Practical investigations
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I am doing my advanced higher chemistry investigation on "Wine analysis" I am doing thin layer chromatography to identify if succinic, tartaric, malic and lactic acids are in red and white wine. I have been advised to use a solvent of n-butanol:formic acid:water in the ratio 8:1:2.5 and use bromocresol green as an indicator. Why? What is the chemistry behind this. I have been unable to find out why.
Igloo writes ...
The choice of solvent for TLC tends to be a hit-and-miss affair. Since you are identifying acids, you need a solvent which can form strong intermolecular bonds with the acids so as to help carry them up the layer. The butan-1-ol, methanoic acid, water mixture can form hydrogen bonds with the acids and is therefore possibly a good choice. Why butan-1-ol rather than any other alcohol, or why methanoic acid rather than any other carboxylic acid is simply a consequence of somebody somewhere trying out this mixture and finding that it gives good results!
I have carried out this experiment successfully with water alone, and it worked well enough, but I suggest that you try the mixture as recommended, and maybe water as well, to see if better separation occurs with the mixture of solvents.
The choice of indicator is easier to explain. Since you are identifying weak acids you need an indicator which “changes” in the region pH=3-5. Bromocresol green is an example of such as indicator, but I have also used bromophenol blue to good effect.
Methyl orange is the obvious choice, but in practice, the red acid spots don’t show up so well on the yellow/orange background. Once again, the choice boils down to what tends to give the best results, so I suggest that you try all three and see for yourself. After all, this is what investigations are all about!
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: 02 March 2006
