In the Laboratory
Carbon Dioxide Absorbers: An Engaging
Experiment for the General Chemistry Laboratory
Thomas M. Ticich
Department of Chemistry, Centenary College of Louisiana, Shreveport, Louisiana 71104, United States
[email protected]
The toxic effects of carbon dioxide (CO2) require its
continual removal from enclosed living spaces (1-3). The
reaction between lithium hydroxide (LiOH) and carbon dioxide
has been used for decades to remove the gas from spacecraft and
submarines (4, 5). The overall reaction is exothermic,
2LiOHðsÞ þ CO2 ðgÞ f Li2 CO3 ðsÞ þ H2 OðgÞ
and occurs in a two-step process:
2LiOHðsÞ þ 2H2 OðgÞ f 2LiOH 3 H2 OðsÞ
ð1Þ
ð2Þ
2LiOH 3 H2 OðsÞ þ CO2 ðgÞ f Li2 CO3 ðsÞ þ 3H2 OðgÞ ð3Þ
Given the importance of the reaction in life-support systems, its
details have been well studied (6-9). An important figure of
merit for a CO2 absorber is its absorption capacity, defined as the
ratio of the mass of CO2 absorbed to the mass of absorber. LiOH
provides a high absorption capacity due to its low molar mass
relative to alternative compounds, a characteristic that has made
it a particularly compelling choice aboard spacecraft where
minimizing payload is critical.
Despite the fact that general chemistry texts (10, 11) and
articles in this Journal (4, 5, 12) describe the reaction and its
applications, it has not been featured as a laboratory experiment
in published manuals or in the literature. Methods for measuring
the absorption capacity of LiOH typically involve flowing a gas
mixture through the absorber and monitoring the presence of
CO2 in the effluent gas by infrared absorption (6-9). This
article describes a simple procedure for determining the absorption capacity adapted from a method for determining the atomic
weight of Zn, a popular general chemistry experiment (13). The
experiment, which is suitable for general chemistry and can be
carried out in a 3-h laboratory period, utilizes the concepts of
stoichiometry and gas laws and serves several learning goals.
Students compare the theoretical predictions based on stoichiometric calculations with experimental data so they can understand why that latter must be used to determine the amount of
absorber required for a spaceflight. In the process, they use the
ideal gas equation to determine the mass of CO2 that reacts to
compute the absorption capacity. They also connect qualitative
observations to species in the chemical reaction to build their
knowledge of descriptive chemistry while engaging in an important
application.
Ethanolamine, along with other alkanolamines, has been used
to remove carbon dioxide from industrial waste gases (14-16)
and offers the possibility of regeneration after use by mild
heating. LiOH, on the other hand, cannot be readily regenerated
so that sufficient material for the entire space mission must be on
hand. Our general chemistry students have successfully applied
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the method to ethanolamine. By analyzing both absorbers,
students can compare their absorption capacities as well as
consider how reversibility can inform the choice of a material
for a particular application.
Experimental Procedure
The experimental apparatus (Figure 1) consists of a 50 mL
glass syringe with moveable piston (Popper #5058 micro-mate
syringe with metal luer slip tip) attached through a metal adapter
(Popper #6165), 12 cm of Tygon tubing (3/16 in. i.d. 5/16 in.
o.d.), and a short piece of 5 mm glass tubing in a #2 rubber cork
to a 10 mL vial that contains a small mass of the absorber
(approximately 0.05 g of anhydrous LiOH or 0.30 g of
ethanolamine). The values suggested for the mass of the absorber
will result in a substantial change in CO2 volume without
exhausting the contents of the syringe.
The absorber is weighed in the vial to within 1 mg, sealed
with a solid rubber stopper and set aside. Carbon dioxide gas is
generated from the sublimation of a pea-sized piece of dry ice
placed inside the syringe barrel. The apparatus is then flushed
with CO2 vapor and filled to the 50 mL mark as described in the
supporting information. As soon as the dry ice has completely
sublimed, the rubber stopper connected to the tubing assembly
should be loosely fitted into the sample vial. The syringe should
be depressed to a volume of about 40 mL (the exact value should
be recorded) to flush out the air in the vial, which should then be
sealed tightly to the apparatus. There are several signs of chemical
change that the students can observe. Most obvious is the
movement of the syringe barrel as the CO2 is absorbed. The
bottom of the vial will become quite warm, reaching 50 °C in less
than 30 s. In the case of the LiOH reaction, droplets of water
produced from the reaction will form inside the vial. Neither of
the absorbers tested undergo any visible changes, although the
Li2CO3 formed from the LiOH reaction can be detected afterward by its reaction with acid.
Once the syringe barrel stops moving, the students should
record the final volume and also note the pressure and temperature
Figure 1. Reaction apparatus for analysis of CO2 absorbers.
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r 2010 American Chemical Society and Division of Chemical Education, Inc.
