J.R. S. answered • 02/16/17

Tutor

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Ph.D. in Biochemistry--University Professor--Chemistry Tutor

No, you don't just subtract from 100. The equation that relates absorbance to % transmittance is

A = 2 - log %T, so e.g. 10% transmittance = absorbance of 1 and 1 % transmittance = absorbance of 2. In your examples, I'm assuming that you have %T not % A, as I've never heard of data obtained as %A, but %T is common.

If that's the case, then 19%T = 0.7212 Absorbance and 20%T = 0.699 Absorbance. If, in fact, your data are actually % absorbance, then the % transmittance would be 100 - that number. That would be very strange however.