Practical investigations
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I am doing A2 coursework on the acids in vinegars, and I did chromatography on the 8 acids in my vinegars. I used 50% ethanol as my solvent and I wondered if there were any standard Rf values for the acids (especially ethanoic, citric, malonic,). I know the Rf value changes with concentration of the solvent, but I wanted to check to see if they are all very similar, because that's what my results showed.
Igloo writes ...
Unfortunately you are unlikely to find any official values for the Rf values. This is because there are so many variables on which they depend, e.g. the nature of the solvent and its composition, temperature, and on the nature of the stationary phase.
All you can do is to carry out chromatography – using the same conditions – with pure samples of each of the acids you have in mind, and then calculate the Rf values and compare these with your chromatogram for the vinegar. The best procedure is to use spots of the pure components and the vinegar sample on the same piece of paper. This will enable you to carry out the chromatography of all of these at the same time.
Incidentally I have found that the acids you mention do tend to produce similar Rf values, so your work is probably sound enough. However, to complete your investigation you should compare your results with samples of the pure acids as described above.
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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.
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updated: 18 January 2007
