorial on wine investigations on the React website.
2. Using an excess of Cr2O72- (plus acid) in excess, followed by a back titration with Fe2+ ions is certainly another approach, and can give good results, but it is a fiddly method, and is not without hazards. Refer to the FAQs on our React website to get more information about this technique.
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.
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updated: 16 March 2007
