Note to all doing this expt!
1. FeCl3 solution is mildly acidic, so attempting to buffer this solution might well not work - just accept that 'urine' + FeCl3 will be acidic (it's already left the body, remember!). Phosphate buffers don't work well in this situation, as they are apt to form other coloured compounds - just what you do NOT want!
2. FeCL3 solution is pale yellow/brown in colour so it will NOT be essential to re-zero the colorimeter with a blank cuvette of pure 'urine' as the difference in colour will be small and the colorimeter unlikely to detect much difference. Good experimental technique suggests it would still be a good thing to do, though, since the yellow food dye might have some effect.
3. FeCL3 solution stains like crazy, so DON'T get it on your clothes/bench etc!!!