In the Laboratory
edited by
Green Chemistry
Mary M. Kirchhoff
ACS Green Chemistry Institute
Washington, DC 20036
A Greener Approach for Measuring Colligative Properties
W
Sean M. McCarthy and Scott W. Gordon-Wylie*
Department of Chemistry, University of Vermont, Cook Burlington, VT 05405; *[email protected]
Developing instructional laboratory experiments that
demonstrate key chemical concepts while simultaneously incorporating green constraints is not trivial. The results of a
joint effort between faculty, graduate, and undergraduate students to develop a new greener laboratory designed to measure colligative properties are presented. Since colligative
properties depend on the number and not the identity of the
molecules being measured, there is no a priori reason why a
colligative properties experiment cannot be made rigorously
green.
The experiment presented here uses freezing point (fp)
depression data to determine the molar mass, M, of an unidentified compound (1, 2). Normally an aromatic solvent
such as p-dichlorobenzene is used because of a high cryoscopic
constant, convenient freezing point, ease of cleanup, and low
cost. Aromatic solvents readily dissolve aromatic compounds,
so the initial choice of solvent locks in the use of other aromatic compounds as unidentified substances, for example,
naphthalene, biphenyl, benzil, p-nitrotoluene, and benzophenone. Despite the educational and practical advantages derived from using aromatic substances, aromatic compounds
pose significant environmental and safety hazards, particularly in chlorinated or nitrated form (3). Safety hazards associated with aromatics include carcinogenicity (4), noxious
fumes, bioaccumulation of halogenated aromatics, and
ecotoxicity of nitroaromatics (5). A list of aromatic and
nonaromatic compounds, cryoscopic constants, and freezing
points, some of which are commonly used in fp depression
experiments, are provided in Table 1.
Green Aspects
As is apparent from Table 1, relatively few compounds
simultaneously possess convenient freezing points and high
cryoscopic constants at a reasonable cost. The list is even
shorter when green constraints such as toxicity and bioaccum-
Table 1. Cryoscopic Constants, kf, and Freezing Points (fp)
of Some Candidate Compounds for Freezing-Point Depression Experiments
Solvent
k f/
(°C kg/mol)
fp/°C
Cost/
(U.S.$/kg)a
Ref
Water
1.86
0.0
---
6, 7
Formic acid
2.38
8.3
40.00
6, 7
Acetic acid
3.63
16.7
34.33
6, 7
p-Xylene
4.3
13.3
59.00
6, 7
Stearic acid
4.5
69.0
16.90
6, 7
Benzene
5.07
5.5
28.62
6, 7
Palmitic acid
5.8
64
64.10
this workb
Phenol
6.84
40.9
31.30
6, 7
Nitrobenzene
6.87
5.8
19.00
6, 7
Naphthalene
7.45
80.3
27.80
6, 7
p-Dichlorobenzene
7.57
52.7
15.60
6, 7
Myristic acid
8.5
55.1
27.10
this workb
46
19.90
this workb
Lauric acid
13.5
Bicyclohexane
14.52
3.6
1535.00
Cyclohexane
20.8
6.5
31.16
6, 7
Camphor
37.8
178.8
29.20
6, 7
Cyclohexanol
42.2
6.5
17.71
6, 7
6
a
Values obtained from 2004–2005 Acros Organics catalog, values for liquids were converted to kg
using the density.
b
Approximate kf values based on average kf’s for addition of three different fatty acids over the range
of 0–10 wt. % .
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In the Laboratory
ulation are considered. Fatty acids, however, simultaneously
satisfy all of the above constraints. The physical properties
of fatty acids are right, the cost is low, and two major green
chemical objectives (8) can be reached. First, the waste stream
is nontoxic, which simplifies utilizing the waste stream as a
feedstock for making materials such as soap, biodiesel, or wax.
By utilizing the waste stream instead of throwing it away, we
are able to satisfy pedagogical laboratory objectives without
the concomitant generation of waste (see Scheme I and the
experimental section) (9). Second, industrial synthesis of fatty
acids is through the hydrogenation of biomass such as palm
oil, cottonseed oil, and other oils from agricultural feedstocks
(4). Therefore, using fatty acids promotes the use of renewable biomass resources as opposed to exhaustible petroleum
derived feedstocks. A comparison of the two laboratories from
a green perspective is shown in Scheme I.
Old Lab Procedure
Cl
+
petroleum
aromatic
unknown
Cl
freezing point
depression data
aromatic or
chlorinated
waste
Materials
New Greener Lab Procedure
All materials were purchased from Acros Organics and
used without further purification.
O
biomass
+
HO
Hazards
fatty acid
unknown
15
Stearic, lauric, myristic, and palmitic acids are nonhazardous, but prolonged skin contact may cause irritation. If
fatty acids are spilled on skin, students should wash affected
areas thoroughly with soap and water. 2-Propanol is flammable and should be kept away from an ignition source.
freezing point
depression data
useful feedstocks
Experimental Procedures
In one laboratory period students determine the fp of
three fatty acid samples: pure stearic acid (ca. 9 g) and stearic
acid with first 1 g, then a total of 2 g of an unidentified fatty
acid (lauric, palmitic, or myristic) added. The students observe the depression in fp relative to the fp of the pure stearic
acid and use the information to determine the molar mass,
M, of their unidentified sample.
