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DESCRIPTION
5.3. PRECISION AND ACCURACY
Precision is how closely repeated
measurements are to one
another. Precision is usually
expressed as standard deviation
(SD).
Accuracy is defined as the
closeness of a test result to the
true value.
Although good precision suggests
good accuracy, precise results can
be inaccurate. The figure explains
these definitions.
For each method, the accuracy is expressed in the related measurement section.
5.4. PRINCIPLE OF OPERATION
Absorption of light is a typical phenomenon of interaction between electromagnetic
radiation and matter. When a light beam crosses a substance, some of the radiation
may be absorbed by atoms, molecules or crystal lattices.
If pure absorption occurs, the fraction of light absorbed depends both on the optical
path length through the matter and on the physical-chemical characteristics of the
substance according to the Lambert-Beer Law:
-log
I/I
o
=
e
l
c d
or
A =
e
l
c d
I
o
= intensity of incident light beam
I
= intensity of light beam after absorption
e
l
= molar extinction coefficient at wavelength
l
c
= molar concentration of the substance
d
= optical path through the substance
Therefore, the concentration “c” can be calculated from the absorbance of the substance
as the other factors are constant.
Photometric chemical analysis is based on specific chemical reactions between a
sample and reagent to produce a light-absorbing compound.