Practical investigations
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I am doing my A2 investigation on wines. I have completed the majority of my write up but am stuck on calculations involving pH and the Henderson-Hasslebach equation and Ka constant.
In one particular experiment I performed a titration of sodium hydroxide against the wine sample in the conical flask. I have average values for the titration but am unsure as to how to use them to calculate the pH.
My teacher mentioned using the Ka constant - I have looked at the suggested website at looking up values but I’m unsure as to which ones to use - do I need to use values for all the acids present in wine?
I would be grateful if you could give me a step by step calculation that relates to my investigation.
Igloo writes ...
I do not think that the Henderson-Hasslebach equation is really relevant for wines, since they are unlikely to be stable and efficient buffers, for which this equation is relevant.
I’m not sure what you are trying to do in any case. If you’ve carried out titrations you should be able to carry out straightforward calculations to find the concentration of acid in mol dm-3. To do this you will need to assume that one particular acid represents all of the acids present. Unfortunately wines contain a complex mixture of acids, neutralising alkali in different ways. The monoprotic acids (with one replaceable hydrogen) will neutralise NaOH in a 1:1 molar ratio, but the diprotic and triprotic acids will require more alkali for complete neutralisation, so there is obviously a problem. The usual convention is to calculate the acidity in terms of the acid which is predominant. Tartaric acid is often chosen in this context. It is a diprotic acid -(HOOC.CH(OH)CH(OH)COOH) - so one mole will be totally neutralised by two moles of NaOH.
I assume that you have your titration readings.
From these (via the concentration of the NaOH) you can calculate the number of moles of NaOH added.
If this value is halved you have the number of moles of “tartaric acid” present.
Knowing the volume of wine used in the titration this value can be scaled up to 1000 cm3
Now you have the concentration of “tartaric acid” in mol dm-3
Using the equation linking the concentration of tartaric acid and the hydrogen ion concentration with the acid dissociation constant (use a data book), you could calculate the pH, but I cannot see why you should want to do this. A good pH probe could be used to measure the pH directly!
To answer one of your questions succinctly: No, you cannot be expected to use values for all the different acids present. There are too many, and you have no idea in what proportion they are present anyway.
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updated: 18 December 2006
