Practical investigations
Read our general notes on Risk Assessment
I have been reading the answers that are given which might help me with my investigation on the concentration of aspirin in various medical products. However, I am a little confused. What exactly does hydrolysis do to the aspirin? Do I have to hydrolyse the aspirin sample before "back-titration" ?
Igloo replies
The process of hydrolysis involves the nucleophilic attack of water (often with an acid catalyst) or hydroxide ions (OH-) on a compound, causing it to be decomposed. Examples of organic compounds which can be hydrolysed include esters, acid chlorides and amides.
Aspirin is both an acid and an ester, so when it is hydrolysed it is converted into the sodium salts of 2-hydroxybenzoic acid (salicylic acid) and ethanoic acid. The hydrolysis can be carried out in acidic solution or with an alkali:
In acid conditions:
CH3COO.C6H4.COOH + H2O -> HO.C6H4.COOH + CH3COOH
With alkali:
CH3COO.C6H4.COOH + 3NaOH -> NaO.C6H4.COONa + CH3COONa + 2H2O
The “back titration” method has to be carried out after the hydrolysis wiht alkali. You use a measured excess of alkali to hydrolyse the aspirin. Then you use a back titration to find the amount of alkali left after hydrolysis. Hence you can calculate the amount of alkali used to hydrolyse the aspirin.
If you use the search facility on the www.chemistry-react.org website, typing in “back titration” or “aspirin” as your search phrase, you will come across several references to the analysis procedure which follows the hydrolysis.
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, 24 ratings so far
, rated at 
updated: 09 December 2004
