Practical investigations
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Hiya, I’m doing my Salters A2 chemistry investigation at the moment. I've been investigating aspirin. I'm doing fine with it but I’ve become a bit stuck when it comes to calculating the percentage error for the evaluation. It’s not a regular problem - I understand how to do it generally. I’m just not sure how to work it out for a colorimeter. I mean I assume there's quite a high intrinsic error but I can't find out what it is.
Also working out the other errors is a little odd as well because our colorimeters go from zero to two rather than zero to one hundred, but I think that's just the log of the percentage absorbance. Thanks in advance. Alison
Igloo writes
I suspect that you’re correct and that your colorimeters work on a logarithmic scale, which is rather unusual in my experience. First establish the error using the instrument and then convert this to a “concentration error” using the calibration curve which you have presumably plotted. Now that you are in concentration units, rather than (possibly) logarithmic ones, you can use these in your estimation of the overall error. If the scale of your colorimeter is not linear it is probably best to estimate an instrumental error using mid-range readings, or perhaps in the sort of region where you took most of them.
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updated: 18 December 2006
