Special studies
Read our general notes on Risk Assessment
I am finding problems measuring the amount of alcohol in fermented starch. I want to use the back titration method with acidified potassium dichromate and in excess KI. The contents are to be titrated with thiosulphate solution. Can you give me some brief ideas about how to go about it efficiently and successfully.
Igloo writes ...
This is a good means of finding the alcohol content of a liquid mixture.
I’ll give you a brief outline of a method that should work, but it is up to you to work out the details.
1 Measure out a known volume of the fermented starch mixture (say 50 cm3, using a measuring cylinder).
2 Distil this (collecting the distillate carefully) until only a little of the liquid remains.
3 Add a known volume of standardised potassium dichromate solution to the distillate. The potassium dichromate needs to be in excess and capable of oxidising all the ethanol. You also need to add an excess of acid (usually a mixture of dilute sulphuric and phosphoric acids).
4 Place the mixture in a water bath maintained at about 100 °C. Leave it for at least 2 hours, and preferably overnight.
5 Add an excess of acidified potassium iodide solution to the mixture once it is cool. The excess dichromate ions should all be reduced by the iodide ions, forming iodine.
6 Titrate the mixture very carefully with a standard solution of sodium thiosulphate, using starch as indicator when the end-point is near. Alternatively you could titrate portions of the mixture, having made up the mixture (with water) to a known volume, using a volumetric flask.
7 In order to be able to carry out 2 or 3 determinations, it is probably better to have several mixtures “on the go” in the water bath, so that all the titrations can be carried out consecutively.
You will need to think about many things, including:
• a sensible (and safe) concentration for the potassium dichromate solution
• how to standardise your potassium dichromate solution
• how to work out how much potassium dichromate to use giving an excess
• how to work out how much acidified potassium iodide solution to use
an appropriate concentration for the sodium thiosulphate solution
In order to carry out the calculations you will need to work out equations for the three reactions:
Ethanol + excess hot acidified dichromate ions
Dichromate ions + acidified iodide ions
Iodine + thiosulphate ions
I hope that this has set you on the right track, but there’s still much planning to be done!
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.
Rate this page or react
Share your views on this page, 5 ratings so far
, rated at 
updated: 19 March 2006
