Practical investigations
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I’m investigating the purity of aspirin using colorimetry. I know how to set up a calibration curve with iron(III) chloride. However once I have hydrolysed my aspirin do I have to add a certain amount of iron(III) chloride to this in order to compare it with my calibration curve?
Igloo writes ..........
I assume that you have calibrated the colorimeter using an excess of iron(III) chloride with a range of different concentrations of pure salicylic acid (2-hydroxybenzoic acid), relevant to the concentrations of aspirin you are expecting to use with your prepared sample.
For example, if you expect concentrations of your aspirin solution to be in the region of 0.01 mol dm-3, you could set up three burettes, one containing 0.20 mol dm-3 iron(III) chloride (liquid A), another containing 0.04 mol dm-3 pure salicyclic acid (SA) solution (liquid B) and the third pure water (liquid C). The calibration tubes could be set up as shown below.
Assuming that your colorimeter tubes hold about 10 cm3 of liquid:
| A: 5.0 cm3 | B: 0.0 cm3 | C: 5.0 cm3 | SA concentration: zero |
| A: 5.0 cm3 | B: 1.0 cm3 | C: 4.0 cm3 | SA concentration 0.004 mol dm-3 |
| A: 5.0 cm3 | B: 2.0 cm3 | C: 3.0 cm3 | SA concentration 0.008 mol dm-3 |
| etc | |||
| A: 5.0 cm3 | B: 5.0 cm3 | C: 0.0 cm3 | SA concentration 0.020 mol dm-3 |
When you come to use your own hydrolysed sample of aspirin, use the same 5.0 cm3 of the iron(III) chloride, 5.0 cm3 of your aspirin solution, and carry out the colorimetry. Read off the concentration of salicylic acid from your calibration curve, and double this value to give the concentration of aspirin in your original solution.
Can you see why the concentration of SA needs to be doubled?
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Risk assessment
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updated: 14 March 2007
