Practical investigations
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I am currently preparing for my A2 investigation, in which I am looking at the ethanoic acid content of vinegar, but am having trouble getting started. I was wondering if you could give me any advice as to how I should go about it. I was considering fractional distillation but I thought that perhaps this was too basic, and wouldn't earn me a high enough grade. I can’t titrate it, because I can’t find anything that would separate the acids from the vinegar.
Igloo writes...
Assuming that you are using a colourless form of vinegar, e.g. “distilled vinegar” (available from supermarkets), you can certainly use titration methods directly. If your vinegar is coloured, e.g. malt vinegar, then you will have to find a way of removing the colour beforehand. This can be done using “activated” charcoal. Use www.google.co.uk on the internet to find out more about activated charcoal, and how it can be used in the context of your investigation.
You are of course correct in realizing that ethanoic acid is only one of several acids present in the vinegar. The acids can in fact be divided into two categories. Some are volatile, that is they will transfer over into the distillate when the vinegar is subjected to distillation. Others are non-volatile in that they will stay behind in the boiled aqueous solution during this process. Methanoic acid, ethanoic acid and propanoic acid (particularly ethanoic acid) are the main volatile acidic components in the vinegar, whereas acids such as tartaric acid, succinic acid and citric acid are among those which account for the non-volatile components.
You will need to research the structures of all these acids and try to find other examples in each category, because the previous lists are by no means exhaustive.
Next you will need to carry out a titration on the original vinegar using “standard” alkali and an appropriate indicator, remembering to decolourise the vinegar first if necessary (see earlier). An appropriate calculation will then give you a measure of total acids present.
Next carry out a distillation (keeping the distillate in an ice bath to minimize escape of the volatile acids) until you reckon that all the volatile acids have been driven off. A subsequent titration can be carried out as before, and then a calculation. At this stage you have to assume that since the main constituent of the distillate is ethanoic acid then the calculation will have to be carried out as if it is the only acid present, i.e. you say at the end of your calculation that this is the concentration of volatile acids in terms of ethanoic acid.
Next you could carry out a titration plus calculation on the liquid remaining after distillation to give the concentration of non-volatile acids present, in terms of, shall we say, tartaric acid (choose the one you believe to predominate - look up references here).
For all these titrations you will need to make up a standard solution (that is, one of which the concentration is accurately known) of an appropriate alkali. Ideally the concentration should be similar to that of the vinegar sample (so you will need to carry out some trials). Next, the chances are that your alkali will not be a reliable standard, e.g. NaOH and KOH react slowly with the CO2 in the air, so, having made up the concentration as accurately as you can, you will have to check this concentration before use, by standardizing it with a standard solution of a reliable acid. I’ll leave you to research an example of the latter!
Here’s another idea: Use paper chromatography on the vinegars using water as the solvent, and subsequently use an appropriate indicator to detect where the spots have reached (hint: you could use the apparatus of Nuffield Chemistry Experiment 18.3b). Many trials will be needed here to discover the “best” choice of indicator (hint: use the Nuffield Book of Data).
I hope you see that though there doesn’t seem to be much to do, there is in fact a great deal, and by the time you’ve carried out trials and all the other practical work, there is still the question of the calculations and a well-argued write-up!
Incidentally there are other practical techniques which can be employed, but it’s up to you to research these for yourself - I have after all given you a start!
There is now a tutorial on this web site that will give your further pointers as to how you might proceed.
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: 22 November 2003
