Analytical Notes
107
Unusual Samples
If a sample is chemically stable and undergoes no physical or chemical change other
than to absorb incident radiation, errors in photometric values should not be caused
by the sample. Many samples are not stable, and special consideration must be given
to them.
Volatile Samples
Some liquid samples are so volatile that their concentration can change while
recording is in progress. If this occurs, the resulting data will lack reproducibility.
If you are analyzing volatile samples, use stoppered cells to prevent this problem.
Samples not Governed by the Beer-Lambert Law
Quantitative analyses utilizing the absorption of spectral radiation are based on the
Beer-Lambert law which states that the absorption is proportional to the
concentration of the analyte.
The law can be expressed in the form
A
= ε c d
Where:
A
is absorbance
ε is molar absorption coefficient
c is molar concentration
d is thickness through which the radiation is transmitted
This law is mostly true for dilute solutions, but at higher concentrations a plot of
absorbance against concentration will be non-linear for a number of reasons.
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