Practical investigations
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I am doing my practical investigation on the concentration of aspirin. For colorimetry using iron(III) chloride, I cannot find out how to set up the standard solution required for dilution to produce samples of known concentration for use in producing the calibration curve, nor can I find out how to set up the solutions for the actual testing. Can you help me please?
Igloo writes
I would advise using hydrated iron(III) chloride, FeCl3.6H2O, because it can be obtained in a fairly high state of purity. According to the Nuffield Book of Data, its solubility in water equates to a concentration of about 3.4 mol dm-3. This solution would of course be a saturated one. I would therefore make up 100 cm3 of aqueous iron(III) chloride with a concentration of, say, 2.00 mol dm-3. in a volumetric flask, using de-ionised water (I’ll leave you to work out the mass of the solute required).
Next, you could fill two burettes, one with de-ionised water and the other with your 2.00 molar iron(III) chloride solution. If you run various known volumes of each liquid into a colorimeter tube and thoroughly mix the contents this will give you a dilute solution of the iron(III) chloride. For example, if 0.4 cm3 of the 2.00 molar iron chloride solution is added to 9.6 cm3 of water, the total volume is 10.0 cm3 and the iron(III) chloride has been diluted 10.0/0.4 times, i.e. 25 times. So the concentration of iron(III) chloride in the diluted mixture is 2.00/25 = 0.080 mol dm-3. The absorbance of this solution can be measured in a colorimeter, and if this experiment is repeated for other mixtures your calibration curve can be plotted.
The concentrations you will need to use must of course cover the range relevant to your experiments with the aspirin. I’ll leave the rest to you.
Always carry out a risk assessment before starting any practical work, and check with your teacher.
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: 25 February 2005
