What does an absorbance value greater than 2 mean?

Answer 1

The definition of absorbance is A=log( I(0) / I), which is the logarithm (base 10) of the initial intensity of the light passed through the sample divided by the intensity of the light after it has passed through.

Let's look at a few values of A and see what these values imply.

If A = 0, then I(0) must equal I, which means the intensity doesn't change, which means no light is absorbed.

If A = 1, then I(0) / I must equal 10. This means that 90% of the light was absorbed.

If A=2, then I(0) / I must equal 100, and 99% of the light was absorbed.

If A=3, then I(0) / I must equal 1000, and 99.9% of the light was absorbed.

And so on.

Answer 2

An absorbance value greater than 2 typically indicates that the sample is highly absorbing the specific wavelength of light being used for measurement. It suggests a high concentration of the absorbing substance in the sample, potentially beyond the linear range of the spectrophotometer. Dilution or re-measurement at a higher dilution factor may be needed to obtain accurate results.