When Is a Desiccator? HAROLD SIMMONS BOOTH
AND
LUCILLE MCINTYRE, Western Reserve University, Cleveland, Ohio
and thereafter was read at intervals of 0.5-mm. change on the manometer. This was the approximate decrease during the 1-minute intervals first recorded. Readings were taken until the pressure change was so slow that it would not affect the curve materially. The apparatus was allowed to stand until the next morning, when the final reading was taken. At least six runs were made with each drying agent, but only the representative ones were plotted. This method, while rough, yielded some interesting information.
HEN the authors completed the study establishing porous barium oxide ( I ) as a drying agent comparable with phosphorus pentoxide, it became the practice in this laboratory to use barium oxide (furnished through the kindness of M. J. Rentschler of the J. H. R. Products Co., Willoughby, Ohio) as a reagent in desiccators, on account of its cheapness and its efficacy. It occurred to them that it would be interesting to learn the rate of drying a space, such as a desiccator, with barium oxide and the other common drying agents.
Results Figure 2 shows representative curves for each drying agent used, the fall in vapor pressure being plotted against the time elapsed. While the results show little significant differentiation between the rates a t which the air in a desiccator is dried by different desiccants, the uselessness of a desiccator as commonly used is clearly revealed when the curves are exam-
Apparatus The method consisted of bringing air saturated with moisture into contact with the drying agent under the conditions which exist in a desiccator, and of observing the rate of decrease in the amount of moisture present in that air, as indicated by the decrease in pressure observed. It was decided that a manometer, on which the pressure could be read directly owing to a Toricellian vacuum on one side, could be used to measure the drop in vapor pressure sufficiently accurately for this purpose. In preparation for a run, moist air was drawn through the gaswashing bottles through stopcocks 4 and 5, while 3 (Figure 1) was closed, into the as holder, D, by applying suction to the outlet a t the bottom o f D. Water was then run into D from vessel C through stopcock 6 until the pressure inside D equaled that of the outside atmosphere. All stopcocks were closed and the moist air was allowed to stand overnight in D. One hundred grams of the fresh drying agent were put into a Petri dish which just fitted the bottom of the desiccator. The lid on the desiccator was greased with a stopcock grease of proved low vapor pressure, and the cross piece of the wooden frame holding the desiccator in place was fastened down. A rubber stopper of a size near that of the handle of the desiccator lid was fastened to the wooden cross piece so that when the latter was in place, the pressure upon the handle of the cover of the desiccator was relieved by the elasticity of the stopper. The desiccator was sealed to the apparatus near stopcock 1 by DeKhotinsky cement. Stopcock 2 was opened, and the manometer, B, and the desiccator were evacuated. Stopcock 2 was then closed and the apparatus allowed to stand overnight to be sure that no slow leaks would develop. The next day stopcocks 3 and 5 were opened and the water-saturated air was forced into the desiccator by allowing water to run into bottle D from vessel C through stopcock 6 until the pressure in the desiccator was about 760 mm. As soon as the stream of air was shut off, the vapor pressure was read on the manometer and the time recorded. The pressure was read at intervals of 1 minute until the fall in pressure was slow
TIME INMINUTES
FIGURE2. REPRESENTATIVE DRYING CURVES ined. The average analyst has a holy respect for the power of a desiccator to keep his ignited crucibles dry during the cooling process, yet is careless about keeping the desiccator closed and about opening for only the briefest time. The writers have seen a student open a desiccator, go to the oven or electric crucible furnace,.get his crucible, return, and put it in the desiccator to “coo1 in a nice dry atmosphere.” Let us assume that half of the air in the desiccator has been displaced, and that the air let in is half saturated, or roughly that a partial pressure of 5 mm. of water vapor is now in the desiccator. The crucible will attain practically room temperature in 10 minutes, in which time the drying agent will have lowered the partial pressure of the water vapor in the air in the desiccator only 2 mm., while the crucible is simultaneously adsorbing the moisture from the air in the desiccator. Generally the air in the analytical laboratory is nearly saturated with moisture, so that the probable moisture content in the desiccator would be much higher. Even some textbooks advise the student not to place the cover on the desiccator until the crucible has cooled somewhat or else the expansion of air will blow the cover off and break it! Why use a desiccator a t all if that is the criterion? How can students following such advice ever get constant weight on a
FIGURE 1. APPARATUS 148
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 it will. The rather facetious title of this paper is inspired by 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-
149
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 product 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 by 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. To 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 dry 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.
