7
6
Colorimetric Mode
The color of every object we see is deter-
mined by a process of absorption and emis-
sion of the electromagnetic radiation (light) of
its molecules.
Colorimetric analysis is based on the principle
that specific compounds react with others to
form a color, the intensity of which is
proportional to the concentration of the
substance being measured.
Block diagram of an ion specific measurement
When a substance is exposed to a beam of
light intensity
I
o
, a portion of the radiation is
absorbed by the substance's molecules and
a radiation of intensity
I
, lower than
I
o
, is
emitted.
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 the sample
Since other factors are known, the concen-
tration "c" can be calculated from the color
intensity of the substance determined by the
emitted radiation
I
.
An LED (Light Emitting Diode) emits radiation
at a relatively narrow spectrum, supplying the
system with the intensity
I
o
.
LED
EMITTED LIGHT
CUVET
MICROPROCESSOR
LIGHT
DETECTOR
PRINCIPLE OF OPERATION
Turbidity Mode
HI 93114 has been designed to perform mea-
surements according to the USEPA’s 180.1
method and the Standard Method 2130B.
The instrument functions by passing a beam
of light through a vial containing the sample
being measured.
The light source is a Pure Green LED to
ensure that any interference caused by a
colored samples is minimized.
A sensor, positioned at 90° with respect to
the direction of light, detects the amount of
light scattered by the undissolved particles
present in the sample. The microprocessor
converts such readings into NTU* values.
NTU units are equal to FTU units. However,
there are other known measurement units for
turbidity, namely the Jackson Turbidity Unit
(JTU) based on the old method of Jackson's
candle, and Silica Unit (mg / L of SiO
2
). The
conversion table between these measurement
units is shown below:
JTU
NTU/FTU
SiO
2
(mg / L)
JTU
1
19
2.5
NTU/FTU
0.053
1
0.13
SiO
2
(mg / L)
0.4
7.5
1
* 1 NTU = 1 FTU