can i just say something....did you lot read the question paper,you're not supposed to use actual urine! read properly. use urine and you'll fail. you only need the standard solutions of different concentrations.that'll act as your urine,and that's how you'll get your calibration. so if you make solutions eg at 1M,0.5M,0.2M and add the Fe,you'll get different shades of the purple which you put into the colorimeter and get your different absorbances and then you can plot your calibration curve. you'll doing this at pH7 as they said it's the highest volume of aspirin that can be obtained. then you use buffers to change the pH,according the question,there is less amount of aspirin in the urine at lower pHs so supposedly,i haven't gotten this far yet,if you have add the Fe to your aspirin solution,somehow the Fe reacts or something,but your colour won't be the same as it was at lets say pH7.
please please do not usee actual urine. you can say at the end,the aspirin aolution will obvisouly be replaced with urine,if an actual test was being carried out in a medical lab.
the actual words of the plan say right at the bottom in bold: You should not trial your plan using human subjects or actual urine so please people,read properly!