Spectroscopic Determination of
Aspirin
 
Purpose(s)
 
 
 
 
become more familiar with the operation of a simple
spectrophotometer,
gain a deeper understanding of the interaction of light
with matter, learn and use the relationship between %T
and A,
experimentally determine λmax for the analyte,
determine the concentration of an unknown aspirin
solution using a Beer’s Law plot
ElectroMagnetic Radiation (EMR)
Spectroscopy
 
 
 
Light can be transmitted, absorbed, reflected by
samples (wavelength dependent).
The amount of light that is transmitted by a
sample can be compared to a “blank” providing
useful information.
The ratio of the intensity of the light transmitted
by the sample (Ix) vs. the blank (Io) is called the
transmittance (T).
T=
€
Ix
I0
Color
Wheel
Concepts
Complimentary
Colors –
Opposite one another
on color wheel
Solution Color
 
related to wavelength(s) are transmitted (or
reflected)
 
 
Example: cobalt (II), Co2+, appears pink in aqueous
solution
wavelength of minimum light transmittance is
called λmax
 
For cobalt (II), Co2+, occurs at about 510 nm (the
wavelength of green light)
Absorbance defined & related to . . .
 
 
Absorbance indicates how much of the light is
NOT transmitted by the sample
Absorbance (A) is related to Transmittance
 
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A = -log (T)
% T = 100 (T)
So . . . . A = -log (%T/100)
When %T = 100.0, A = 0.000
When %T = 10.0, A = 1.00
When %T = 1.0, A = 2.00
Absorbance defined & related to . . .
 
 
Absorbance indicates how much of the light is
NOT transmitted by the sample
Beer-Lambert Law
  Absorbance (A) is related to pathlength and
concentration of the absorbing species
  A = εlc
 
 
 
ε = molar absorptivity
l = pathlength
c = concentration (M)
Absorptivity and concentration can be in “non-molarity” units
The Chemistry
A complex is formed by reacting the aspirin with sodium
hydroxide to form the salicylate dianion
The addition of acidified iron (III) ion produces the violet
tetraaquosalicylatroiron (III) complex
Using Spectroscopy to find
Concentration
 
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 
 
 
Prepare standard solutions of the analyte (aspirin
in our lab)
Use one of these standards to determine λmax
Working at λmax, measure the Absorbance of all
the standards and an unknown
Prepare a graph of A vs. Concentration (a Beer’s
law plot) using the data from the standards
Determine the concentration of the unknown
Example Beer’s Law Plot
0.8
y = 0.0083x - 0.0021
R² = 0.99896
0.7
Absorbance
0.6
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
60
Aspirin Concentration (mg/L)
70
80
90
Spec 20 or Genesys 20
Directions will be provided in the laboratory.
LEARN to use this equipment; you will see it again.
Spec 20 or Genesys 20
Some Tips
 
Cuvettes are not test tubes – treat them with care
 
 
 
Fresh Iron Buffer is necessary for good chemistry
Begin search for λmax at 450 nm, end at 650 nm
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 
Do not scrub, rinse and wipe with KimWipes®
You can use either Spec 20 or Genesis 20 specs
The “color” of light at λmax should be reasonable based on
the color of your solutions – the solutions are “violet-ish”
and the complimentary color will be greenish-yellow
Keep track of dilutions, mass of tablet used, etc.
 
You may use just one tablet even if it has a mass of < 400
mg