ANALYTICAL EDITION
MARCH 15, 1936
gravimetric calcium determination? The authors tell students to hold the cover on and let some of the heated air escape if i t will. The rather facetious title of this paper is inspired b y these observations, in the hope that it will serve to arouse analysts to this unsuspected source of error. However, in view of the moisture always inadvertently admitted, no matter how rapidly the desiccator is opened and closed, it is useless to use a desiccant of the highest absolute drying power such as phosphorus pentoxide, since the object cooling in the desiccator mill ordinarily be removed for weighing before the space can be dried completely. Save for the danger of spilling, sulfuric acid should be satisfactory. If a neutral desiccant is desired, calcium chloride free from calcium hydroxide is useful. Where a n alkaline drying agent is permissible, porous barium oxide is excellent, and is particu-
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larly valuable in determinations affected by carbon dioxide, such as gravimetric calcium. Since barium oxide swells considerably a n absorbing moisture, the bottom of the desiccator should not be more than half full. Porous barium oxide is an industrial product, being produced in the first step in manufacturing barium peroxide, and is available more cheaply than anhydrous calcium chloride, which is difficult to prepare. The exhausted barium oxide can be used for the preparation of standard barium hydroxide solutions for alkalimetry.
Literature Cited (1) Booth and McIntyre, IKD. EKG,CHEX.,Anal. Ed , 2, 15 (1930).
RECEIVED April 1, 1935.
A Versatile Low-Temperature Thermostat G. B. HEISIG, University of Minnesota, Minneapolis, Minn.
T
HE commercial produc-
t i o n of s o l i d c a r b o n dioxide has made available a n economical a n d convenient method of obtaini n g temperatures d o w n t o -78" C. A constant low temperature is desirable for inany operations, such as the FIGURE1 study of reaction rates, trhe saturation of a gas with a definite quantity of a vapor, and the determination of vapor pressure, temperature, composition diagrams, etc. A number of investigators have described thermostats which may be operated a t low temperatures (1,2,&6). Some are rather elaborate pieces of apparatus, while others are relatively simple. The thermostat described here is easily made from materials found in any well-equipped laboratory, and h.as been used t o maintain a constant temperature in the range +25" to -75" C. The fluid used in the bath may be acetone, alcohol, or kerosene, depending on the temperature to be maintained. Acetone freezes at -94.6' and retains its mobility at the temperature of solid carbon dioxide. Alcohol becomes viscous below -40". Kerosene may be used for temperatures to about -40" C. The container for the bath is a 3-liter Pyrex beaker, A , which is set in a 4-liter beaker, B. The air space between the two beakers serves t o prevent the bath from cooling too rapidly. The formation of ice between the beakers may be prevented by sealing the crack with rubber cement used to fill cuts in the tread of automobile tires. The nested beakers are placed in a can, C, whose diameter is 5 cm. (2 inches) greater than that of the larger beaker, and centered by a ring made from rubber tubing of appropriate size. The can containing t'he beakers is placed in a wooden box, D, fitted with cork lining, E , built around the square tin can, F. If a constant temperature is wanted for several hours only, the space between the outer beaker and can C is filled with a slush of well-crushed dry ice and alcohol. If a constant temperature is wanted for a considerable time, the space.,G, is also filled with crushed dry ice. The bath is stirred with a turbine paddle attached to a Bakelite or metal rod, the other end of which is connected to the shaft of a small induction motor. An induction motor is desirable to avoid igniting the vapors from the bath. Since the bath is constantly being cooled, it is necessary to supp1.y heat to maintain a constant temperature. This is done by means of a heating coil made From 10 cm. of No. 24 resistance wire welded or clamped to leads made of No. 18 iron wire, which are connected to the secondary terminals of a 25-m.att toy transformer. The current in the primary circuit of the transformer is made and broken by a relay as the temperature varies. The proper amount of heat necessary to offset the cooling is obtained by adjusting the voltage.
The thermostatic element shown in Figure 2 is a bimetallic strip, A , made of brass and invar and consisting of four bows with the open end of the bows directed toward the long axis. The distance between the open ends of t,he bow will increase or decrease on warming, depending on Rrhether the invar is on the inside or outside of the born. A brass or preferably glass or Bakelite rod, B , rests on the lower end of the last bow. The upper end of the rod fits into a cup attached t o a screw, E, threaded onto a piece of spring brass, C, one end of which has a fixed position. Screw E is adjusted so as t o place a slight tension on the spring arm, C. The other end rises and falls with a change of temperature in the bath, making and breaking the grid current of a radio relay circuit ( 3 ) . A thermionic relay control of some sort is required, since no sparking should occur between the contacts, To start the thermostat, the bath is cooled to a temperature slightly below that wanted b y pieces of dry ice in a small wire sieve, such as that used for straining tea or coffee, or a cup made of wire screen, immersed so that the liquid comes in contact with the cooling agent. The gas bubbles have only a short distance to go before reaching the surface of the liquid and the usual overflow is thus avoided. The stirring motor is started and screws E and D are adjusted so that heat is being supplied to the bath. When the proper temperature is reached the screws are adjusted so that the heat is disconFIGURE2 tinued. T o increase the temperature of the thermostat, it is only necesrary to cause the heater to go on, and when the proper temperature is reached to adjust screw E , place a slight tension on rod B , and turn screw D so that the heater goes off. The temperature of the bath is constant * 0.01" C. About 5.5 kg. (12 pounds) of d r y ice were required to maintain a temperature of -38.1" C. for nearly 18 hours and 45 kg. (100 pounds) were used during a week.
Literature Cited Andrews, D. H., J . Franklin Institute, 206, 285-99 (1928). Cameron, Rev.Sci. Instruments, 4, 610-11 (1933). Hcisig and Gernes, IND.ENQ. CHEM.,Anal. E d . , 6, 155-66 (1934). (4) Lundstrom and Whittaker, Ibid., 4, 294-5 (1932). (5) Scott and Brickwedde, Bur. Standards J . Research, 6,401 (1931). (6) Ubbelohde, Trans. Faraday Soc., 26,236 (1930). R E C ~ I V EDecember D 23, 1935.
