Hanna Instruments HI 83218-01 - 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 83218 is based on special subminiature tungsten lamps and narrow-band
interference filters to guarantee both high performance and reliable results.
Three measuring channels 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 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) cuvette are optically identical to provide the
same measurement conditions. Most methods use the same cuvette for both, so it is important that measurements
are taken at the same optical point. The instrument and the cuvette cap have special marks that must be
aligned in order to obtain better reproducibility.
The surface of the cuvette must be 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 cuvette walls with hands.
Furthermore, in order to maintain the same conditions during the zeroing and the measurement phases,
it is necessary to cap the cuvette to prevent any contamination.