Microchemistry An Electric Furnace for Automatic Combustion in Microelementary Analysis L. T. HALLETT, Eastman Kodak Co., Rochester, N. Y.
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small furnace tilted hack awayfrom thetube. Combustion tubes, in all cases, are of clear fused quartz having an outside diameter of 9 mm. and an inside diameter of 7 mm. The furnace shells are i o made that the ends are in the form of a 10.1-cm. square.
the past few years work has been done in these laboratories on adapting micromethods to the rapid and accurate routine elementary analysis of organic compounds. The electrical heating of combustion tubes (4) has proved satisfactory and convenient. It lias been found that a large portion of the time required to complete a n analysis is taken u p with the constant attention required in moving the furnace which burns the sample. An even burning by hand manipulation is often difficult and tiresome. B y making this part of the analysis automatic, more time for calculation, meighing, or titrating is given and, therefore, a greater number of samples can be analyzed per day with less fatigue.
Description of Apparatus
FURNACE FIGURE2. ENDPLATEOF COMBUSTION
FURXACES. The electric furnace is simple in design and Figure 2 shows a bottom end plate. There are two such plates (Figure 1, B and C) for each end of the furnace unit. The three sides of each half-section are madeof 1.6-mm. thick aluminum and are attached by means of screws to t\vo pieces of aluminum (Figure 2, C and D). The top and bottom halves are hinged at the back and each section contains a refractory unit. The furnace shells are insulated with shredded asbestos packing. The furnaces are so arranged that they are attached to one rod at their base at a point (Figure 1, M , and Figure 2, G) such that, on pushing away from the combustion tube, they de.Gcribean arc which allows both top and bottom sections to clear the combustion tube. The dimensions can be obtained from Figures 1 and 2. Rods K , N , and 0 are made of a material which is a poor heat conductor, such as Synthane, and allow the hot furnaces to be opened and closed without burning the fingers. Figure 2, H , shows the hole into which the Synthane rod fits. REFACTORY UNIT. The refractory unit is the same as that previously described ( 2 ) . It consists of an alundum tuhe with a 2.5-em. bore and a 3.2-mm. wall cut to form two semicylindrical pieces (Figure 3, B ) . At the end of the shell and held in place by the two pieces of aluminum (C and D,Figure 2) is placed transite or pressed asbestos, B , 6.1 mm. thick, machined to fit and support the semicylinder of refractory. Another piece (Figure 2, A and Figure 1, P) of asbestos 3.2 mm. thick is inserted in the end plate. Figures 1 and 2 show the dimensions. The 6.4-mm. asbestos is fastened by means of screws to the end plate and the 3.2-mm. is, in turn, fastened to the 6.4-mm. asbestos. The terminals (Figure 3, H a n d I) of the electric heating elements are fastened into the asbestos and are placed on the sides near the large end plates (Figure 1, L a n d R ) . On either side of the alundum semicircle (Figure 3, B ) are placed alundum pieces, A and E, 4.8 mm. thick held in place by screws. Figurc 2, E and F , shows the support for these pieces. The heating element consists of No. 24 Nichrome wire wound in the form of a pencil coil with ti 3.2-mm. inside diameter and supported on a 3.2-mm. diameter alundum rod which fits inside the coil, thus giving it rigidity. There are two of these elements running lengthwise of each half-section of the furnace and connected to binding posts at the end (Figure 3, Cand D). The coils thus radiate their heat onto the combustion tube.
