Practical investigations
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I am currently doing my personal investigation for A2 on the analysis of vinegars and the ethanoic acid content. Having read through some of the already answered questions on the website I was still confused as to how I can calculate the total acid content in a vinegar using the results from my titration of vinegar using 0.4M NaOH. Can you help??
Igloo writes ...
I can see why you’re confused. The problem is that ethanoic acid is by no means the only acid present in vinegar, so when you carry out your neutralisation the alkali reacts with all the acids present. Some of these, such as ethanoic acid, are monobasic (monoprotic) and contain one acidic hydrogen atom per molecule, whilst others such as tartaric acid are dibasic (diprotic) and a few, such as citric acid, are tribasic (triprotic). As a consequence, there are three types of equation for the neutralisation:
1 mole of monoprotic acid needs 1 mole of NaOH for neutralisation
1 mole of diprotic acid needs 2 moles of NaOH for neutralisation
1 mole of triprotic acid needs 3 moles of NaOH for neutralisation.
You can see there are far too many variables here and that it’s impossible to calculate the total acidity since we don’t know the proportion of monoprotic to diprotic to triprotic acids in the vinegar. So, in a sense, we’re stuck!
However, if we assume that all the acidity is due to monoprotic acids, which is the best we can do in the circumstances, then we can carry out a calculation. We then say at the end that we have calculated the total acidity in terms of monoprotic acids. This is the most reasonable way round the problem since ethanoic acid is monoprotic and is almost certainly the acid which predominates in the vinegar.
As for detail, you do the following:
1 From the titration reading calculate the number of moles of 0.40 molar NaOH used.
2 This equals the number of moles of monoprotic acids present.
3 Finally scale up to 1 dm3 to calculate the concentration in mol dm-3.
Since a few of the acids are diprotic or triprotic I hope you can see that your calculation will overestimate the total concentration of acids present.
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.
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updated: 29 November 2007
