Practical investigations
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In the wine titration experiment to determine ethanol content do I need to acidify the potassium dichromate and add phosphoric acid to the diphenylamine sulphonate indicator or is it acidify one or the other?
Igloo writes ...
The potassium dichromate(VI) needs to be acidified with sulphuric acid before the titration is carried out AND the indicator needs to be made up in phosphoric acid, so the short answer to your question is that both components need to be acidified. The reason for using an acid with the indicator is that the solid sodium diphenylamine-4-sulphonate dissolves to a greater extent in acid than it does under neutral conditions.
I thought that it would be helpful if I take you through the practical details, since the preparation of the sodium diphenylamine-4-sulphonate indicator is not widely documented.
Sodium diphenylamine-4-sulphonate is normally used in the form of a 0.01 mol dm-3 solution, and this needs to be prepared beforehand. In this back titration it is usual to make up a solution in 25% phosphoric acid.
First make up the solution of phosphoric acid. Use about 25 g of the pure acid, place it in a 100 cm3 beaker, and add water until the beaker is about two-thirds full. Stir until thoroughly mixed, and then top up with more water. The quantities do not need to be precise.
Next measure out about 0.3 g of solid sodium diphenylamine-4-sulphonate and transfer it to a 100 cm3 volumetric flask. Add the phosphoric acid from the beaker (via a funnel), swirl the flask to dissolve the powder and when all the acid has been added, top up to the mark if necessary with water. Upend the flask several times to ensure thorough mixing. This is the indicator solution you will need for your titrations – use about 5 cm3 for each titration.
As for the initial oxidation of the alcohol in the wine by the potassium dichromate(VI), use enough sulphuric acid (in perhaps a 1.0 mol dm-3 solution) to ensure complete oxidation and to provide enough hydrogen ions for the subsequent back titration with Fe(II) ions. There should be no need to add any more sulphuric acid at a later stage. As for the reaction conditions, the longer you heat the mixture in the water bath, the better. Maybe 30 minutes is enough, but I would recommend leaving it longer to ensure that the reaction is complete. Remember that you can make up several mixtures and have them all in the heated water bath at the same time. At the end of your practical session the bath can be switched off and the flasks can be left inside overnight, and then, on the next occasion, the back titrations can be carried out successively in one go.
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: 11 May 2007