the construction inirolves a minimum of machining. The furnaces are of the split type and can be tilted back away from the combustion tube. T h e materials used for construction are aluminum alloy and stainless steel. Figure 1 gives a general view of the apparatus. It shows the large furnace, A , in a position corresponding to that over a combustion tube (not shown in the figure) and the
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FIGURE 1. GENEUL VIEW OF APPARATUS 101
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FIGURE 3. REFRACTORY UNIT The heating element rods with coil are held in place by two small 6.4-mm. thick semicircles of refractory notched to accommodate the heating elements and hold them in place against the alundum refractory tube (Figure 3, F and G ) . They are also notched to accommodate the combustion tube. The top half-section has heen removed in Figure 3. The furnace temperature is Calibrated with a thermocouple and a suitable resistance is placed in the circuit to maintain the desired temDerature. The average temperature is 700" to 750" C. M O U ~ ~ T IOF N QFURNACES. The baseplate on which the apparatus is mounted is 50.8 cm. long by 17.8 cm. wide by 4.8 mm. thick. On this plate are placed two end plates of 6.4mm. aluminum, 38.1 cm. apart, 24.1 cm. h i g h , a n d 1 7 . 8 cm. wide. At a point 5.7 em. from the base, the plate is cut at an angle of 40'. One of these end plates is shown in F i g u r e 4. The end plates are fastened by s c r e w s t o t h e base plate a n d a r e h e l d rigid by a 9.5-mm. rod at the top and two at the bottom, o n e 9 . 5 mm. and the other 6.4 FIGURE 4. END PLATETO FURmm indiameter. The NACE SUPPORT Dosition of thece rods ;an be seen in Figures 1 and 4. The 6.4-mm. rod acts as a stop for the furnaces when in the position away from the combustion tube. The large end plates are also drilled to support two 1.3-cm. diameter stainless steel rods, one of which supports the furnaces and on which they are free to move (Figure 1, M ) . The other (Figure 1, F ) 1.3-cm. shaft has at one end a case-hardened, threaded sleeve 1.9cm. in diameter, 7.6cm. longwith 18 threads to the inch. This shaft is connected to a Bodine motor (1/80 h. p., Universal type, gear reduction 1120-1) by means of a Universal joint. The motor is bolted t o the base plate on the remaining 12.7-cm. space outside the end pIates. To the bottom small burning furnace is bolted a segment of screw (Figure 1, G ) which meshes with the screw on the revolving shaft when the furnace is brought over the combustion tube. As the shaft revolves, the furnace moves forward. All bearings subject to wear have steel bushings. The combustion tube is supported by a notched plate attached to each large end pIate (Figure 1, E ) . The top bar which is shown in Figure 1, D, provides a safety device so that the furnaces cannot be pushed back until they are opened. Should the small furnace tend to "ride up" on the screw, a piece of brass, weighing about 200 grams, placed inside the upper half of the furnace shell, will prevent this. ELECTRICAL UNITS. The electrical connections for the heating units are situated at one end of the furnace (Figure 5). The furnace which burns the sample has an insulated brush contact on the bottom (Figure 1, I). When the furnace is opened and brought over the combustion tube, it makes contact with a brass segment which starts the motor; the proper speed is maintained by means of a suitable resistance in the circuit. At the same time the furnace meshes with the revolving shaft and so moves for-
ward. The initial speed for average burning is 2.5 cm. in 15 minutes. $fter the furnace has passed over the boat and the sample has carbonized, the brush contact reaches another brass segment xhich increases the speed to 2.5 cm. in 3 minutes. When the small furnace has reached the end of a predetermined point, which is the large furnace, it is so adjusted that it runs off the screw and makes contact with a segment which allows the motor to run at slow speeds. This last segment is usually of such a length that the final 1.3 cm. that the furnace moves is at this slow speed. The segments are mounted in a strip of Synthane which, in turn, is supported at a suitable height by means of small holts screwed into the base (Figure 1, H ) . Some compounds sublime or distill as the furnace moves forward and, if the speed of the furnace at the end of the burning is too fast, the compound will burn explosively or incompletely. By having a final slow speed, they dissipate slowly and are completely burned. The slow speed is attached to a variable 350-ohm Ward Leonard resistance (Bulletin 1103), so that the speed of burning may be varied as required. Figure 6 gives the details of the motor wiring. By varying the posit'ion, length, and number of the segments, the Peculiarities of burning for carbon, hydrogen, nitrogen, sulfur, and halogen may be met. The whole unit is mounted on an oak board, 121.9 by 35.5 by 2.5 cm. The board has a 5.1-cm. strip on the bottom on three sides, the back being left open. The electric wires may then be run underneath the board and are readily available for changes or repairs.
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Applications DETERMISATION OF HALOGENS AND SULFUR. Figure 1 is the drawing of the apparatus used for the determinations of halogens and sulfur. T h e furnace heating the platinum contacts is 17.8 cm. long. T h e burning furnace is 7.6 cm. long. T h e space allowed between the two combustion furnaces at the start of a run is 6.3 to 7.6 cm. Three contact segments are generally used: the first for slow or initial burning, the second, fast after the sample has carbonized, and the third, slow for the final 1.3 cm. of travel. The regular Pregl combustion tube with the glass spiral at the end can be used, b u t a modified tube, which will be described in a later paper, has the advantages of simpler operation and of a n essential saving of working time (3). The time for combustion of the sample is about 30 minutes. Twelve iodine determinations may be made per day, including calculating. The chlorine and bromine determinations require slightly more time for titrating and, in the case of sulfur, for precipitating and weighing the barium sulfate, but, of course, the burning time is the same in all cases. Iodine is determined by the Goldberg method (I). Bromine and chlorine are determined by the t o l h a r d m'ethod and sulfur by the absorption of the oxides in dilute hydrogen peroxide and the precipitation of barium sulfate.
