ANALYTICAL EDITIOD;
FEBRUARY 15, 1940
Literature Cited (1) ddams, ISD. ENG.CHEM.,rlnal. Ed., 6, 277 (1934). (2) Chalmers, Ihid., 4, 1 (1932).
Ibid.,4, 143 (1932). Clarke and Hermance, Ibid., 9, 597 (1937). (5) Kirk and Williams, Ibid.,4, 403 (1932). (6) hlohlman and Williams, Ihid., 3, 119 (1931).
(3) (4)
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Nernst, 2. Eleklrochem., 9, 622 (1903). (8) Pollard, ISD. EKQ.CHEM..Anal. Ed., 7, 77 (1935).
(7)
PRESENTED before t h e Division of AIicrochemistry a t the 94th Meeting of the American Chemical Society, Rochester, N. 'Y. Abstract of a portion of a thesis submitted t o the faculty of Purdue Un;versity by T. Z. Ball in partial fulfillment of t h e requirements for the 1RI.S. degree.
A Furnace for Micro-Carius Determination JULIUS A . KUCK
iND
JI.4URICE GRIFFEL, City College. College of the City of S e w - York, S e w York. 5. Y.
A
LTHOUGH the micro-Carius determination offers one outstanding advantage-great accuracy-it is often avoided h y professional analysts because it is time-consuming. Siederl (private communication) has found it possible to reduce to one hour the time required in heating the organic substance with nitric acid in t h e glass bomb in order to decompose it. However, the time required for the furnace t o reach t h e necessary 300" C. and to cool d o v n t o room teniperature still constitutes a drawback. Gas furnaces are unsuitable in crowded laboratories and give unequal heating. Commercially available electric furnaces are expensive. This article describes a simple, homemade furnace v h i c h heats u p to 250" C. in 15 minutes and cools down from 300' C. t o room temperature in the same time.
General Design The furnace (Figure 1) consists of three heating units: A , which is contained within tTyo concentric steel c\hnders, and B and C, the space between the two being filled Iyith Sil-0-Gel as a heat-insulating medium. The space between A and B is empty and is utilized for cooling the furnace by passing through it a stream of cold compressed air. The ends of B and C are sunk to a depth of 0 125 inch into properly channeled Tranqite plates, D
(0.25 inch thick), n-hich have three holes drilled through them as shovn in Figure 2, to accommodate the ends of A which lie flush with the outside surface. The dimensions of the steel cylinders are as follows: Cylinder
Length Inches
; C
Outer Diameter Inches 0.625
12 11.75 11.75
Gage 16 16 16
2.75 3.5
All cylinders are of Shelby steel tubing and A is soft-annealed. I n the center of one of the Transite plates, TThich serves as the
bottom of the furnace, is inserted a brass inlet for compressed air. There is another hole for the electric cord and tn-o smaller ones on either side for binding posts. The upper Transite plate has a single hole in the center for an air outlet. At each of the four corners of both Transite plates is a hole to accommodate a steel rod, E, which serves to bind the furnace together and to hold it down to its stand. The stand may be of any design but it should be about 10 inches high, rather heavy, and mounted on a flat Transite board. -4Transite cover, hinged a t the top, and three separate plunger rods, F , topped with brass pistons fitted to the inner walls of tubes A , complete the furnace. These rods are 12 inches long and serve not only to support the glass bomb tubes in the furnace but also to push them out. They may be equipped with wooden handles.
Windings and Connections The three heating units are separately M-ound and should be as identical as possible. For each a strip of asbestos paper 3 inches wide and 11.5 inches long is thoroughly wetted and firmly wapped around the steel tube. Repeated xinding of thin copper wire (later removed) serves to secure the soft, wet asbestos until it can be baked dovin hard by drying in the oven. This asbestos must be thoroughly dry before the electrical resistance is \vound. The heating wire used is No. 37 Nichrome ribbon, which has a resistance of 2 ohms per foot. This ribbon is fastened about 1 inch from the end of the asbestos-covered tube by binding it doxn with heavy copper wire vhich is wrapped over a part of one turn and twisted on the other side. 'The resistance of the coil should finally be 30 ohms, representing a stretched length of about 15 feet. .4 second asbestos wrapper (4.5 inches wide) is now put on, baked, and secured n i t h two or three turns of heavy copper wire. After the completed coils are set into the bottom plate, the Sichrome ends are welded together to form series connections. TABLEI.
DATAON FURNACE
Room temperature
300' C. 300' C. with auxiliary resistance
Total Time of Heating Min. 0 12 15 20 25 30 45
Temperature
c.
30 230 250 280 300 320 358
R Ohms 78 89.5
ELECTRIC MICRO-CARIUS FURNACE
1.41 1.23 1.02
107.5
Time of Cooling .Win.
Temperature O
c.
0 10 15
333 100 40
..
... ...
..
..
..
Auxiliary Resistance On 60 70 80 90 100
I .4mpe~es
...
... ... ... ... ... ...
INDUSTRIAL AND EKGINEERIKG CHEMISTRI-
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B and C are put on, with the Sil-0-Cel packing, and finally the upper Transite plate is fitted on. The plunger rods are inserted and the furnace is mounted in its stand.
Manville Co. T h e total cost of the furnace, excluding the labor, was about five dollars.
Use and Operation
Modifications on Original Construction
This furnace consumes about 150 watts at 110 volts, the current being 1.41 amperes at room temperature and 1.23 amperes at 300" C. To maintain a const,ant temperature of 300" an auxiliary resistance of about 18 ohms must be used. The exact amount can be determined by experiment. After the furnace has reached 300" (as determined by the time elapsed) the auxiliary resistance is switched in. The auxiliary resistance, consisting of 9 feet of the KO.37 Nichrome wire ribbon, is wound around three pieces of glass tubing which terminate in the ends of a Transite box forming an equilateral triangle (Figure 4). Large holes facilitate cooling by permitting air convection. Two toggle switches are mounted on the box; one turns on the current and the other throws in the auxiliary resistance after the desired temperature is attained. To cool the furnace the current is shut o f f and the furnace is connected to the compressed air line. A rapid st'ream of cold air quickly dissipates the heat from tubes A and cylinder B .
Cost of Materials and Source of Supply T h e steel tubing was obtained from the Peter A. Frasse Co., Inc., New York, K.Y., and the Sil-0-Cel from Johns-
Instead of B and C with Sil-0-Cel packing between them, a larger outer steel casing and an inner pipe of Sil-0-Cel cement (Johns-Maniille Co.) may be used. An automatic thermostat may be used instead of the manually operated auxiliary resistance. A strip of Invar brass costing a few cents can be fastened to the inner side of the Sil-0-Cel pipe SO as to switch in the auxiliary resistance (Figure 3). Below 300" the auxiliary resistance is shunted by the brass-Invar bar, G, which makes contact with the pin projecting from the metal rod, H. Above 300' this contact is broken and the auxiliary resistance is put in series. T h e authors are considering the construction of a furnace in which a reduction of the resistance of the main coil windings with a corresponding increase of the auxiliary resistance will result in a still shorter heating time. This auxiliary resistance should be internal and separately wound on the second asbestos wrapping of the A units.
