A New Vacuum-Furnace Design KENNETHK. KELLEY,Pacific Experiment Station, U. S. Bureau of Mines, Berkeley, Calif. in this recess. A narrow XPERIMENTAbearing s u r f a c e 0.95 cm. TION u n d e r conwide e n a b l e s satisfactory ditions of high temcompression of the gasket' perature and high vacuum T to be s e c u r e d . A simple involves numerous difficularrangement of rubber and ties. M a n y of the older metal bands (not shown in t y p e s of f u r n a c e s are a the diagram) is incorporated source of continual trouble for filling annular space G b e c a u s e of m e c h a n i c a l with m e r c u r y to a s s u r e imperfections and unsuitthe g a s - t i g h t n e s s of this able operating characterisjoint for work at very high tics, and necessitate a convacuums. stant battle by the experiThe cooling water enters menter a g a i n s t v a c u u m the jacket through tube H, leaks and electrical derangepasses out tubes J, enters ments. Consequently the the head through tube K , efficiency of such e x p e r i and leaves by tube L. A mentation suffers. metal spiral (shown in the The Pacific Experiment plan of the head) assures Station, in connection with the absence of dead spaces, its program of specific heat measurements a t high tema precaution that experience p e r a t u r e s on metals and has proved to be essential for satisfactory cooling of metallurgically important FIGURE1. DIAGRAM OF FURNACE AND IMPORTANT PARTS the g a s k e t s and lead-out compounds, recently comwire seals. pleted the construction and HEATER. At present the furnace contains a copper block, testing of a new vacuum furnace for calorimetric measurements a t high temperatures. The design adopted has several N , of some 22 kg. mass, which is heated by a coil of nichrome new features which greatly improve the ease and certainty of ribbon of 8 ohms resistance, wound helically, and insulated by alundum cement of the grade used for platinum-wound experimentation. furnaces. This block is supported by framework 0 of 2.54DESCRIPTION OF FURNACE cm. (1-inch) square cold-rolled steel suspended from the The furnace is an electrically heated, water-cooled,type, 76 head, and is held in place and thermally insulated by porcecm. long and 40.5 cm. in diameter. It has been built of lain tubes P . For other purposes any suitable heating coil unit parts in such a manner that it may be repaired or altered may be similarly installed. The heater lead wires pass through the compression-joint easily and economically. Figure 1 shows a cross section, a plan of the jacket-head, and the design of the construction tubes Q, which are of a special type and will be described later. for bringing out electrical lead wires. The heater unit is surrounded by two thin 0.04-cm. (27VACUUMJACKET. The vacuum jacket consists of two concentric cylindrical shells of heavy steel pipe, the outer gage) monel metal radiation shields, R, which also are supshell, A , being 0.64 cm. thick, and the inner shell, B, 1.59 ported by frame 0. JOINTS AND SEALSFOR LEADWIRES AND VACUUM CONcm. thick, surmounted by a heavy, removable head. The cylindrical shells have welded boiler-plate bottoms and are NECTION. The heater lead tubes &, evacuation tube S, and held together a t the top by a circularly split ring, C, which fitting T,through which thermocouples and other lead wires is screwed on the outside of the inner shell and on the inside pass, are threaded and screwed into holes in the bottom plate of the outer shell. The two parts of this ring telescope with of the head and are soldered to it on the upper side. This a driving fit, which enables the jacket to be taken apart by is necessary since all the joints must be vacuum-tight a t this applying a few pounds of air pressure between the walls point. At the junctions with the upper plate of the head, and to be assembled by applying a vacuum similarly. The which need be water-tight only, two nuts screwed to each pressure or vacuum required is less than one atmosphere. tube and a gasket below the plate, such as shown at M , This arrangement has been found to be of great value in were found to make satisfactory, removable, and waterthat it permits easy repair of minute leaks which may develop. tight seals. The tubes for the heater leads are beveled in at the top The jacket-head consists of a bottom of 1.59-cm. boiler plate and a top of 0.95-cm. boiler plate separated by a ring to form wells and hold close-fitting Pyrex glass tubes, which cut from a section of 30.5-cm. pipe 0.95 cm. thick, the width extend just below the head. The heater wires are enclosed being 3.18 em. The head is assembled with 0.64-cm. (l/4- by the glass and the wells filled with de Khotinsky cement. Fitting T for the thermocouple leads consists of an outer inch) S. A. E. cap screws, E, which pass through the top plate and screw into the bottom plate. A cork gasket and cement heaLy copper tube, U,and a close-fitting copper cylinder, are used on each side of the ring. The head is fastened Ti, held in place by split-ring nut IT. The thermocouples to the jacket by 1.1l-cm. ('/te-inch) S. A. E. cap screws, D, are brought out through Pyrex glass tubes held in holes and is recessed a t F to fit the projecting upper end of the X , and well Y is filled with deKhotinsky cement. 2 is a inner jacket shell. A fiber gasket was found to be satisfactory hollow copper cylinder set in V and equipped with tubes
E
-nr
391
392
ANALYTICAL EDITION
for water cooling, which gives additional protection to the de Khotinsky seal. OPERATION OF FURNACE
-
