Practical investigations
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I am trying to find out why it is necessary to use a monobasic (monoprotic) acid in the vinegars titrations? Is it so that a 1:1 molar ratio can be established? I used distillate solutions and vinegars (in the conical flask) with 0.1M NaOH in the titrations. I just want to know why the ethanoic acid being monobasic has any relevance?
Igloo writes
Carrying out acid-base titrations on wines is fine as far as the practical work goes. Problems arise when you come to carry out meaningful calculations afterwards. If the acidity of wine were simply caused by the presence of ethanoic acid, the calculation would be relatively simple, but it’s not. Wine contains a complex mixture of all sorts of acids, including ethanoic, tartaric, succinic, malic, citric and succinic acids, to name but a few. What complicates the situation yet further is that some of these acids are monoprotic (e.g. ethanoic and lactic acids), whilst others are diprotic (e.g. malic, succinic and tartaric acids) and citric acid is triprotic.
Each type of acid will neutralise your alkali differently,
i.e. one mole of the monoprotic acids reacts with one mole of sodium hydroxide:
HX + NaOH -> NaX + H2O
whereas one mole of the diprotic acids neutralises two moles of sodium hydroxide:
H2X + 2NaOH -> Na2X + 2H2O
and one mole of the triprotic acids neutralises three moles of sodium hydroxide:
H3X + 3NaOH -> Na3X + 3H2O
I hope that you are beginning to see that it is impossible to carry out a calculation, even for the total acids present, given that you do not know the relative proportions of the different types of acids present.
However, there is a way out. The accepted convention is to make the assumption that one of these acids represents all the different acids present, and then to carry out a calculation based on this assumption. If, for example, you choose ethanoic acid as being representative of all the acids present, then you can carry out a calculation based on the monoprotic acid equation above, and when you come to quote your answer, you say that the total acidity content is based in terms of their all being monoprotic acids.
Incidentally, since tartaric acid is usually the most abundant acid in a typical wine, you might think that it would be more appropriate to carry out your calculations on a diprotic acid basis, but I’ll leave you to carry on from here, and I hope that I have helped you to understand better the problems involved!
Risk assessment
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updated: 09 February 2005
