Practical investigations
Read our general notes on Risk Assessment
For the A2 investigation I am synthesising my own aspirin and testing its purity. One of the methods is the back titration and I am aware that 2 moles of NaOH will react with one mole of aspirin. However, salicylic acid impurities in the sample will also react with one mole of NaOH. Using the formula that you have suggested does not take into account the NaOH used up by the salicylic acid impurity and so it is not possible to work out the purity of my synthesised aspirin. Can you suggest a method or equation with its derivation for working out the purity of my synthesised aspirin in this case? Please bear in mind that I have to use stand alone methods for the determination of purity
Igloo writes ...
You are correct. The back titration method is ideal for commercial aspirin tablets where the main impurity is “filler” and it is assumed that little or no hydrolysis to salicylic acid has occurred. If the tablets are fresh then this is a perfectly fair assumption to make.
In your case, the unreacted salicylic acid will indeed interfere with your results in the way you have described.
A method for lab prepared samples of aspirin is as follows:
1 Take a known mass of pure salicylic acid and titrate it with a standardised solution of sodium hydroxide.
2 Carry out the same procedure, using the same mass of pure acetylsalicylic acid (pure aspirin) using ice-cold conditions so that the ester linkage in unaffected.
3 Finally use the same procedure yet again with the same mass of your sample (using ice-cold conditions).
You will have three titres. The first and the second will represent minimum and maximum values and the titre for your sample should be somewhere between the extremes. Interpolation of the third titre between the two extreme values will enable you to deduce the composition of your sample.
Have a search through the FAQs on React to find a more detailed and diagrammatic response to this question.
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, 1 ratings so far
, rated at 
updated: 21 January 2008