The furnace operates for all except the highest vacuums without using the mercury seal. No difficulty was encountered in pumping out to a pressure of 0.002 mm. of mercury a t room temperature with a good oil pump. I n the neighborhood of 1000° C., which is about as high a temperature as may be reached safely with the copper block, the apparatus pumped down to 0.1 mm. of mercury after small amounts of impurities in the copper had been removed by distillation and the original rapid degassing of metal and alundum cement had diminished. The heating unit and monel metal shields proved to be entirely satisfactory, 4.5 and 13.8 amperes through the 8-ohm coil giving, respectively, temperatures of 412" and 906" C. The experience with this furnace has definitely proved the great value of radiation shields. Monel metal was used because it retains a good reflecting surface after being heated in vacuo to temperatures around 1000" C., and has a relatively low thermal conductivity. On dismantling the apparatus, the outer shield had the appearance of never having
Vol. 4, No. 4
been hot, which is substantiated by the fact that the temperature of the cooling water was only slightly higher on leaving the furnace than on entering. I n reducing the heat losses, the radiation shields also protect the de Khotinsky seals, which were a great source of trouble in a previously used furnace which was not equipped with radiation shields completely surrounding the heater unit. The method of bringing out lead wires also proved very satisfactory and is considered a marked improvement over methods formerly employed. Fitting T was constructed to give good thermal contact between copper cylinder V and copper tube U,which is surrounded by the cooling water. This construction, with the cooling unit 2, makes practically certain the permanence of the deKhotinsky seal in well Y . ACKNOWLEDGMENT The success of this furnace is largely due to designs contributed by C. C. Maier, supervising engiqeer of the Pacific Experiment Station, and to the excellent workmanship of C. M. Bell, mechanician a t the Pacific Experiment Station. RH~CEIVED April 30, 1932. Published by permission of the Director, U. 8. Bureau of Mines.
(Not subject to oopyright.)
Determination of Fluorine and Boron in Organic Compounds DANIELJ. PFLAUM AND HERMAN H. WENZKE,University of Notre Dame, Notre Dame, Ind.
I
N SOME of the researches being carried on in this department on organic boron trifluoride compounds, the problem of finding a dependable method of analysis for both fluorine and boron arose. A number of more or less unsuccessful attempts to do this have been made in the past. Kraus and Brown (4) prepared various amino derivatives of boron trifluoride, and analyzed them for fluorine, boron, and nitrogen, in establishing their structures. Their analyses for fluorine and boron indicate the need of a more reliable method. Bowlus and Nieuwland ( 1 ) were able to analyze for boron with an average error of about 6 per cent, by decomposition of the sample through continued heating with fuming nitric acid in a sealed tube, and titrating the boric acid formed with base and mannitol, according to the standard procedure (6). These authors did not report fluorine on the analyses. Vaughn and Nieuwland (10) presented a new method of analysis for organic fluorides, using sodium in liquid ammonia, but boron was not present in their compounds. There are a number of difficulties peculiar to the analysis of compounds containing boron and fluorine. Fusions of the samples with alkali carbonates (6) proved to be unsuccessful, as there was difficulty in decomposing the compounds, and there was evidently some loss through volatilization. It was finally decided to destroy the organic matter by combustion in a Parr sulfur bomb with an oxidizing mixture of sodium peroxide, potassium chlorate, and sugar. Most of the organic boron trifluoride compounds are deliquescent fuming substances, liquids or solids, which interact instantly with the fusion mixture. Recourse was had to weighing them in gelatin capsules and placing them in the bomb, thus inclosed. The capsules were completely destroyed by the ignition and did not interfere with quantitative combustion of the compound.
When the products of the fusion were dissolved in water, a solution containing a high concentration of alkali hydroxide and carbonate was obtained, and these interfered with the precipitation of calcium fluoride, When an attempt was made to remove them by treatment with hydrochloric acid, low results for fluorine were invariably obtained. According to Mellor (7),it is possible that the stable fluoborate ion was formed and not completely broken up by the weakly alkaline solutions permissible for the precipitation of calcium fluoride. Whatever the explanation, any appreciable concentration of hydrogen ion resulted in low values for fluorine. Although acetic or other weak acids are suitable for destroying hydroxide and carbonate without causing too high hydrogen-ion concentration, they interfere with the subsequent titration for boron. The method finally chosen was to destroy the carbonate and alkali by boiling the solution with ammonium chloride. Some trouble was had in the proper coagulation of the precipitate of calcium fluoride, but this was overcome by the use of Fisher "filter accelerators." It was found that calcium fluoride was filterable when these were used, even without the addition of ammonium hydroxide, acrording to the method of CarriBre and Rouanet ( 2 ) . Coprecipitation of fluoride and carbonate (9) was unnecessary, and has been attacked recently on theoretical grounds by Mougnaud and others (8). Clarke and Bradshaw ( 3 ) have shown that the calcium fluoride method can be made to give accurate results by substituting these paper-pulp filter-aids for the coprecipitation and subsequent washing with acetic acid. The next difficulty occurred through the presence of the excess ammonium chloride in the filtrate containing boron. Ammonium ion causes the end point to appear too late, through buffer action. It was removed by the addition of
