Practical investigations
Read our general notes on Risk Assessment
I am doing wine for my individual investigation and am finding the acidity by titrating it with NaOH. The book where I got my method from said that when I find the amount of NaOH needed to reach a pH of 8.3, I can work out the amount of tartaric acid in the wine. However I am stuck as to how to do that. Is there an equation or a ratio I can use? Thank you.
Igloo writes ...
The pH of a solution of sodium tartrate at the concentrations you are using appears to be about 8.3. In reality I suspect that you will be using a suitable indicator to detect the end-point, rather than a pH meter, but both approaches are acceptable.
First you need to know the formula for tartaric acid, which is HOOC.CH(OH).CH(OH).COOH. This shows that it is a diprotic acid with two "replaceable hydrogen atoms" and that one mole of tartaric acid will need two moles of NaOH for neutralisation.
You will need to construct your own equation to verify this - it will be expected of you when you come to write up your report.
I assume that the wine sample is being titrated with the NaOH (in the burette). If so, your titration reading, together with knowledge of the concentration of the NaOH will enable you to calculate the number of moles of NaOH used. Knowing the mole ratio which I have already mentioned you will have to divide (or multiply) this quantity by 2 [which is it?] and from this you will be able to deduce the concentration of tartaric acid in the wine.
One very important matter to consider. Your answer will be an overestimate, since there are other acids in the wine as well as tartaric acid, which will be neutralised simultaneously. Tartaric acid is probably the most abundant acid in the wine, which is why the acidity in wines is often expressed in terms of tartaric acid, i.e. assuming that all acidity is due to tartaric acid (which is non volatile).
Thank you for your reply regarding the way to work out the amount of tartaric acid in wine. The second part of my question was that I also worked out the non volatile acidity by first evaporating the wine. Do I presume that the acids that were neutralised there were also tartaric acid or could it be another e.g. acetic acid? If so, is the ratio the same as with the tartaric acid?
A titration carried out on the original wine will give you a measure of the concentration of ALL the acids in the wine.
When you distil the wine, a titration on the liquid NOT distilled over gives a measure of the non volatile acids present, e.g. tartaric acid, whereas a titration on the liquid which distils over measures the volatile acids present, e.g. acetic acid.
The calculations for volatile acids are different in that acetic acid reacts with sodium hydroxide in 1:1 molar ratio. Remember too that these calculations will be carried out on the basis that acetic acid represents all the volatile acids present. There will be others too, e.g. methanoic acid and propanoic acid.
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.
back to Practical investigations
Rate this page or react
Share your views on this page, 0 ratings so far
updated: 02 March 2007