Colligative Properties Measurements
A hot water bath (85–90 ⬚C) is used to melt an accurately weighed sample of about 9 g of stearic acid in a test
tube. The test tube containing the molten stearic acid at approximately 85 ⬚C is removed from the heating bath and
placed in an insulating jacket to cool (see the Supplemental
MaterialW). An alcohol-based thermometer is used to stir and
measure the temperature of the mixture over a period of 8–
10 minutes. Students record temperature data every 30 seconds. After completion of the first cooling trial, the measuring
procedure is repeated 2–3 more times.
Next, an accurately weighed sample of about 1 g of an
unidentified fatty acid is added to the stearic acid and the
measuring procedure repeated. An additional aliquot of about
1 g of the same unidentified fatty acid is added to the mixture and the measuring procedure again repeated. Collected
data are then analyzed (see the Calculations section below
for instructions and equation) to yield the M of the unidentified sample.
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SOAP
cleaning
products
biodiesel
wax
Scheme 1. Comparison of conventional and green laboratory procedures for measuring colligative properties.
Utilizing the Waste Stream
The fatty acid mixture is recovered by reheating to
85 ⬚C and pouring the molten mixture into a clean, foodgrade waste container for further use as a feedstock for making soap, biodiesel, or wax (see Supplemental MaterialW).
Residual fatty acids remaining in the test tube and on the
thermometer are dissolved with hot 2-propanol and transferred to a separate container. Cooling of the 2-propanol mixture or removal of the 2-propanol via rotary evaporation
allows an additional crop of fatty acids to be recovered. The
2-propanol can be recycled and reused if desired. Even if the
waste stream from the experiment is not utilized, recycled,
or reused, it is at least benign.
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90
25
85
20
80
15
∆Tf = 4.5 m
Tf
Temp / °C
In the Laboratory
freezing point
75
10
70
5
65
0
0
100
200
300
400
500
600
0
Time / s
1
2
3
4
5
6
Molality (m) of Myristic Acid / (mol/kg)
Figure 1. Example of data collected by students using a mixture of
stearic and myristic acids. The squares represent temperature
changes greater than 0.5 ⬚C per 30 seconds and the circles represent temperature changes by less than 0.5 ⬚C per 30 seconds. The
intersection of the two best-fit lines estimates the fp of the mixture.
Figure 2. Plot of temperature change versus molality of the myristic
acid for addition of myristic acid to stearic acid. The initial slope
of the line yields a kf value that matches the literature value over a
wide range of molalities.
Calculations
2. This is the same number of trials performed by students
in the actual laboratory experiment.
The accuracy of the M values determined by freezing
point depression of fatty acid mixtures is directly comparable
to M values determined using aromatic hydrocarbon solvents
with aromatic unidentified samples. Most students can determine M’s to within about 5–10% using these methods,
just accurate enough to definitively assign which fatty acid is
the unidentified sample. Typical student data demonstrating
colligative behavior for addition of varying quantities of
myristic acid to stearic acid are shown in Figure 2. Stearic
acid makes a good solvent choice for the experiment because
it exhibits a linear change in freezing point over a wide concentration range of added solute (see the Supplemental
MaterialW).
For each trial, a cooling curve is constructed as shown
in Figure 1. Data are plotted in two separate series. The first
series is where the temperature changes by more than 0.5 ⬚C
per 30 seconds and the second series is where the temperature changes by less than 0.5 ⬚C per 30 seconds. A best-fit
line is drawn through each series. The temperature at which
the lines intersect is approximately the fp of the mixture.
Freezing points for each trial are determined, averaged, and
used for calculations based on the equation,
∆Tf = kf m
where ∆Tf is the change in fp, kf is the cryoscopic constant
for stearic acid, 4.5 ⬚C kg兾mole, and m is the molality of the
unidentified sample. The M of the unidentified sample is then
determined for each trial (see Supplemental MaterialW).
Results and Discussion
Experimentally determined M’s for an average of four
independent trials using stearic acid as the solvent and palmitic, myristic, or lauric acid as the solute are shown in Table
Conclusion
Using green techniques in a teaching laboratory setting
allows pedagogical objectives to be attained without the concomitant generation of hazardous wastes. The approach is
both practical and cost effective. This green lab has been successfully implemented for about 250 students per semester.
Table 2. Student Obtained M values from ∆Tf Measurements
Solvent
Unidentified
Sample
Detd M/
(g/mol)
Std Dev
in Detd M
Actual M/
(g/mol)
Percent Error
Stearic Acid
Lauric Acid
203.13
1.47
200.32
1.4
Stearic Acid
Myristic Acid
235.77
6.43
228.37
3.2
Stearic Acid
Palmitic Acid
252.79
7.17
256.24
1.3
NOTE: Four trials were performed for each unidentified sample.
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In the Laboratory
Acknowledgments
tions and quizzes, are all available in this issue of JCE
Online.
We would like to thank members of the Spring 2002
and 2003 introductory chemistry classes Denise Pisani,
Rosemarie Candido, Meredith Kocur, Emily King, Colt
Lorson, Patty-Ann Czismesia, Rosalie Wilson, Lisa
Rinaldi, members of the GreenCats student organization,
the ChemCats student organization president Anya
Gushchin, and the chemistry laboratory supervisor John
Sharp for their help in developing and implementing this
laboratory. We would also like to thank the University of
Vermont and VT EPSCoR for their support of this work.
Finally, acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American
Chemical Society, Grant #36567-G3 for partial support
of this research.
W
Supplemental Material
Comprehensive instructor notes, a detailed discussion
of the freezing behavior of various fatty acid mixtures, examples of typical data obtained, and a detailed description
of the experimental protocols, including student calcula-
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