The Student Room Group

Colorimetry of Salicylic acid

For my aspirin coursework, I did some colorimetry, and to do that I needed to create a calibration curve for different cncentrations of salicylic acid. The salicylic acid was purple because I complexed it with iron (III) Chloride. The concentrations I used were 0, 0.1, 0.2, 0.3, etc up to 1 mol dm3.

However, ater doing this, I got a sort of s shaped calibration curve... Can anyone tell me whether that's the sort of thing I SHOULD be seeing of not? I'm a little unsure because, especially for the higher concentrations, precipitate formed when diluting, so the readings aren't very accurate... plus I don't really trust the colorimeter XD

If I shouldn't be seeing an s shaped curve, what kind of thing should I be seeing? I tried looking on the internet but I can't seem to find anything on what it should look like...
Reply 1
Ok, I've found out that there's supposed to be a straight line relationship... but since I have an S shaped curve what should I do? Should I draw a line of best fit through it or should I keep the curve and use that to determine the concentration of my unknown solution?

And if I keep the curve what should I say about it, because it's SUPPOSED to be a straight line? Maybe something like how unreliable colorimetry is? Or should I pretend to not know that there's supposed to be a straight line and not say anything about my curve... XD
Calibration curves are just that.. curves. They may be straight lines in theory but given all of the vaguaries of experimentation they are usually curves, and that is precisely why they are used. If the relationship was experimentally a straight line there would be no need for calibration...
Reply 3
So shall I keep to my S-shape and use that to determine my unknown concentration? And then in evaluation and stuff explain why it might not be very accurate?
Reply 4
I am doing my salicylic aspirin colorimetry but I'm not sure how small to make my concentration of both salicylic acid and of iron (III) chloride. At the moment I am planning on using a 0.01M standard salicylic acid solution, and a 0.001M standard Iron (III) chloride solution. Are those concentrations too low?????