NOTES
Fcb., 1959 THERMAL CONDUCTIVITY OF POLYCRYSTALLINE BORON1 BY CLAUDE P. TALLEY Experiment Incorporated, Richmond 2 , Virginia Receii,ed Augzrst 8 , 1958
To the author’s knowledge there have been no previous reports of the thermal conductivity of boron. In the course of preparing samples of boron to study its oxidation properties, cylindrical polycrystalline boron rods about 1 mm. in diameter and several centimeters long were made which contained a 0.025 mni. diameter tungsten core. Such specimens are well suited to the measurement of thermal conductivity, and it was decided to take this opportunity to obtain a t least an order-of-magnitude estimate of this property for boron. The boron rods were prepared by the reduction of boron tribromide by hydrogen near a 0.025 mni. diameter tungsten filament a t about 1250”. Wet chemical analysis for total boron showed that the boron content exceeded 99% by weight. The main impurity in the rods was that due to the tungsten core, which amounted to about 0.7% by weight. Emission spectrographic analysis indicated small amounts of Cs, Fe, Cu, Mg and Si amounting to a total of 0.02% by weight. The thermal conductivity of a polycrystalline boron rod was measured in the following manner. The tiny 0.025 mm. diameter tungsten core was heated electrically and served as the heat source. The circuit consisted of battery, variable resistor, ammeter, voltmeter and the specimen immersed in a constant temperature water-bath. At the temperatures used in these experiments the electrical conductivity of the boron was negligible, and therefore practically all the electrical current that passed through the rod flowed through the tungsten filament located in the core. Electrical contact wns made to the tungsten by fusing on platinum leads a t each end of the rod. Heat generated in the tungsten was conducted through the cylindrical casing of boron and into a surrounding water-bath. The temperature of the inside surface of the boron was assumed to be equal to the tempernture of the tungsten. The temperature of the tungsten was obtained by computing its resistance from the measured voltage and current supplied to the rod and comparing this resistance with a previously experimentally determined curve on the same rod, a t negligible power input, of resistance versus temperature. The temperature of the outside surface of the boron vas nasnmed equal to that of the stirred nrater-bath. In a t>,pical esperinient Kith a 1.19 mm. diameter by 27.4 min. length rod, the power supplied to the 0.025 mm. diameter tungsten core wns 0.iO cal./sec., the resistance of the tungsten corresponded to a temperature of 80°, and the ternperature of the water-bath was 22”. In these preliminary experiOC./cm.) was ments an average value of 0.003 cal./(sec. ohtained for the thermal conductivity between 20 and 80’. The equ:it8ionfor the calculation of t h i thermal conductivity IVaS2
I‘ =
Q In r h 27rL(T1 - Tz)
where
K = thermal conductivity
Q = rate of heat conduction
rz = outside radius of boron rod rl = radius of the tungsten core L = length of boron rod TI = temperature a t T I Tz = temperature a t rz (1) This work was supported by the Office of Naval Research. (2) W. H. RlcAdams, ”Heat Transmission,” 3rd Ed., McGraw-Hill Rook Co., Inc., New York, N. Y., 1954, Chapter 2.
31 1
Heat loss a t the ends of the rod was neglected because of the high length-to-diameter ratio of the rod. By electrically probing a polished cross section of a polycrystalline boron rod for electrical resistance, it was found that the tungsten filament in the center had remained essentially unaltered during deposition, and therefore rI was taken as equal to the starting radius of the tungsten filament. For example, a t a distance of about 0.010 mm. from the center of the rod the resistance was only a few ohms, whereas a t about 0.025 mm. from the center of the rod the resistance increased to about 150,000 ohms. Also the tungsten core appeared under the microscope to be a maximum of 0.035 mm. in diameter. Using this same technique a value was obtained for the thermal conductivity of Pyrex glass rods containing a central 0.025 mm. diameter tungsten filament which agreed with published values within a factor of three. Considering that the thermal conductivity of Pyrex IS about the same as obtained on polycrystalline boron and considering the accuracy of measurement of the individual quantities, the thermal conductivity value obtained for boron is thought to be accurate to within a factor of three also.
