Hanna Instruments HI 83214 - Manual (Page 5)

5
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
HI 83214 is based on special subminiature tungsten lamps and narrow-band
interference filters to guarantee both high performance and reliable results.
Three measuring channels (at three different wavelengths) allow a wide range of tests.
Instrument block diagram (optical layout)
A microprocessor-controlled special tungsten lamp emits radiation which is first optically conditioned and
beamed through the sample contained in the vial. The optical path is fixed by the diameter of the vial.
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 vial 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) vials are optically identical to
provide the same measurement conditions. Most methods use the same vial for both, so it is important that
measurements are taken at the same optical point. Instrument measurement principle ensures this.
It is necessary that the surface of the vial is clean and not scratched. This is to avoid measurement
interference due to unwanted reflection and absorption of light. It is recommended not to touch the vial
walls with hands.
Furthermore, in order to maintain the same conditions during the zeroing and the measuring phases, it is
necessary to cap the vial to prevent any contamination.