Practical investigations
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I am doing an investigation involving wines and want to do a back titration to determine which esters are present in wine. However, the idea of doing a back titration is confusing could you please explain some of the theory behind it and why a back titration is needed.
Igloo writes
In order to determine to concentration of esters present in wines it is necessary to choose a suitable reaction in which they take part. The usual choice is alkaline hydrolysis with a solution of sodium hydroxide. The mixture needs to be boiled for the hydrolysis to take place.
The procedure here involves measuring out a known volume of aqueous sodium hydroxide of known concentration and adding it to a known volume of the wine. The quantity of sodium hydroxide used must be an excess, so it may be necessary to carry out trial experiments to ensure that the final mixture is in fact still alkaline at the end of the process.
The quantity of alkali remaining in excess can be determined by carrying out a titration with a solution of acid, e.g. hydrochloric acid, of known concentration, using phenolphthalein as indicator.
The calculation involves the following steps:
[1] The titration reading enables you to determine the number of moles of NaOH remaining in excess.
[2] By subtraction from the quantity of NaOH added initially, the number of moles used up in the reaction with the wine can be calculated.
[3] The NaOH used up will have destroyed not only the acids present, but also the esters of course. If you have not already done so, you will need to carry out a straightforward titration of the wine with the sodium hydroxide solution – “in the cold” - in order to estimate how much alkali is needed to destroy the acids alone. This will enable you to deduce how much alkali was used up to destroy the esters.
[4] Finally you can determine the concentration of esters in wine in terms of, for example, a chosen ester, e.g. ethyl ethanoate.
In our wine tutorial on this website we did imply that this procedure was complicated and that the calculation was tricky. You will probably agree with this!
By the way, we call the first procedure mentioned a “back titration” because it starts with (acidic) wine, then, once excess alkali has been added and the mixture has been boiled it becomes alkaline, and finally the titration takes us “back” to a neutral solution.
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: 18 April 2005
