In the Laboratory

Determination of the Heat of Combustion of Biodiesel W

Using Bomb Calorimetry
A Multidisciplinary Undergraduate Chemistry Experiment
Stephen M. Akers, Jeremy L. Conkle, Stephanie N. Thomas, and Keith B. Rider*
Department of Natural Sciences, Longwood University, Farmville, VA 23909; *riderkb@longwood.edu

Bomb calorimetry is a popular component of many ther- Experimental

modynamics laboratory courses and is included in most labo-
ratory manuals (1, 2). In its simplest incarnation, the bomb Synthesis of Biodiesel
calorimetry experiment consists of measuring the heat of com- The synthesis of biodiesel was done in a 500-mL three-
bustion of compounds, then comparing the results with tabu- neck round-bottomed flask fitted with a thermometer, reflux
lated values. This type of experiment provides a valuable condenser, and glass stopper on a magnetic stir plate (Figure
introduction to thermochemistry, but students typically take 1). A thermostatically controlled, recirculating hot water bath
thermodynamics courses in their third or fourth years, so they (Neslab RTE-220) provided a convenient way to maintain a
are more challenged by a thermochemistry experiment that 65 ⬚C reaction temperature (9, 10). Used peanut cooking oil
combines bomb calorimetry with another technique to solve was obtained from the university food services. Food particles
a richer, more complex problem. One physical chemistry were removed from a sample of oil by vacuum filtration. So-
laboratory manual (2) and two articles in this Journal (3, 4) dium hydroxide (NaOH, EM Science) was dried in an oven
successfully combine bomb calorimetry with structural analy- at 100 ⬚C for 24 hours, then ground using a mortar and pestle
sis, food science, or computational methods, respectively. This to speed its dissolution in methanol. Petroleum diesel was
article presents an experiment for the undergraduate labora- purchased from Watts Petroleum Corporation (11) and was
tory that allows advanced students to apply their organic syn- used as delivered.
thesis skills and their understanding of thermodynamics to Filtered peanut oil (260 g), methanol (75 g, 8:1
an increasingly important green technology—biodiesel. methanol:oil mole ratio1; ref 12 ) and NaOH (2.6 g, 1% wt
Biodiesel is a diesel fuel substitute derived from vegetable of oil) were combined in the round-bottomed flask (9, 10).
oil that has several environmental benefits. Biodiesel and pe- The reaction was allowed to proceed at 65 ⬚C for one hour
troleum diesel both burn to produce greenhouse gasses like with continuous stirring. At the end of this time, most of
carbon dioxide, but growing peanuts or soybeans to make the bottom glycerol layer was removed by pipet. The mix-
biodiesel removes carbon dioxide from the atmosphere, so it ture was then allowed to react for an additional 30 minutes.
has less impact on the climate than petroleum diesel. Cur- Upon termination of the reaction, the product mixture was
rent EPA regulations (5) require diesel to have a low sulfur
content, which necessitates costly sulfur removal from the oil.
Biodiesel, on the other hand, is naturally low in sulfur (6,
7). One of the most complex parts of a diesel engine is the
fuel pump and injector assembly, which is lubricated only
by the fuel itself. Biodiesel has better lubricity than petro-
leum diesel (6, 7), so it prevents wear in these critical engine
components. Finally, engines burning biodiesel have been
shown to produce exhaust that is less toxic and reportedly
smells like French fries (6, 7).
In the proposed laboratory experiment, students synthe-
size biodiesel by transesterification of waste vegetable oil us-
ing common glassware and reagents, and then characterize it
by measuring heat of combustion, cloud point, and density.
Measuring the heat of combustion and density together al-
lows the students to calculate the energy density of the fuel
on a per-gram and a per-milliliter basis for comparison with
petroleum diesel.
When diesel or biodiesel is cooled, wax crystals begin to
form (7, 8). The temperature at which this occurs is known
as the cloud point and is an indicator of the thermal stability
of a fuel. The measurement is usually done with a dedicated
instrument that is not commonly found in undergraduate
laboratories, but we describe a method of measuring the cloud Figure 1. This diagram represents the experimental apparatus used
point of biodiesel using a UV–visible spectrometer with a during the synthesis of biodiesel. A recirculating hot water bath is
temperature controlled sample cell. used to maintain a constant reaction temperature.