T H E DESICCATION AND DENSITY OF ACETONE B Y K.s. HOWARD AND F.P. PIKE Department of Chemical Engineering North Carolina Slate College Raleigh, N. C . Received August 1 1 , 1068
The dehydration of otherwise pure acetone has been a troublesome problem for many years. The ordinary inert desiccants, such as CaC12and CaSO4, are extremely slowacting1 and are ineffective2 in the complete removal of water, since snlall amounts of water are retained very tenaciously by the acetone phase. Formation of an addition compound between acetone and sodium iodide3 with subsequent regeneration of ((anhydrous” acetone was used by Young4 t o obtain a product with a density of 0.79053 g./nil. at 20”. Timmermans2 has used PzO5 for dehydration, a process which involves great loss of material through condensation reactions and requires an efficient distillation column for separation of the product. Iiiterpolation of the Timmermans data gives a density of 0.7904 g./ml. a t 20°, and this value was accepted for many years as the density of anhydrous acetone. However, Thirion and Craven5 used desiccation by acetic anhydride, folloved by distillation, to obtain a product with an average density at 20“ of 0.78990 f 0.00006 g./ml., significantly lower than these previous values. The results of Thomas and McAllister’ confirm this lower density. The current work was begun in an attempt to find a simple procedure, employing ordinary laboratory equipment, for complete dehydration of acetone. Use of the sodium iodide adduct was no more successful here than it had been iii Young’s hands.4 Dehydration by CaH2 or acetic anhydride led to great material loss and separation problems. An attempt to titrate the water by modifications of the Karl Fischer technique,6 followed by distil(1) K. T. Thomas and R. A. MoAllister. A . I . Ch. E. J . , 3 , 161 (1957). (2) J. Timmermans, “Physico-Chemical Constants of Pure Organic Compounds,” Elsevier Publ. Co., Inc., New York, N. Y . , 1950, p. 354. (3) K. Shipsey and E. A. Werner, J. Chem. S o c . , 103, 1255 (1913). (4) W. Young, J . Soc. Chem. I n d . , 5 2 , 449 (1933). ( 5 ) P. Thirion and E. C. Craven, J . A p p l . Chem., 2 , 210 (1952). (6) J. Mitchell, Jr., and D. 11. Smith, “Application of the Karl F~scller Reagent to Quantitative Analyses Involving Water,” InterBcience Publishers, Inc., New York, N. Y., 1948.
312
Vol. 63
NOTES
lation of the anhydrous acetone, was unsuccessful. Use of the acetone-urea comples7 did not appear promising. The authors were, however, able to obtain high yields of acetone with an average density a t SO.OOo of 0.78994 f 0.00003 g./ml., using a synthetic zeolite as a desiccant, followed by distillation with ordinary equipment.
pares favorably with the 1.35596 & 0.00003 reported by Thomas and R’lcAllister.’ Although the reproducibility of the densit’y results was within the precision of the method (* 0.00005 g./ml.), the accuracy of the results is claimed only to f 0,0001 g./ml. because of the difficulty of precisely evaluating the purity of the acetone. The assumption for insignificantly low water content of the samples is based on the idenExperimental tity of samples prepared by the method from difMost reagent grade acetone is unlikdy to contain significant impurities, with the exception of water; “Baker An- ferent starting materials, and the failure of SYCalyzed” reagent grade material (J. T. Bakcr Co., Phillips- cessive repeated applications of the desiccatlon burg, N. J.) was found to be a particularly suitable starting procedure to change the measured properties. material, with a water content of about 1.2 mole yo. The Density and refractive index were the analytical synthetic zeolite Type 5A Molecular Sieve (Linde Air Prod- methods for evaluating both these criteria. In ucts, New York, N.Y.) was regenerated by heating a t 670’ F. for 3 hours in an air oven: the hot material was sealed addition, the assumption of the absence of any and allowed to cool. This regenerated material was added impurity other than water appears valid in view of to the starting acetone in a weight ratio of 1 part Molecular the close correspondence of both density and reSieve to 7 parts acetone; this amount represents a 3- to 4fold excess over the calculated capacity of the zeolite. If fractive index values of acetone from these prepathis operation is carried out in the flask to be used for Rub- rations to those reported for material prepared sequent distillation, the possibility of water contamination through processes employing more rigorous sepafrom the atmosphere during future transfer operations is 01,ration techniques. It seems improbable that any viated. The mixture was allowed to stand 18 to 24 hours, with occasional swirling, and then was quickly connected t o significant impurity would remain undetected by a Vigreux column equipped with a straight water-cooled con- both analytical methods. denser and receiving flask, all of which had been purged with Acknowledgment.-This work was carried out a steam of CaS04-dried air to remove water adsorbed on the with funds made available by the National Science glass surfaces. All junctions were non-lubricated ground glass joints, and the apparatus was vented through a CaSOI Foundation, and the authors gratefully acltnoxldrying tube. The center cut, comprising 80 to 90% of the edge this support and encouragement.
