Absorbance is
important in determining concentration of a substance in a sample through
colorimeter analysis. Colorimeter measures the intensity of colour and light
transmittance by the sample to achieve the concentration. When a beam of light
passes through a coloured solution, the amount of light absorbed depends on the
nature of the molecules absorbing the light, their concentration and thickness
(path length) of the solution. The ratio of transmitted intensity to original
intensity s known as the “transmittance”, T.
Transmittance
(T) = I/I0
I =
intensity of the transmitted light
I0
= intensity of incident light
The Beer- Lambert law
states that there is a logarithmic dependence between the transmittance and the
absorbance. Therefore the transmittance is expressed in terms of absorbance;
Absorbance
(A) = -log10 T
= -log10
(I/I0)
According to this, the
absorbance becomes linear with concentration considering;
A =
ℰ ℓ C
ℰ = Molar absorbance coefficient
ℓ
= path length
C =
concentration of the solution
Therefore in dilute
solution,
A = -log10
(I/I0) = ℰ ℓ C
Molar absorbance
coefficient indicates the absorbance under a standard set of conditions, i.e.
the light travelling 1cm through a solution of 1moldm-3. In a
material with a low absorption coefficient, light is poorly absorbed and vise
versa. This depends on the material and on the wave length of the light.
When using the
colorimeter the path length i.e. the width of the glass cell is constant. Also the
concentration of one solution used at one specific wave length. Therefore ℰ is also constant through the measurements. This shows
out clear relationship between the absorbance and the concentration.
A ∝ C as ℓ and ℰ are constant
The glass cell/
container with plane parallel faces are transverse by monochromatic radiation
in the colorimeter. If the glass cell is filled with non absorbing solution,
there is 100% transmittance; therefore the absorption would be zero.
Colorimeter applies
only in relation to the visible region. Also Beer- Lambert law is applicable
for 0.800-0.200 absorbances.
In the experiment,
firstly the absorbance reading of the colorimeter should be zeroed using
distilled water as distilled water is used to prepare the solutions.
Also
before taking the measurements of the absorbance value in each solution, the
glass cell should be washed with distilled water in order to prevent
interferences to the reading. It is important not to touch the two smooth surfaces
of the glass cell and wipe out the additional drops remain on the surfaces of
the glass cell, using a tissue. Otherwise the beam of the radiation would be
scattered incorrectly and interfere the accuracy of the reading.
When filling the cell,
air bubbles should not be remained inside the cell as it would decrease the
absorbance value.
When refilling a glass
cell with a different solution, small amount of the new solution should be used
to rinse the cell before filing as it would give more accurate results.
Spectrophotometer also
uses a monochromatic light to pass though a solution and measure its
absorbance. The principle of spectrophotometer and colorimeter is same but a
colorimeter can only use one wavelength at a time and have a fixed number of wavelengths
that can be used. Also they have to be in visible range only.
A spectrophotometer
on the other hand can not function like a colorimeter but take a spectrum of a
solution across the entire wave spectrum especially in UV – IR. Therefore use
of spectrophotometer is beneficial than a colorimeter and useful to determine
concentration of unknown solutions.
Your blog is always useful. Thank You
ReplyDelete