Chemistry, Third edition

372 20 · LIGHT AND SPECTROSCOPY

Fig. 20.6Absorbance spectra for the case shown in Fig. 20.5. (a) The
absorption spectrum of the spectrometer with an empty sample
chamber (or with a non-absorbing sample). The small peaks are due to
electronic ‘noise’ from the detector. (b) The absorption spectrum of a
sample which absorbs at three wavelengths.

Example 20.2

40% of the light emitted by a spectrometer source at a

wavelength of 460 nm is absorbed by a sample. What is the

absorbance of the sample at 460 nm?

 Answer

If 40% of the light intensity is absorbed, 60% is transmitted. Since

100 I()

%T= ————–

I0()

then

100 I()

60% = ————–

I0()

or,

I0() 100

—— = —— = 1.7

I() 60

Therefore, the absorbance is log(1.7) = 0.23.

Comment

It is usually unwise to deal with samples whose absorbances are greater than 2.0.

An absorbance of 2.0 corresponds to a percent transmittance of 1% so that 99%

of the incident light has been absorbed at that wavelength. At higher absorbances,

the percentage of transmitted light is so small that it becomes comparable in size

with light losses in the spectrometer optics, and experimental measurements are

therefore subject to considerable error. (Now try Exercise 20D.)

Absorbance

(i) Calculate the absorbance of a sample where 10% of the light is absorbed at the monitoring

wavelength.

(ii) The absorbance of a solution is 3.00. What percentage of the incident light is transmitted

by the sample at this wavelength?

Exercise 20D