Practical investigations
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I am doing wine analysis for my A2 investigation. My TLC for volatile and non-volatile acids has not worked well, and I was wondering if you could tell which acids out of tartaric, malic, lactic, acetic, citric and succinic are volatile and which are not. I have looked everywhere for this. Also I have done the pH experiment of titrating wine with sodium hydroxide. I have a graph so that I can read how much I needed to add to get a pH of 8.2. However I have no idea why I’ve had to do this and what it shows. What does this tell me and what results can I obtain from it? Any help is appreciated.
Igloo writes ...
Volatile acids are those which tend to vaporise easily, and the ability to be volatile depends in turn on the intermolecular forces between the molecules. Acids with a large molecular size have relatively high van der Waals' forces, and, as it turns out, five acids in your list above also have more than one functional group capable of forming hydrogen bonds. These are the ones which have such strong intermolecular forces that they happen to be non- volatile solids at room temperature. I’ll leave you now to deduce which are volatile and which non- volatile!
As for your second question, this is all to do with what is known as the hydrolysis of salts. Salts formed from weak acids, e.g. sodium tartrate, sodium acetate, sodium citrate, etc. all form slightly alkaline solutions. Consider sodium ethanoate (sodium acetate), for example:
In solution, sodium ethanoate is present in the form of a mixture of sodium and ethanoate ions. The ethanoate ions (derived from the weak acid, ethanoic acid) are bases and react with water as follows:
CH3COO-(aq) + H2O(l) <----> CH3COOH(aq) + OH-(aq)
This proton transfer reaction is reversible, but, as you can see, some hydroxide ions will be present at equilibrium. Now, once you have reached the end-point of your titration, you effectively have a dilute solution consisting of a mixture of salts, such as sodium ethanoate. So, as explained above, this is bound to contain a small number of hydroxide ions, causing the pH to be slightly higher than 7. Somebody somewhere has obviously estimated that this value is in the region of 8.2, which is why your pH curve should have its “vertical portion” in the region of 7.5 – 9.0, and why you need to add alkali to the wine until this pH is reached. If you look up “hydrolysis of salts” in any good A-Level Chemistry text book or on the web, via Google, you will be able to read up about all this in greater detail.
Since the volume reading at this pH gives the end point for the titration you should now be able to calculate the total acid concentration within the wine.
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updated: 18 December 2006
