Practical investigations
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I'm doing my chemistry project on the concentrations of volatile and non-volatile acids in 4 different supermarket vinegars. Although I have conducted the experiment differently from the method on your site, I was wondering if you could advise me on the final calculations.
After I had separated my volatile and non-volatile acids, 10 ml of each were made up to 100 ml in a standard flask and then titrated against standardised NaOH. I am slightly confused about finding the concentration from this. I have found the number of moles of hydrogen ions for each vinegar in both the volatile and non-volatile portions by determining the no. of moles of OH- required for neutralisation (as they react in a 1:1 ratio (OH-) = (H+) and multiplying by 10 to take into account the dilution.
However, I was wondering how to calculate the concentration from this. Using the equation c = n/v is the volume the original 10 ml of the vinegar or the 100 ml that it was diluted to and are the units simply moles per litre.
Also, if after doing my chromatography I find for example that a diprotic acid is present, would I simply say that the acid concentration is half that of the total hydrogen ion concentration?
Igloo writes ...
Rather than trying to substitute into a formula which you are finding difficult to apply here I would recommend the following.
1 From your titre and knowledge of the concentration of the NaOH, calculate the number of moles of NaOH used.
2 As you say, this is equal to the number of moles of OH- used and also equals the number of moles of H+ ions neutralised.
3 Assuming that you used a 10 ml pipette for the vinegar sample, this number of moles of H+ needs to be scaled up (by 100) to 1000 ml to give a concentration of H+ in mol dm-3.
4 Next you need to realize that the vinegar sample was diluted by a factor of 10 before the titration was carried out, so the concentration in 3 needs to be multiplied by 10 to give the concentration of H+ ions in the original sample.
5 Assuming that all the acids in the vinegar are monoprotic, you can then say that the total concentration of acids is whatever you calculated in 4.
6 However, if you assume that all the acids are diprotic, then yes, you have to divide this value by 2 in order to determine the concentration of acids in the vinegar.
7 Of course in practice, the vinegar is a complex mixture of many different types of acids, including triprotic acids, such as citric acid.
8 The conventional approach is to choose what you believe is the predominant acid (let’s say tartaric acid, which is diprotic), and to give the halved value as your answer, emphasising that your calculation has been carried out in terms of the representative acid - tartaric acid. However, many people choose ethanoic acid as their representative acid since it’s the obvious one present (due to its smell), and please remember that volatile acids do not tend to show up in chromatograms, so you might be mislead if this fact is forgotten.
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Risk assessment
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updated: 17 April 2007
