B.Sc. Second year Undergraduate degree course (CBCS Pattern)
Semester Third
Physical Chemistry (CHE-312)
Chapter – Colorimetry
Principle
When
an incident light beam with intensity I0 passes through a solution,
a part of the incident light is reflected (Ir) and absorbed (Ia) while the
remaining incident light is transmitted (It).
i.e.,
Io = Ir + Ia + It
The
measurement of (I0) in the colorimeter eliminates (Ir),
and it is sufficient to calculate the (Ia). Using cells with the same
characteristics maintains a steady amount of light reflection (Ir). Then (I0)
& (It) are measured.
The
operation of the colorimeter is based on Beer-Lambert’s law which states that
the amount of light absorbed by a color solution is directly proportional to
the solution’s concentration and the length of a light path through it.
A ∝ Cl
A
= ∈Cl
A
= Absorbance/ Optical density of the solution
∈ = Coefficient of
absorption
C
= Concentration of solution
l
= Length of the path
A
series of lenses in a colorimeter guide a beam of light with a particular
wavelength through a solution as it makes its way to the measuring apparatus.
This compares the colour to a current standard to examine it. The absorbance or
% transmittance is then calculated using a microprocessor. By measuring the
difference between the amount of light at its source and that after
passing the solution, it is possible to determine the concentration of the
solution and how much light will be absorbed.
Construction
and components
Fig.Components of colorimeter
1.
Light Source: The light source must generate sufficient energy over the entire
visible spectrum (380-780nm). Tungsten lamps are commonly used.
2.
Slit: It permits a beam of light to pass through and reduces unwanted light.
3.
Condensing lens: produce a parallel light beam.
4.
Monochromator: It produces monochromatic radiation (one wavelength band) from
polychromatic radiation (white light) supplied by a light source. It allows the
required wavelength through. It is possible to employ prisms, gelatin fibres,
grating monochromators, and interference filters.
5.
Sample Holder (Cuvette): The monochromatic light from the filter travels
through the cuvette containing the colourful sample solution. Their sizes range
from square to rectangular to circular, and their diameter is a constant 1cm.
Glass, quartz, and plastic cuvettes are the three varieties based on their
respective compositions.
a)
Glass cuvettes are inexpensive and
absorb light with a wavelength of 340 nm
b)
Quartz cuvettes allow both UV and
visible light to pass through.
c)
Plastic cuvettes are less expensive,
scratch easier, and have shorter lives.
6.
Photo detectors: Photodetectors assist in turning the resultant transmitted
light rays into an electrical signal once they pass through the sample
container. It is also known as a photocell. In colorimeters, numerous types of
sensors are utilised based on the material employed. Photocells made of
selenium, phototubes, and photocells made of silicon are examples of regularly
employed detectors.
a)
Selenium photocell: Selenium photocell
is the simplest sort of detector and operates without the need for external
power sources.
b)
Phototube: The phototube consists of a
glass bulb coated with photosensitive substances such as cesium or potassium.
c)
Silicon photocell: When a photon of
light meets the semiconductive surface of the silicon photocell, electrons are
produced.
7.Filter:
The types of filters vary depending on the manufacturer of the colorimeter.
Monochromatic (just one wavelength) or polychromatic (many wavelengths)
depending on the wavelength (white light). There are four choices available:
gelatin, interference, grating, and prisms.
a)
Gelatin filters: Gelatin filters
consist of a small layer of coloured gelatin sandwiched between two thin glass
plates. These filters are inexpensive, but they can absorb 30-40% of all
incident radiation, reducing the energy throughput of the detectors.
b)
Glass filters: tinted glass filters
with wide bandpasses up to 150 nm are another form of filter. By combining
several glass filters, specific wavelengths can be attained.
c)
Interference filter: It consists of
several semi-transmissive, semi-reflective silver sheets separated by thin
layers of transparent dielectric material. Multiple reflections are created
between the semitransparent mirrors as white light passes through the
dielectric layers. Here, some light ray energy travels directly through the
filter. This is the wavelength required for analysis. The thickness of the dielectric
layer determines the light’s resulting wavelength.
d)
Grating monochromator: It generates
monochromatic light and consists of several parallel grooves closely spaced on
a polished surface made of steel, glass, or quartz. A common grating may have
500-600 lines/mm, whereas research-based equipment may have 1200-2 000
lines/mm. When white light strikes the grating, different wavelengths are
deflected at different angles.
e)
Prism: It splits white light into its
component wavelengths. Selecting the desired spectrum is accomplished by
rotating the prism. The colorimeter’s prism is composed of glass and operates
within the wavelength range of 350-800 nm.
8.
Display: It monitors and measures the electrical signal and produces output
that is visible.
9.
Measuring device: The current from the detector is supplied to the measuring
device, the galvanometer, which displays a metre value that is directly
proportional to the light intensity.
10.
Control panel: A control panel is used to operate the colorimeter and input the
necessary parameters for the measurement. The control panel may include
buttons, switches, and other controls.
11.
Power source: A power source, such as a battery or AC adapter, is used to power
the colorimeter.
Working
Step 1: Before starting the experiment it is important to calibrate the colorimeter. It is done by using the standard solutions of the known solute concentration that has to be determined. Fill the standard solutions in the cuvettes and place it in the cuvette holder of colorimeter.
Step
2: A light ray of a certain wavelength, which is specific for the assay is in
the direction of the solution. The light passes through a series of different
lenses and filters. The coloured light navigates with the help of lenses, and
the filter helps to split a beam of light into different wavelengths allowing
only the required wavelength to pass through it and reach the cuvette of the
standard test solution.
Step 3: When the beam of light reaches’ cuvette, it is transmitted, reflected, and absorbed by the solution. The transmitted ray falls on the photodetector system where it measures the intensity of transmitted light. It converts it into the electrical signals and sends it to the galvanometer.
Step 4: The electrical signals measured by the galvanometer are displayed in the digital form.
Applications
1.
The colorimeter is commonly used for
the determination of the concentration of a colored compound by measuring the
optical density or its absorbance.
2.
It can also be used for the
determination of the course of the reaction by measuring the rate of formation
and disappearance of the light-absorbing compound in the range of the visible
spectrum of light.
3.
By colorimeter, a compound can be
identified by determining the absorption spectrum in the visible region of the
light spectrum.
4.
Colorimetric measurement is used for
identification of complex formation.
5.
It is used to determine Pk value of an
indicator.
Colorimetry: Interaction of electromagnetic radiation with matter
Colorimetry:Lamberts Law,Beer's Law,Lambert-Beer's Law, Molar absorptivity
Colorimetry: Limitations of Beer –Lambert’s law,Deviation from Beers Law,Reasons for Deviation from Beer's Law.
Colorimeter: Principle, Construction and components, working, Applications
Colorimetry Multiple Choice questions
