4
The quantity of radiation absorbed is given by the Lambert-Beer
Law:
log
I
o
/
I
=
ε
λ
c d
Where
log
I
o
/
I
= Absorbance (A)
ε
λ
= molar extinction coefficient of the substance at
wavelength
λ
c = molar concentration of the substance
d = optical distance light travels through sample
Therefore, the concentration "c" can be calculated from the color
intensity of the substance determined by the emitted radiation
I
, as
the other factors are known.
BLOCK DIAGRAM OF A COLORIMETER
A monochromatic LED (Light Emitting Diode) emits radiation at a
single wavelength, supplying the system with the intensity
I
o
.
Since a substance absorbs the color complimentary to the one it emits
(for example, a substance appears yellow because it absorbs blue
light), Hanna colorimeters use LEDs that emit the appropriate
wavelength to measure the sample.
The optical distance (d) is measured by the diameter of the cuvet
containing the sample.
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 done in two phases: setting the meter to
zero and the actual measurement.
The cuvet 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) cuvets are optically identical to provide the same measure-
ment conditions. Whenever possible use the same cuvet for both.
It is also necessary that the cuvet's surface is clean and not scratched,
in order to avoid measurement interference due to unwanted reflec-
tion and absorption of light.
LED
EMITTED LIGHT
CUVET
MICROPROCESSOR
LIGHT
DETECTOR