B.Sc. Second year Undergraduate degree course (CBCS Pattern)
Semester Third
Physical Chemistry (CHE-312)
Chapter – Colorimetry
Interaction
of electromagnetic radiation with matter
When a beam of light
interacts with matter, numerous changes occur in both light and matter. These
changes provide the basis for several research tools such as spectrographs,
colorimeters, polarimeter and refractometer etc. Electromagnetic radiation is
characterized by wavelength λ, frequency Vo. intensity Io
and direction. Important changes that may occur are summarised below.
1.
The direction of the incident beam of light can be changed by reflection and refraction.
2.
The beam of light can be transformed into other beams by diffraction, double
refraction and scattering.
3.
If scattering occurs, the scattered light may exhibit the same frequency as
incident light. This type of scattering is referred to as Rayleigh
scattering.
4.
If the scattered beam exhibits either higher or lower frequency, it is called
Raman scattering or Raman Effect.
5. If the incident beam is plane polarised,
the plane of polarisation may be rotated by passing through the compound. It is
known as optical rotation and is measured by polarimeter.
6.
The intensity of the incident beam gets reduced or even disappear when passed
through the substance. It is absorption of light.
i.
If there is an exchange of energy
between the light beam and the molecules, a pattern of wavelength of light
absorbed with an indication of the energy absorbed at each wavelength constitute
the absorption spectra.
ii.
Absorption occurs only when the
radiation supplying the right packet of energy impinges on the matter. This
forms the basis of molecular spectra. The absorption of radiation depends on
the molecular structure of the compound.
iii.
The extent of absorption may depend on
the orientation of the plane polarization in the incident beam of radiation.
This is called dichroism.
iv.
The absorption of radiation causes the
atom or molecule to be in an excited state since excited states are short lived
(10-8 s), the electron may return to its ground state with the
emission of certain amount of energy. When this emission of light is
instantaneous, the phenomenon is known as fluorescence, if delayed it is called
phosphorescence.
v.
If the light absorption produces
chemically reactive substances, the process called photo-activation and
photochemical reaction.
7.
Matter can be made to emit light if it is properly excited. The resulting
radiation may contain several discrete and reproducible wavelengths in
ultraviolet and visible regions. Thus a pattern of wavelength of radiation
emitted constitute emission spectra.
The
absorption and emission spectra provide the same information about the energy
level separation in the molecule. Interaction of radiation with matter provide
significant information’s for the determination of the molecular structure. The
phenomenon associated to frequency and intensity of radiation include (i)
Transmission (ii) Reflection (ii) Absorption (iv) Scattering Shown in figure
Fig.Interaction
of electromagnetic radiation with matter
Transmission
Transmission is the process by which incident
radiation passes through matter without measurable attenuation; the substance
is thus transparent to the radiation. Transmission through material media of
different densities (e.g., air to water) causes radiation to be refracted or
deflected from a straight-line path with an accompanying change in its velocity
and wavelength; frequency always remains constant. In Figure, it is observed
that the incident beam of light is
deflected toward the normal in going from a low-density to a denser medium (θ2).Emerging from the
far side of the denser medium, the beam is refracted from the normal (θ3)
θ 1> θ2 and
θ1 =θ3
The change in EMR velocity is explained by the index of
refraction (n), which is the ratio between the velocity of EMR in a vacuum (c)
and its velocity in a material medium (v):
n =c/v
The index of refraction for a vacuum (perfectly transparent medium) is
equal to 1, or unity. Because v is never greater than c, n can never be less
than 1 for any substance. Indices of refraction vary from 1.0002926 for the Earth’s
atmosphere to 1.33 for water to 2.42 for diamond. The index of refraction leads
to Snell’s Law:
n1 sin θ1 =n2 sin θ2
Reflection
Reflection
occurs when a beam of radiation is allowed to cross an interface between media
of different refractive indices. When a beam travel normal to the interface,
the fraction of reflected beam increases with increasing difference in
refractive index and is given by
Where
I0 and Ir are the intensities of incident and reflected
radiation, n1 and n2 are the refractive indices of two
media
Absorption
Absorption
is the process by which incident radiation is taken in by a medium. For this to
occur, the substance must be opaque to the incident radiation. A portion of the
absorbed radiation is converted into internal heat energy, which is
subsequently emitted or re radiated at longer thermal infrared wavelengths.
Scattering
If
the incoming radiant energy strikes upon particles which are suspended in a
medium having refractive index different from that of the suspended particles,
the light which is transmitted at angles other than 180' from the incident
light is said to be scattered as the radiation passes through the sample. The
size, shape and concentration of colloidal particles and suspensions may be
determined from this property. Nephelometry and turbidimetry are based upon
this ability of particles to scatter light. Scattering by molecules or
aggregates of molecules with dimensions smaller than the wavelength of the
radiation is called Rayleigh scattering
Radiant
power
"Radiant
power" typically refers to the amount of electromagnetic radiation, such
as light or other forms of electromagnetic waves, emitted or transmitted from a
source in a given period of time. It's a measure of the energy carried by
electromagnetic radiation.
Radiant
power is usually measured in units like watts (W), which represent the rate of
energy transfer. In the context of light, radiant power can be thought of as
the amount of light energy emitted per unit of time. For example, a light bulb
might have a certain radiant power rating, indicating how much light energy it
emits per second.
Radiant
power is a fundamental quantity in the study of electromagnetic radiation. It
is important to understand the concept of radiant power in order to understand
the behavior of electromagnetic radiation and its applications.
Here
are some additional things to keep in mind about radiant power:
·
Radiant power is a scalar quantity,
meaning that it has magnitude but no direction.
·
The SI unit of radiant power is the
watt (W).
·
Radiant power can be measured using a
variety of instruments, such as a radiometer or a calorimeter.
Radiant
power can be calculated using the following equation:
P=E/t
where P is the radiant power, E is the radiant energy, and t is the time.
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
