Photometers; Photometry - wtw photolab S12 Operating Instructions Manual

photoLab ® Series

1 Photometers

1.1 Photometry

2
When a beam of light is transmitted through a colored solution, then this
beam loses its intensity, in other words a part of the light is absorbed by
the solution. Depending on the substance in question, this absorption
occurs at specific wave lengths.
Monochromators (e. g. narrow-band interference filters, lattices) are used
to select the wavelength from the total spectrum of a tungsten-halogen
lamp (VIS spectrum), a deuterium lamp (UV spectrum) or, respectively, a
xenon lamp.
The intensity of the absorption can be characterized using the transmit-
tance T (or, respectively, T in percent).
T = I/I
0
I = Initial intensity of the light
0
I = Intensity of the transmitted light
If the light is not absorbed at all by a solution, then this solution has a
transmittance of 100 %; a com plete absorption of the light in the solution
means 0 % transmittance.
The measure generally used for the absorption of light is the absorbance
(A), since this correlates directly with the concentration of the absorbing
substance. The fol lowing connection exists between absorbance and
transmittance:
A = – log T
Experiments by BOUGUER (1698–1758) and LAMBERT (1728–1777)
showed that the absorbance is dependent on the thickness of the ab-
sorbing layer of the cell used. The relationship between the absorbance
and the concentration of the analyte in ques tion was discovered by BEER
(1825–1863). The combination of these two natu ral laws led to the deri va-
tion of Lambert-Beer's law, which can be described in the form of the fol-
lowing equation:

A = · c · d

 
= Molar absorptivity, in l/mol

d = Path length of the cell, in cm
c = Concentration of the analyte, in mol/l
cm
x
Release 01/2012