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FURNACL WIRING
FIQURE 5
DUMAS KITROGEN.The changes in apparatus neccssary when determining nitrogen according to t h e Dumas method are: an increase of 3.8 cm. in the over-all length of the frame so t h a t the large furnace is 21.6 em. in length and the inclusion of four segments-(I) a fast speed to bring the furnace up to the burning position, (2) a slow burning speed, (3) a fast
ANALYTICAL EDITION
FEBRUARY 15. 1938
speed, and finally (4) the last 6.4 111111. a slow or idling speed. Segment 3 is so arranged t h a t by means of a switch it may be thrown into the slow burning speed, 2, if the sample has not burned completely while passing over 2. In the case of liquids where the length of the capillary and its position cannot conveniently be the same each time, the furnace must be watched occasionally to maintain proper burning conditions. The slow speed as before may be varied by Y rheostat. Solids rarely require attention. To complete a run requires 30 to 35 minutes. Nine determinations can be done in a n 8-hour day, including calculating and weighing out the samples for the next day. CARBON AND HYDROGEN. I n the deterniination of carbon and hydrogen, some laboratorieq now use electric furnaces for heating that section of the tube containing the lead dioxide filling and the copper oxide-lead chromate mixture. A unit for the automatic combustion of the sample in such a case can readily be built. It consists of only the sample-burning furnace and the mechanism for moving it along the combustion tube. Figure 7 shows such a unit. T h e frame is the same except that it is shorter. T h e furnace is round instead of square, and the safety bar at the back is eliminated. The screw is 10.1 instead of 7.6 cm.
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FIQURE 7. UNITFOR ACTOMATIC COMBUSTIOS mined point after the sample has been burned. The application of this type of furnace to such determinations as carbon, hydrogen, nitrogen, halogen, and sulfur is described. The unit is designed primarily for laboratories having a large number of routine determinations. As the combustion of the sample is done automatically, the operator has more time to carry out calculations, titrations, and weighings. About 25 per cent more work can be done per day than is possible with burning the sample by hand, and the operator is less fatigued.
Acknowledgment -3
FIGURE 6. MOTORWIRING
It is provided with 3 speeds: slow initial speed, fast, and then slow. A 350-ohm variable rheostat allows the slow speed to be changed if burning conditions are not suitable. With this apparatus, nine carbon and hydrogen determinations may be done in a n 8-hour day, including calculating and weighing out samples for the next day.
Conclusion T h e automatic combustion unit has allowed a n average increase of 25 per cent in the number of analyses completed per day and the strain occasioned by the constant attention during the burning of the sample has been eliminated.
The author wishes to thank Mr. Ord, in charge of t h e Research Laboratory instrument shop, for working out the d e tails of the design. Without his continued help, the simplicity of the design could not have been achieved.
Literature Cited Goldberg, J. L., Xikrochemie, 14,161 (1933-1934). Hallett, L. T., “Electric Furnaces for Micro-Organic Technical Analysis,” read at the 1936 Meeting of the American Chemical Society, Pittsburgh, Pa. (3) Hallett, L. T.,IND.ENO.CHEM.Anal. Ed., 10, 111 (1938). (4)Kemmerer, G., and Hallett. L. T.. ISD. ENO. CHEM.,19, 173 (1927). RECEIVEDNovember 22, 1937. Presented before the Microchemical Section a t the 94th Meeting of the American Chemical Society, Rochester, N. Y., September 6 to 10, 1937. Communication No. 652 from the Kodak Research Laboratories.
Summary An apparatus using electric furnaces of new design for the automatic combustion of microsamples is described. The furnaces fabricated from aluminum alloy are of the split type and may be conveniently opened and pushed aside so that the combustion tube may be cooled if necessary. A small electric motor with gear reduction drives a screw which in turn moves the sample-burning furnace along t h e combustion tube. B y means of a brush contact passing over a series of insulated metal se-ments, the speed of the furnace is automatically varied to give slow initial burning and accelerated burning after the sample has carbonized. T h e furnace automatically stops when i t has reached a predeter-
Correction In the article entitled “A New Method in Pycnometric Analysis” [IND. ENG. CHEM.,Anal. Ed., 9, 592 (1937)] two term definitions have been confused. Under the heading “Theory” the definitions should read:
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weight of pycnometer plus preripitate plus liquid necessary t o fill PYCnometer V = capacity of pycnometer d density of precipitate in grams per cc.
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W. WALKERRUSSELL