260 Journal of Chemical Education • Vol. 83 No. 2 February 2006 • www.JCE.DivCHED.org

 In the Laboratory

transferred to a separatory funnel where it was allowed to were perfectly transparent as long as no emulsion was present.
stand for one day. Afterward, the remaining glycerol layer The temperature of the sample in the spectrophotometer was
was removed from the biodiesel product. The biodiesel was decreased and transmittance measurements were taken over
transferred to separatory funnels and washed dropwise with a temperature range of 0–25 ⬚C. In the 25 to 5 ⬚C range
water until the wash water became neutral. This ensured that there was little change in the transmittance and the sample
all of the remaining catalyst was removed from the product. appeared clear. As the temperature dropped below 5 ⬚C, the
We found that an emulsion formed very easily during samples became cloudy as wax crystals formed. Figure 3 shows
the washing process. We produced emulsions with 100-mL the transmittance of biodiesel as a function of sample tem-
samples of biodiesel and tested two common methods for perature. Note that the wax crystals in these samples scat-
breaking emulsions: treatment with methanol and treatment tered short wavelength light more effectively than long
with saturated sodium chloride solution. The emulsion was wavelength light. The strong, wavelength dependent scatter-
combined with 5 mL of methanol or sodium chloride solu- ing in these samples indicated that the crystals were close to
tion in a separatory funnel, then gently inverted several times. the wavelength of visible light, a few hundred nanometers
The product was allowed to stand for five days, then the (13).
biodiesel fraction was collected and excess methanol was re-
moved by rotary evaporation. When sodium chloride was Hazards
used to break the emulsion, the samples were rewashed with
distilled water until the wash water tested negative for the Hot vegetable oil and methanol are both flammable.
chloride ion using 0.1 M silver nitrate. Sodium hydroxide is caustic and should be handled with
The density of biodiesel was easily measured by pipet- gloves. The bomb calorimeter requires handling high pres-
ting 10.00-mL aliquots into a beaker on an accurate balance. sure oxygen, which is a strong oxidizer.
Determination of Heat of Combustion of Biodiesel Results and Discussion
A Parr 1261 bomb calorimeter was used to measure the
heat of combustion of biodiesel, petroleum diesel, and two Calculation of Heats of Combustion
samples of biodiesel recovered from emulsions: one with The heats of combustion of the samples were calculated
methanol (biodiesel–MeOH) and one with sodium chloride using the procedure recommended in the Parr manual. The
solution (biodiesel–NaCl). After standardization with ben- biodiesel prepared by the conventional method and the
zoic acid, 0.5 g samples of each fuel were burned in the bomb. biodiesel recovered from the emulsions using two different
All measurements were repeated seven times. techniques produced 41.2 ± 0.2 kJ兾g with no measurable dif-
ference between the three preparations. Petroleum diesel
Determination of Cloud Point yielded 47.0 ± 0.2 kJ兾g. The fuel injectors in a diesel engine
An HP8453A UV–visible diode array spectrophotom- deliver a premeasured volume of fuel to each cylinder and
eter equipped with a temperature control apparatus was used the densities of petroleum diesel and biodiesel are the same,
to measure the cloud point of the biodiesel, biodiesel–MeOH, so engines running on biodiesel should produce less power.
and biodiesel–NaCl. Transmittance spectra (300–900 nm) for This has, in fact, been observed in laboratory test engines
the three biodiesel products are shown in Figure 2. All of the (5). The heats of combustion for the biodiesel products are
samples absorbed light in the 380 to 400 nm range, which reported in Table 1 and are compared to the heat of com-
accounts for its faint amber color. Carefully prepared samples bustion of petroleum diesel.

Figure 3. This plot demonstrates the decrease in transmittance as

the biodiesel-MeOH is cooled from 25 to 0 ⬚C. From 25 to 3 ⬚C
Figure 2. This graph compares the transmittance spectra of the three there is little change, but the transmittance drops sharply between
biodiesel products at room temperature. 3 and 0 ⬚C as wax crystals form in the fuel and scatter the light.

www.JCE.DivCHED.org • Vol. 83 No. 2 February 2006 • Journal of Chemical Education 261

