6
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
I
= intensity of light beam after absorption
ε
λ
= molar extinction coefficient at wavelength
λ
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 known.
Photometric chemical analysis is based on the possibility to develop an absorbing compound from
a specific chemical reaction between sample and reagents. Given that the absorption of a
compound strictly depends on the wavelength of the incident light beam, a narrow spectral
bandwidth should be selected as well as a proper central wavelength to optimize measurements.
The optical system of Hanna's
HI 95
series colorimeters is based on special subminiature tungsten
lamps and narrow-band interference filters to guarantee both high performance and reliable
results.
HI 95 series block diagram (optical layout)
PRINCIPLE OF OPERATION
15
• Turn the meter on by pressing ON/OFF.
• When the LCD displays “761”, it is ready.
• Place the CAL CHECK™ Standard Cuvet
A
into
the holder and ensure that the notch on the cap
is positioned securely into the groove.
• Press ZERO and “SIP” will blink on the display.
• After a few seconds the display will show
“-0.0-”. The meter is now zeroed and ready for
validation.
• Remove the cuvet.
• Place the CAL CHECK™ Standard HI 95761-11
Cuvet
B
into the holder and ensure that the
notch on the cap is positioned securely into the
groove.
• Press CAL CHECK and “SIP” will blink during
measurement.
Use the validation procedure to ensure that the instrument is properly calibrated.
Warning
: do not validate or calibrate the instrument with standard solutions other than the Hanna
CAL CHECK™ Standards, otherwise erroneous results will be obtained.
VALIDATION PROCEDURE
Temperature correction for Validation: corrected value = read value
×
[1
+
0.002
×
(20
−
T(°C))]
corrected value = read value
×
[1
+
0.001
×
(68
−
T(°F))]
• Wait for a few seconds and the display will show the validation standard value.
The reading should be within specifications as reported on the CAL CHECK™ Standard
Certificate. If the value is found out of specifications, please check that the cuvets are free of
fingerprints, oil or dirt and repeat validation. If results are still found out of specifications then
recalibrate the instrument.
