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 “717”, 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 95717-11
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.
