Determination of Carbon and Hydrogen An Electrically Heated, Thermostatically Controlled Constant-Temperature Device for the Pregl C-H Determination FRANK SCHNEIDER AND H. L. VAN MATER School of Chemistry, Rutgers University, New Brunswick, N. J.
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T IS well known that in the microanalytical determination of carbon and hydrogen, the lead peroxide portion of the combustion tube filling must be kept a t a constant temperature, which must be over 170" or under 200" C. It has been found (1) that the higher its temperature, the more rapidly the lead peroxide gives up its retained water, provided that the temperature never exceeds 200" C. A
the liquid in the system and minimizes leakage a t the junction between the double-walled vessel and condenser. The tendency to gum is lessened (gumming is retarded, although not eliminated), since the metallic surfaces, which apparently catalyze the polymerization, are eliminated. However, this device still requires careful regulation of the gas flame and necessitates the use of bits of porous tile, etc., to ensure quiet boiling. The authors have attempted to overcome these difficulties by electrical heating and thermostatic control of the electric current.
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Their a paratus consists of a double-walled tube of glass, as in the SchoeEel device. The condenser is replaced by a three-way stopcock and capillary tube, as shown in Figure 1. The doublewalled tube is filled with mercury through tube B, the mercury acting both as heating medium and as its own thermoregulator. Contact is made and broken in capillary tube A , as in the familiar thermoregulator. Tubes A and B are fitted with hard-rubber stoppers, the one in A carrying a screw attached to the platinum wire contact for fine adjustment of temperature. The circuit is completed by means of a platinum wire fused into the tube. The double-walled tube is wrapped with asbestos paper and then wound with about 6.5 meters (20 feet) of No. 24 Nichrome wire and the whole is encased in refractory cement. The tube, thus insulated, together with an external resistance, relay, and light bulb, is enclosed in an asbestos board case. The function'of the externa1:resistance is to slow down the rate of heating. The refractory cement retains the heat so well that, if the heating circuit is connected directly to the winding on the double-walled tube, the tem erature continues to rise for a time even after the current has gee, cut off. This prevents sharp control of the temperature. The lamp, which is placed in Fries with the magnet circuit of the relay, acts as a resistance and indicates that the apparatus is functioning. A Dunco Midget relay is used, since both the magnet and heating circuits operate on 110 volts and its compactness allows it to beeasily fitted into the asbestos case.
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The apparatus may be set for any temperature desired, as suggested by Saschek, and will easily maintain that temperature within *0.5" C. It is entirely automatic and needs only to be plugged into the 110-volt line. Since the boiling liquid has been eliminated, all the disadvantages encountered in the previous types of apparatus have been overcome.
FIGURE 1. DIAGRAM OF APPARATUS
The usual devices employed for this purpose consist of a double-walled metallic or glass tube, the space between the walls being filled with a liquid whose boiling point lies within the range of 170" to 200" C. T o this double-walled tube is attached a reflux air condenser. The temperature inside the tube is maintained constant by keeping the liquid boiling by means of a small gas flame. This type Of apparatus has disadvantages' There is first the Xm3?hdcal difficulty of making the apparatus leakproof, which has been overcome to some extent by the constant-temperature device of Verdino (2). Another difficulty arises from the fact that cymene and decalin, usually employed as boiling liquids, always polymerize and form gummy deposits that prevent proper heat transfer from the flame to the liquid and make frequent adjustments of the gas flame necessary. They also cause superheating. Moreover, it is impossible to determine the amount of liquid remaining in the all-metal apparatus and to observe if even boiling is being maintained. The all-glass device of Schoebel makes it possible to observe
Literature Cited Sasohek, William, private communication. (2) Verdino, Makrochemie, 4, 123 (1931). RECEIVED February 16, 1937. Presented before the Division of Physical and Inorganic Chemistry, Symposium on Recent Advances in Microchemi. cal Analysis, at the 89th itteeting of the Amerioan Chemical Society, New York, N. Y., April 22 t o 26, 1936.
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