6
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
=
ε
λ
c d
or
A
=
ε
λ
c d
Where:
-log
I
/
I
o
=
Absorbance (A)
I
o
=
intensity of incident light beam
PRECISION AND ACCURACY
Precision
is how closely repeated measurements
agree with each other. Precision is usually
expressed as standard deviation (SD).
Accuracy
is defined as the nearness of a test
result to the true value.
Although good precision suggests good accuracy,
precise results can be inaccurate. The figure
explains these definitions.
In a laboratory using a standard solution of the
parameter and a representative lot of reagent
(for each parameter) the following standard
deviations were obtained:
Parameter
Standard Solution
Standard Deviation
pH
7.0 pH
0.1 pH units
Free Chlorine
1.00 mg/L
0.03 mg/L
Total Chlorine
1.00 mg/L
0.03 mg/L
Cyanuric Acid
20 mg/L
1 mg/L