1
Ken Yokoyama
CHE 331
January 30, 2003
EXPERIMENT 1: Simultaneous UV Analysis of a Three-Component Mixture
Objectives
This experiment introduces the simultaneous analysis of a mixture of components
that have similar properties. Mixtures of three isomers of cresol will be analyzed using an
ultraviolet spectrophotometer. From the UV spectra, each component and an unknown
mixture will be determined.
Theory
This experimental technique relies upon the absorption of ultraviolet light. Atoms
and/or molecules in the sample absorb incident energy, enter an excited state, and may
dissipate their absorbed energy via thermal, radiant, or chemical processes. Beer’s law is
a simple formulation relating the absorbance of incident energy to the particular chemical
species, the concentration of that species in solution, and the distance the incident energy
must travel through the solution as shown in figure 1.
Figure 1
where A is the absorbance of the solution, ε is the molar absorptivity (L/molcm, specific
to the chemical species and wavelength of light used), b is the cell path length (cm), and c
is the concentration of absorbing species (mol/L). Traditionally, measurements using
Beer’s law have been made at particular wavelengths of light where there exists a linear
relationship between concentration and absorbance as shown in figure 2. Usually the
pathlength, b, is known; thus, we can solve these simultaneous equations for the
concentrations of different species if the absorptivity coefficients of both species at both
wavelengths are known. The absorptivity coefficients of each cresol isomer will be
determined at different wavelength by Beer’s Law plots. The absorbance of the sample
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mixture will then be obtained for the same wavelengths; solution of simultaneous
equations will thus enable us to obtain estimates of the cresol concentrations.
Procedure
I.
First, determine the suitable wavelengths for the analysis by obtaining an
absorption spectrum for m-cresol, p-cresol, and o-cresol separately.
A. Prepare the following solutions:
1.
Pipet 5, 10, 15 and 20 mL of the 0.05 g/L stock solution of
p-cresol into a 25-mL volumetric flask and diluting to the
mark. Mix well.
2.
Repeat for m-cresol and o-cresol.
3.
Follow the SOP for operation of the UVspectrophotometer.
4.
Obtain the spectra for both solutions from 240 nm to 300
nm (or as narrow a range as possible).
5.
Generate a Beer's Law (A vs. C) for all species
II.
Next, prepare mixtures of the following solutions:
Mixtures
#1
p-15
#2
m-15
#3
o-15
III.
o-cresol
5 ml
5 ml
15 ml
m-cresol
5 ml
15 ml
5 ml
p-cresol
15 ml
5 ml
5 ml
Finally, obtain unknown(s) from the instructor. This solution contains a
mixture of the cresol isomers. Obtain its absorption spectrum, recording the
absorbance at the wavelengths of interest.
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Data
4
5
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6
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7
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Results
o-Cresol
Conc.(mg/L)
10
20
30
40
50
1.2
Conc.(M)
0.0000924
0.000185
0.000277
0.000370
0.000462
Absorbance
0.16
0.32
0.5
0.76
0.82
y = 2391.2x + 0.005
R2 = 0.9994
p-Cresol Standard Curve
Absorbance
1
0.8
0.6
0.4
0.2
0
0.0000
0.0001
0.0002 0.0003 0.0004
Concentration (M)
0.0005
9
m-Cresol
Conc.(mg/L)
10
20
30
40
50
1.2
Conc.(M)
0.0000924
0.000185
0.000277
0.000370
0.000462
Absorbance
0.2
0.39
0.59
0.78
0.97
y = 2088.3x + 0.007
m-Cresol Standard Curve
R2 = 0.9999
Absorbance
1
0.8
0.6
0.4
0.2
0
0.0000
0.0001
0.0002 0.0003 0.0004
Concentration (M)
0.0005
10
p-Cresol
Conc.(mg/L)
10
20
30
40
50
1.2
Conc.(M)
0.0000924
0.000185
0.000277
0.000370
0.000462
Absorbance
0.22
0.45
0.67
0.9
1.1
y = 2391.2x + 0.005
R2 = 0.9994
p-Cresol Standard Curve
Absorbance
1
0.8
0.6
0.4
0.2
0
0.0000
0.0001
0.0002 0.0003 0.0004
Concentration (M)
Isomer Molar Absorptivity Constant
o-Cresol
1904.3 M
m-Cresol
2088.3 M
p-Cresol
2391.2 M
0.0005