A CARBON DIOXIDE "GENERATOR" CHARLES R. CONARD Harvard College, Cambridge, Massachusetts
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OR the past two years thislaboratoryhasused solid carbon dioxideas a source of gas for the preparation of sodium bicarbonate. The accompanying diagram, Figure 1, shows the arrangement of apparatus that has proved successful in the hands of firstyear students. The bottle containing the solid carbon dioxide is fitted with a tight rubber stopper carrying a Bunsen valve and a delivery tube. It is usually necessary to wire the stopper in place. The Bunsen valve is closed a t the top with a spring pinch-clamp. The rate of sublimation (generation) may be con- FIGURE1.-THE CARBON Droxros GENERATOR trolled by varying the amount of insulation as well B-Bunsen valve D-Dry ice as the type of container. A 250-cc. bottle filled with dry ice and wrapped with a loosely wound towel, held.in place with a beaker as shown in the diagram, will furnish a steady supply of gas for ten hours at the rate of about one-half mole per hour. A metal container (tin can) standing in air a t room temperature will supply the gas a t the rate of about two moles per hour. With the aid of these figures one may easily design a container that will furnish gas a t the desired rate. The Bunsen valve is the important part of the apparatus and t h e safety of the experimenter depends upon its successful operation. Students are directed to make a slit '/8-L/4 inch in length along the wall of a rubber tube.* A file point should be inserted through the opening, from the inside, to make ceffain that the slit actually goes through the wall of the tube. The top of the tube is closed with a spring pinchclamp rather than a glass plug, since it was found that students may push the glass past the slit and thereby prevent the escape of the gas. Because of the great danger involved when solid carbon dioxide is placed in a closed container each valve should be tested by means of a manometer. In this laboratory a mercury manometer, *The tubing used was the laboratory quality red tubing, '/,-inch diameter and '/rsinch wall. Valves made of this tubing maintained pressures between ten and twenty-five cm. of merc w .
Figure 2, was set up with sufficient height to balance the pressure in the compressed air line used to test the release pressure of the valve. In order to prevent too sudden pressure changes, a capillary tube? was inserted between the air line and the manometer. This arrangement bas been entirely satisfactory for student use. In short, this apparatus will furnish a t a reasonable cost a steady source of carbon dioxide gas a t a constant pressure for several hours without attention or adjustment.
FIGURE &-THE MANOMET'& B-Bunsen valve C-Thermometer tubing
Note:-The increased industrial use of "dry ice" has brought the cost of carbon dioxide gas from this source below the cost of material for the marble-hydrochloric acid method. Our records show that the cost of marble chips and hydrochloric acid used in the generation of carbon dioxide for the preparation of sodium bicarbonate was over ten cents per student, whereas the dry ice cost about four cents per student. The cost of dry ice depends somewhat on the locality. How-
t, One inch of a broken thermometer tube allows a flow of air whlch corresponds to the flow of carbon dioxide from the generator.
ever, the standard fifty-pounB"cakes may be obtained almost anywhere in the United States a t s maximum cost of five cents per pound. The consumer should calculate on a fifteen per cent. shinoine loss Der twentv-four hours. Shinment is usuallv made by truck or railway exprcs,. lrlforrnation regarding the I ~ a t i o n of shipuing pointsmay he obtained by writing to thc ofkerof the Liquid Carhonic Corporation, 52 Vandcrhilt Avenue. New Vnrk
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City. Cutting the fifty-pound blocks adds considerably t o the expense and the local ice cream stores may be of service in filling small orders. The d m ice mav be stored cheaDlv in a nest of . , and convenientlv ~~, three or four cun