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Vol. 88 No. 2 February 2011
10.1021/ed100826p Published on Web 11/23/2010
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Journal of Chemical Education
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In the Laboratory
Table 1. Student Data for CO2 Absorber Experiment
Absorber
Number of Trials Average Absorber Mass/g Average CO2Volume/mL Absorption Capacity/(g CO2 absorbed/g absorber)
LiOH
80
0.051
17.4
0.61 ( 0.14
Ethanolamine
38
0.330
20.5
0.115 ( 0.036
in the laboratory. The mass of CO2 absorbed is readily computed
from these data using the ideal gas equation and, together with
the mass of the absorber, the absorption capacity can be
determined. The Li2CO3(s) is easily disposed of by reaction with
2 M HCl, which also confirms its presence. The products of the
ethanolamine reaction are diluted with water and sequestered for
subsequent disposal.
Hazards
LiOH causes burns and is toxic if swallowed or inhaled.
Ethanolamine is a corrosive liquid that causes burns. It is harmful
by inhalation and irritating to the eyes, respiratory system, and
skin. To minimize handling of this substance by students, it is
recommended that the instructor or laboratory assistant dispense
the appropriate volume of the liquid for them (∼0.3 mL) using
an automatic pipet. The students will still need to accurately
weigh the liquid dispensed in a stoppered vial. Dry ice can cause
frostbite upon contact with skin and should be handled with
tongs or forceps. The syringe barrel in the apparatus should move
freely to prevent a build-up of pressure.
Results and Discussion
The raw data from five general chemistry sections are shown
in Table 1 as well as the average absorption capacity obtained and
the standard deviation of the aggregate set. The 1-2 s required to
flush the vial and seal the stopper introduces error into the
method. Direct observation shows that the reaction with LiOH
proceeds at a rate of about 10 mL CO2 absorbed/min, so that the
corresponding error in the CO2 volume is less than 0.5 mL. The
reaction with ethanolamine proceeds at approximately twice
the rate of the reaction with LiOH, which increases the error in
the measured CO2 volume to nearly 1.0 mL. Thus, one would
expect greater variability in the results for ethanolamine than for
LiOH. The observed standard deviations, however, do exceed the
percent variation expected solely due to flushing errors. The
author has obtained results with far less variability than those of
the general chemistry students. Thus, the greater range of values
the students obtain are likely due to procedural errors such as
failing to maintain a sealed system during the absorption process,
failing to check that the syringe barrel moves freely along its
entire path, or initiating the absorption experiment before all of
the dry ice has sublimed. Less experienced experimentalists are
also prone to misreading the volume scale on the syringe as well as
other recording errors.
The value for LiOH deviates from its theoretical maximum
of 0.92 that assumes a complete reaction. The actual value of the
absorption capacity of LiOH can depend on a variety of factors,
including particle size, humidity, temperature, and CO2 concentration. For flowing systems, the flow velocity and absorber
bed geometry can also play a role. Although the reaction is
diffusion controlled, the 170 mesh particle size of the LiOH is
too small to affect the absorption capacity (9). The best-fit curve
to published data of the absorption capacity at various temperatures,
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Vol. 88 No. 2 February 2011
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obtained using a flowing-gas method, gives a value of 0.67 at
35 °C (7). This result compares favorably with values obtained
by our students at room temperature. The absorption capacity
of ethanolamine depends on whether it is pure or in solution.
Data in refs 17 and 18 predict an absorption capacity for pure
ethanolamine of 0.12, which likewise shows excellent agreement
with the average of the student data.
At the beginning of the laboratory period, we show short
clips from the film Apollo 13 (19) that portray engineers solving
the problem of retrofitting LiOH canisters. Students use stoichiometry to predict the mass and volume of CO2 that react as
well as the absorption capacity for each absorber. Each pair of
students is required to complete two trials on LiOH and two
trials on ethanolamine. Class data are compiled and shared
during the post-laboratory discussion so each team can assess
its results. They compare theoretical and experimental values and
discuss which to use for planning a space mission. Students are
then asked to compute the mass of LiOH required for a 10 day
space shuttle mission with a crew of seven astronauts and for a
3 month trip to Mars for a crew of three. They will need to obtain
information on the amount of CO2 expired per day per person.
Finally, they compare the performance of both absorbers and
consider how reversibility enters into these comparisons.
The learning goals were assessed through a question on the
final laboratory exam as well as the laboratory reports. The
laboratory exam question requires students to apply their knowledge of why the experimental absorption capacity should be used
to determine the amount of absorber required for a space flight.
That question is given in the supporting information. The
average score on the question for three classes (a total of 51
students) was 7.6 (out of 10 points) with 71% of the students
earning 8 or more points on the question. Lab reports were used
to assess the other learning goals. These reports show that 100%
of the students are able to determine the mass of CO2 absorbed
from the ideal gas equation. Of course, all students must perform
this calculation for the post-laboratory discussion to commence,
so some receive assistance to accomplish the task. The reports
also show that 83% of the students who attempted to connect
their qualitative observations to species in the reaction did so
correctly. (Some students omitted this from their reports,
however.) Finally, 95% of the students correctly used the class
data to compare the absorption capacities of the absorbers. That
performance was likely bolstered by the post-laboratory discussion of the question that precedes report submission.
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Supporting Information Available
Student handout and notes for the instructor. This material is
available via the Internet at http://pubs.acs.org.
pubs.acs.org/jchemeduc
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