Practical investigations
Read our general notes on Risk Assessment
Hello, I am carrying out my investigation into the purity of synthesised aspirin. I encountered a problem when setting up my calibration curve using iron (III) chloride. I came across another method that suggested making a buffer solution of HCl and KCl with the iron (III) chloride. When I did this it worked and a calibration curve could be drawn. Can you tell me why the buffer solution was used and the difference between it and just the normal iron (III) chloride? Does it interact differently with the 2-hydroxybenzoic acid? Thanks in advance :)
Igloo writes ...
Whatever your instructions say, the mixture of HCl and KCl is not a buffer since it is a mixture of a strong acid with one of its salts. Nevertheless I can see why this mixture will help to give you more reliable results. Solutions of iron(III) compounds tend to hydrolyse reversibly:
Fe3+(aq) + 3H2O(l) <---> Fe(OH)3(s) + 3H+(aq)
The creation of solid Fe(OH)3 tends to make the solutions somewhat murky, and this isn't conducive to the process of colorimetry, which in this instance works most reliably with transparent solutions (coloured or not).
Addition of the HCl/KCl adds H+ ions and causes the equilibrium above to swing to the left, destroying most of the Fe(OH) 3 and restoring transparency.
The complex formation between iron(III) ions and 2-hydroxybenzoic acid is as before: there is no different interaction. I do not know why KCl is added as well. I feel sure that the procedure would work equally well without it.
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