starting volume, was collected. This material had a density of 0.78993 g./ml. Using this sample, the entire desiccation procedure was carried out a second time, with fresh Molecular Sieve, and the twice-treated distillate had a density of 0.78996 g./ml., indicating that no further drying had been effected. An additional preparation, from another sample of “Baker Analyzed” acetone, had, after one desiccation treatment, a density identical t o those reported above. The refractive index of the various preparations was similarly constant. The densities were determined using the apparatus and procedure described by Thomas and McAllister,’ with the modification that all glass surfaces were thoroughly dried with a CaSOrdried air stream, and exposure of the acetone t o ordinary air at any point during loading of tho pycnometers was eliminated. Refractive index measurements were made using a Bausch and Lomb precision refractometer capable of giving results accurate to f 0.00003 unit.
TABLE I THE DENSITY OF ANHYDROUS ACETONE Temp.
Density (dml.)
(OC.)
20.00
0.78989 .78997 .78906 .78419 .78426 .78429 .76943 .76944 .75482 .75481
25.00
37.80 50.05
Results and Discussion The densities given in Table within d=O.0lo. this acetone was
of the desiccated acetone are I. Temperature control was The refractive index ( n Z 5 ~ of) 1.35589 0.00003, which com-
+
(7) W. Schlenk, .Jr., A n n . , 666, 204 (194Q): 0. Redlich, C . \I. Gahle, A. IC. Dunlop and R . \V. Alillar, J. Am. Chem. Soc., 78, 4153 (IOXIj.
T H E EFFECT OF DROP SIZE ON THE ACCURACY OF SURFACE TENSION DETERMINATIONS BY THE SESSILE DROP METHOD BYEDWARD B. DISMUEES Southern Reaearch Institute, Birminpham 6 , Alabama Received Aupust 1.8, 1968
A classictzl method for determining surface tension, particularly of molten metals and salts, is the sessile drop method. If a liquid drop rests on suitable flat surface and assumes an obtuse contact angle, the dimensions of the drop silhouette, namely, the maximum radius r and the distance h from the maximum diameter to the vertex, are determined by gravitational acceleration 9, the liquid density d and the surface tension y. No analytical solution of the differential equations that relate these quantities has been obtained, but the surface tension may be calculated from approximation formulas such as the Worthington formula’ y
=
h2gd/(l
+ 0.6094 h / r )
(1)
or from the Bashforth and Adams tables.2 The use of these tables has been reviewed by Ellefson and Tay10r.~ IGngery and Humenik4recently pointed out that the accuracy of surface tension determinations by the sessile drop method becomes less as the drop (1) A . M. Worthington, Phil. Map., 20, 51 (1885). (2) F. Bashforth and S. C. Adams, “An Attempt to Test the Theories of Capillarity,” Cambridge University Press, 1883. (3) B. 9. Ellcfson and N. W. Taylor, J . A m . Ceram. Soc.. 21, 193 (1938). (4) W. D. Kingery and ill. Humenik, Jr., THISJOURXAL, S7, 359 (1‘333).