 In the Laboratory

Calculation of Cloud Points Summary

Cloud points were determined by creating a plot of the Synthesizing and analyzing biodiesel can serve as a chal-
percent transmittance of a sample as a function of the sample lenging and enlightening introduction to thermochemistry,
temperature. Figure 4 shows the transmittance of the three fuel science, and green chemistry for physical chemistry stu-
biodiesel products at 540 nm as a function of temperature. dents. In addition to the physical chemistry skills, most stu-
The cloud point occurs in the temperature range where the dents find that this experiment requires fundamental organic
transmittance rapidly decreases toward zero. In methods pre- chemistry laboratory skills that are both valuable and easily
viously described, the value of the cloud point is defined as forgotten. It also encourages careful planning, data process-
the inflection point of a curve that represents the gathered ing, and critical thinking. Most importantly, though, this ex-
data (14). Each data set in Figure 4 was fitted with a fifth- periment teaches basic chemistry skills and knowledge in the
order polynomial and the inflection point of the polynomial context of an important and timely environmental topic.
was taken as the cloud point. This procedure can be expected
to give a rough approximation of the cloud point, which can W
Supplemental Material
only be properly measured by controlling the cooling rate of
the sample per the ASTM D 2500 method (8). Notice that Instructions for the student, a three-week timetable for
the cloud point for biodiesel was much higher than for pe- the experiment, and notes to the instructor are available in
troleum diesel, which suggests that biodiesel may cause cold this issue of JCE Online.
weather performance problems. Wax crystals can clog fuel
filters and will increase the fuel viscosity, requiring the use Note
of fuel line heaters. Cloud point data obtained for the biodie-
sel products are reported in Table 1 and compared to a re- 1. Vegetable oil does not have a uniform molecular weight
ported value (11) for petroleum-based diesel. because it is a natural product. Reference 12 has a table of fatty
acids found in many common vegetable oils that can be used to
calculate an average molecular weight.

Literature Cited
1. Crockford, H. D.; Baird, H. W.; Nowell, J. W.; Getzen, F. W.
Laboratory Manual of Physical Chemistry; Wiley: New York,
1975.
2. Garland, C. W.; Nibler, J. W.; Shoemaker, D. P. Experiments
in Physical Chemistry; McGraw-Hill: New York, 2003.
3. Stout, R. P.; Nettleton, F. E.; Price, L. M. J. Chem. Educ. 1985,
62, 438.
4. Salter, C.; Foresman, J. B. J. Chem. Educ. 1998, 75, 1341.
5. Pouton, M. L. Alternative Fuels for Road Vehicles; Computa-
tional Mechanics Publications: Boston, 1994.
6. National Biodiesel Board Home Page. http://www.biodiesel.org
Figure 4. This plot shows the transmittance at 540 nm as a func-
tion of temperature for the three biodiesel products. (accessed Nov 2005).
7. Pacific Biodiesel. http://www.biodiesel.com/ (accessed Nov
2005).
8. Chevron Corp. Home Page. http://www.chevron.com (accessed
Nov 2005).
Table 1. Calculated Heat of Combustions, Densities, 9. Encinar, J. M.; Gonzalez, J. F.; Sabio, E.; Ramiro, M. J. Ind.
and Cloud Points for Three Biodiesel Products Eng. Chem. Res. 1999, 38, 2927–2931.
and Petroleum Diesel Fuel 10. Encinar, J. M.; Gonzalez, J. F.; Rodriguez, J. J.; Tejedor, A.
Heat of Energy Fuels 2002, 16, 443–450.
Density/ Cloud Point/
Substance Combustion/ 11. Watts Petroleum Corporation, 1505 Rutherford St.,
(g mL−1) ˚C
(kJ g−1) Lynchburg, VA 24501.
Diesel Fuel 47.0 ± 0.2 0.85 ± 0.02 ᎑31.7a 12. Cyberlipid Center. http://www.cyberlipid.org (accessed Nov
Biodiesel 41.2 ± 0.2 0.87 ± 0.02 2.6 2005).
13. Nassau, K. The Physics and Chemistry of Color; John Wiley &
Biodiesel–NaCl 41.1 ± 0.2 0.87 ± 0.02 0.5
Sons: New York, 1983.
Biodiesel–MeOH 41.2 ± 0.2 0.87 ± 0.02 1.6 14. Yin, X.; Stover, H. D. H. Macromolecules 2003, 36, 9817–
a 9822.
This value provided by Watts Petroleum Corporation (11).

262 Journal of Chemical Education • Vol. 83 No. 2 February 2006 • www.JCE.DivCHED.org