Hanna Instruments HI 96733 - Manual (Page 4)

6 7
1) GLP/ key
2) CAL CHECK key
3) ZERO/CFM key
4) READ/ /TIMER key
5) ON/OFF key
6) Liquid Crystal Display (LCD)
7) Cuvette alignment indicator
8) Cuvette holder
PRINCIPLE OF OPERATION
FUNCTIONAL DESCRIPTION
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 96733 colorimeter is based on a tungsten lamp combined with
a narrow-band interference filter to guarantee both high performance and reliable results.
HI 96733 block diagram (optical layout)
A microprocessor controlled special tungsten lamp emits radiation which is first optically
conditioned and beamed to the sample contained in the cuvette. The optical path is fixed by the
diameter of the cuvette. Then the light is spectrally filtered to a narrow spectral bandwidth, to
obtain a light beam of intensity
I
o
or I .
The photoelectric cell collects the radiation I that is not absorbed by the sample and converts
it into an electric current, producing a potential in the mV range.
The microprocessor uses this potential to convert the incoming value into the desired measuring
unit and to display it on the LCD.
The measurement process is carried out in two phases: first the meter is zeroed and then the
actual measurement is performed.
The cuvette has a very important role because it is an optical element and thus requires
particular attention. It is important that both, the measurement and the calibration (zeroing)
cuvettes, are optically identical to provide the same measurement conditions. Whenever possible
use the same cuvette for both. It is necessary that the surface of the cuvette is clean and not
scratched. This to avoid measurement interference due to unwanted reflection and absorption of
light. It is recommended not to touch the cuvette walls with hands.
Furthermore, in order to maintain the same conditions during the zeroing and the measuring
phases, it is necessary to close the cuvette to prevent any contamination.
INSTRUMENT DESCRIPTION