Isaias Raw and Anita Bromley Harvord School of Public Health 665 Huntington Avenue Boston, Mossochusetts 02115 Gerald Holleman,' Ernst R. Pariser, and John Vournakis2 Education Research Center Massachusetts Institute of Technology Cambridge, 02139
An atmospheric pressure calorimeter was developed at the Education Research Center of M.I.T. as part of a curriculum development project3 aimed a t students having poor background in mathematics, no previous experience in natural sciences, but with intellectual potential for success in the health science areas, and motivation to learn about their own diet. Students are required to perform a quantitative analysis on a duplicate of their meals of one day. The total duplicate is homogenized, and a portion of the uniform suspension is dried, providing the dry matter content and producine " a stable s a m ~ l ethat is further analvzed for ashes. carbon, hydrogen, nitrogen, traces of lead and mercury, oroteins. carbohvdrates. lipids. saturated and unsaturated ?atty acids, and iholesterol: Students perform these analyses by acceptable methods with low cost instruments, using a methods guide. To interpret the results, motivated by the open-ended exercise of the analvsis of their own meal. the students go to a selfinstruct;onal, heavily cxperimenral lexI.< The calurimeter. inexpensive tu huild and operare, was designed to measure the heat of combustion of food samples and pure organic chemicals, and to perform the carbon and hydrogen analysis on these same substances. Assembly of the Calorimeter The calorimeter consists of two major components (see Fig. I), the "tin can" assembly and the asbestos firing
platform. The "tin can" assembly is constructed as follows: A 2-in. diameter hole is cut into the bottom of a 2-lb coffee can and a %in. hole is cut in the side 'k in. from the base. A cup, 2 in. in diameter, 1% in. high, is constructed from &,-in. copper sheet. A 3/16-in. hole is cut in the cup near the base. A piece of %in. 0.d. copper tuhing, 30 in. long, is welded perpendicularly to the side of the cup over the hole. The tubing is coiled closely around the cup and the end of the tubing is put through the Wsin. hole in the side of the can. The copper cup is welded to the bottom of the can, over the pre-cut hole, and the tubing is welded to the can where it passes through the hole. The "tin can" assembly fits over an asbestos firing platform. A piece of plywood 10 in. X 15 in. with four ruhber feet is covered with a layer of asbestos hoard. A ruhber stop'Present address: 1600Ardmore, Hermosa Beach, California. ZPresent address: Biology Department, Syracuse University, Syracuse, New York. The work was supported by a grant from the Carnegie Corporation of New York. ' A preliminary version of the methods guide and part I of the text are available and on trial at Bennett College, Greensboro, N.C.; Brandeis University, Waltham, Mass.; Detroit Institute of Technology, Detroit, Michigan; Federal City College, Washington, D.C.; SUNY at Old Westbury, Long Island, N.Y.; Washington Technical Institute, Washington, D.C.
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Figure 2. Wiring diagram.
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Figure 1 . Diagram of the calorimeter assembly.
per, size 12, is mounted small side up on the center of the platform and is covered with a layer of rubber cement. The stopper is topped with a disk and over it a ring of asbestos board. Two brass screws are introduced through the rubber stopper from below through the board. An %in. piece of KG-in. 0.d. copper tubing is also introduced throueh the board., stomer. .. . and rine.-. extendine.. 'A.- in. above-the ring. The comer tuhine that oasses through the firing ~ l a t form can bk connected to an oxygen tank with rubbertubing. The two brass screws extending below the platform are wired to a 12-V transformer, a small light bulb, GE 1634, and a push button switch (see Fig. 2). This circuit is comnleted with a niece of Nichrome wire across the brass screws above the iring platform a t the time of the analysis. The transformer is connected to a cord and plug. The circuitry is enclosed in a small box (for safety) and mounted on the platform. The "tin can" assembly rests over the rubber stopper on the firing platform and is held firmly in place with three long screws and crossing aluminum bars. The calorimeter apparatus is complete with a simple stirrer which may be a wooden spoon. Determination of Heat of Combustion
A dry sample, 0.5-1.0 g, is placed in the center of the asbestos ring. A piece of 30-32-gauge Nichrome wire is connected to the two brass screws so that it touches the sample. The "tin can" assembly is placed over the stopper, secured with the aluminum bars, and filled with 500 ml of water a t room temperature. The copper inlet tube below the platform is connected to an oxygen tank. Oxygen is allowed to flow in at a slow, barely noticeable rate. The initial temperature of the water in the tin can is measured as accurately as possible, preferably to within 0.lsC, and is recorded (TI). The transformer is plugged into a
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wall electrical outlet and the push button switch is closed until the light bulb goes out, thus igniting the sample. The water in the can is stirred constantly and the temperature measured until it no longer increases. This maximum temperature is recorded (Td. A known volume, about 80 ml, of boiling distilled water is added to the calorimeter, and the mixture stirred until it no longer increases. This temperature is recorded fT3). The second rise in temperature fT3 - T2) is due to the addition of the boiling water. Since the decrease of 1°C by 1 1 of water corresponds to approximately 1 cal (the exact value is defined for a change from 15.5"C-14.5"C), the heat released by the burning sample can be calculated by calculating (a) the heat added by the boiling water and (b) the temperature change of the water in the calorimeter per calorie added. For oil samples it was found that burning is facilitated by the presence of a small asbestos wick, and spattering is reduced by placing inside the asbestos ring a small watch glass, made by cutting the bottom from a large Pyrex tube. Typical student5 results are: sucrose, 3.72; tripalmitin 7.90; and gelatin 4.07 kcal/g. Usually food calorimeters are standardized with a known substance, such as sucrose which has a standard value of 3.976 kcal/g. This would raise the values of fat and protein by 7%. The usual accepted values are 4.15 kcal/g for carbohydrates, 9.4 kcal/g for lipids, and 5.65 kcal/g for proteins. Determination of Carbon and Hydrogen Content
The copper tubing outlet from the burning chamber is connected to two cartridges with rubber tubing. The cartridges, either glass or plastic, are charged with dehydrite and oscarite, respectively. Ready made cartridges are available (Mallinckrodt; brand names, Aquasorb and Mallcosorb). The cartridges are weighed prior to burning the sample. The sample is burned as before, except 500 ml of boiling water is added in place of the original 500 ml to prevent vapor given off during burning from condensing in the tubing. The first cartridge collects water and the second absorbs carbon dioxide. Weighing the cartridges after burning allows a rapid determination of the carbon and hydrogen content of the sample. Assuming the presence of only oxygen, hydrogen, and carbon, oxygen content is also determined by difference. In the case of food samples separate nitrogen and ash determinations provide other needed data. Using this method students have arrived a t the following formula for sucrose: C12H20-24011-13.
Judy Cook,Keene State College, New Hampshire
