Practical investigations
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I am doing a red and white wine analysis experiment. We are asked to do it by pH titration, detection of non-volatile acids by TLC and ethanol content analysis. For the pH titration, I am quite confused with the terms: total acidity, fixed acidity and volatile acidity. What role do they actually play in the wine? For the non-volatile acids by TLC, we used a solvent mixture: n-butanol-formic acid-water, in the ratio 10:1:2.5 by volume. How come formic acid is present in the solvent? Can I use others instead? Also, why does the TLC plate have to be dry before spraying bromocresol green?
Igloo writes ...
First, the different types of acidity. There are many different types of acids in the wine. Some are volatile, and tend to evaporate relatively easily – these are the volatile acids (e.g. ethanoic acid, propanoic acid) – whereas others, with greater intermolecular attractions, are not. I assume that fixed acidity refers to these acids, though they are more usually known as the non-volatile acids, such as tartaric acid, succinic acid and citric acid.
When you carry out the TLC you need to use solvents which are capable of drawing the non-volatile acids up the stationary layer. Hydrogen bonding is the principal type of intermolecular attraction here, so solvents capable of forming strong hydrogen bonds with the acids should be chosen. Butan-1-ol, formic acid (methanoic acid) and water are three such liquids, and a mixture of these has been shown to work well. If you decide to choose another mixture, then go ahead and try, but do select liquids which can form hydrogen bonds with the carboxyl group of the acids in the wine, i.e. alcohols or acids. Given that the official mixture is the recommended one, then you are unlikely to find a better mixture!
The TLC plate needs to be dry so that when you carry out the spraying the acids on the plate are disturbed as little as possible, and are more likely to stay in place. Later you will need to measure their Rf values as accurately as you can.
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: 01 May 2007
