ENVE 201
Environmental Engineering
Chemistry 1
COLORIMETRY
Dr. Aslıhan Kerç
Colorimetry
• Colorimetric methods are applicable to dilute
solutions.
• For a colorimetric method to be quantitative,
it must form a coumpound with definite color
characteristics.
• Color amount must be directly proportional to
the concentration.
• Colored compound must obey Beer’s Law and
Lambert’s Law.
Lambert’s Law
• Relates the absorption of light to the depth or
thickness of the colored liquid.
• Each layer of equal thickness absorns an equal
fraction of the light which traverses it.
• Monochromatic light?
Lambert’s Law
I
Io
Cell length l
• Intensity decreases
exponentially with
length.
• This law is valid for
homogeneous solutions
Lambert’s Law
I
 k 'l
T   10
Io
I
Io
Cell
length l
Io
A  log  k ' l
I
l: length of absorbing layer
T: transmittance of solution
A: Absorbance or optical density
k’ : constant for particular solution
Beer’s Law
Intensity of a ray of monochromatic light
decreases expponentially as the concentration
of the absorbing medium increases.
I
 k ''C
T   10
Io
Io
A  log  k ' ' C
I
k’’ : constant for particular solution
C: concentration
Beer’s Law
If light is absorbed exponentially with
concentration, the colored material is said to
confirm Beer’s Law.
Prepare a series of samples in desired
concentration range, measure light
transmission. If it forms a straight line on a
semi-log paper  obeys Beer’s law
Lambert – Beer Law
Two absorbtion laws are combined:
I
 kCl
T   10
Io
Io
A  log  kCl
I
Spectrophotometer
Spectrophotometer
Colorimeter - Spectrophotometer