High Vacuum Distillation KENNETH HICKMAN
I36 Pelham Road, Rochester IO, N. Y.
A
MONG the gains that have accrued since the 1952 review
(28) there is a loss. Saul Ilushman died on July 7, 1054, ( 3 6 ) leaving his many friends saddened but leaving also a monumental legacy in high vacuum. Born in Russia in 1883, he was graduated from the University of Toronto, Canada, in 1904, and joined the General Electric Co. in 1912, where for many years he played Boswell to Langmuir’s Johnson and in 1922 puhlished a small but epoch-making book “High Vacuum.” Of his own many contributions we mention three: the invention of the ionization gage, the teaching and proselytizing in advanced physics (he coined the suffix, tron--e.g., kenotron, magnetron, cyclotron), and his second book, “High Vacuum Tcchnique,” which survives him as the standard of reference. The gains include the emergence of the First Vacuum Syinposium, sponsored by the American Instit’ute of Chemical Engineers a t French Lick, Ind., in 1952 as the fullyfledged Committee on Vacuum Techniques which met a t Asbury Park, N. J., in June 1954 and again in Pittsburgh a t t,he Uellon Institute in Oct,oher 1955. The new society has established a, long overdue committee on standards and nomenclature and has isslued its first volume of transactions (62). A third focus for meeting, which evoked favorable comment), TI-asthe hchema Eshibit,ion stagcd in Frankfort-am-Main ( 1 ) where large pieccs of vacuum equipment n-ere dieplayed, including at least one new mechanical pump, the overFize, low-pressure Roots blower.
Vacuum Distillation
Ta-o groups which have been occupied for many years, chiefly with the practical side of short-path distillation, have recessed for intellectual renaissance. Burrows, at, hfetropolita,n I-ickers, England, has authored three related papers (8-10) and coauthored three more (11-13) Jvhile Hickman, Trevoy, and Torpey have, severally, continued their studies on the niechanisin of evaporation in particular relation t,o the liquid surface. A third group at the University of Madrid has entered t,he field with an important series of seven papers (59-46), reviewed in detail bclow. -4s a practical matter a fractionating short-path still inay be defined as a cascadeof individual stills in countercurrent dist’illateresidue flov. Two kinds of assembly are favored, one which might be termed “engineered” where thc streams are mechanically pumped and metered between units, and “laborat,ory” where the residue falls from unit to successively lower units and the distillates flow upwards, the work of elevation being done by the vapor, with the ceiling of each still situated above the floor of the one nest higher. Both types have been built and operated successfully, for instance, at the National Bureau of Shndards, but t,he complexity and the large volume of liquid in transit make the engineered type unattractive for labile substances. The first promising embodiment of the laboratory type, conceived by Brewer and Madorski and in modified form by Kollner (see previous reviews), involved a practically continuous sloping container, the bott.om being heated and the top, which is periodically Pectioned off, cooled. r)ist,illates are led upstream 133‘ sloping
projections from the ceilmg I n a rccent modification, glass alembic stills are arranged in a slanting group and arr interconnected with all-gravlty flow. A 20-alembic still has been built in a petroleum laboratoq- ( $9)and tested on the usual binary milture Octoil-Octoil S (EHP-EHS), which indicated an rffciency a t total reflux of 0.8 theoretical plate per stage. A 50-stage still of the slanting tube pattern has been built a t the Carnegie Institute of Technology. Both stills achieved separations of relatively high m o l e c Lila r weight hydrocarbons, differing by one CHz group-- e.g., n-heneicosane from n-docosame. The assemblies would be left under total reflux for 12 to 14 hours, a fraction reinovcd froiii the top end and another 12 to 24 hours nlloived t o elapPe before removing the n e s t fraction--.--a, lengthy over-all proccss. ‘if t,he thermal hazard to ivhich the distilland is cxposcd is considered, approximately, t o be the product of tempera t w e and time of distillation, the S a u l Dushman, who died in ha,zarti in thesc multiple J u l y 1954, left a monumenoperated units is a t least it tal l e g a c y in high vacuum inillion times greater than a singlc pass through a ccntrifuaal - still. The slanting tube fractionating still desciibed by 13003’ and others ( 5 ) differs in an impoitant paiticular from the Carnegie Institute still in that the condenser 1s an internal concentric watercooled tube which 1s rotated mechanically through a vacuum seal, for the p r p o s e of transferring viscous condensate to the slanting feed-back members. Kine distillation stages are provided and the apparatus i s emplo:, ed to examine a variety of reduced petioleuni crudes. In 1919 Rooy and TI-,iteiinan dcscribcd a micro short-path still consisting of a miniatuie pan suspcntled on a helical spring in ’t tall, evacuated tubc The pan n as heated by an infrared laiiip, and the rate of distillation n as deduced from the rate of rise of the pan. A variation in n hich the sample is heated by a tubular radiant coil and tempei atures are noted by thermocouple has been described by Sims (68),and a somewhat simllar dcsign (65),in which the coil is replaccd 1 3 ~ -a cavity in a warmed aluminum block, has been built for research in vegetahle oil derivatives. I n Sinis’ still, samplee 11 eighing 30 nig. gave a liquid depth of 0 4 to 0.5 mm. in the pan nhich lifted 1 em. for each milligram loss, permitting a precisloll of zk 1%. Using a distillation indicator such as quinizarin glcaen, elimination maxima of a conventionalized type were determined for a number of cholesterol esters,
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,,,, , ,
Vol. 48, No. 3
HIGH VACUUM DlSTlLLATlON fatty acids, and sugar esters: hents of vnporization were also dcducecl. The most precise single-stage v a c i i u ~ still ~ i yet devised was built by Trevoy and Torpey ( 6 2 ) who coupled a falling stream tensimeter with a stirred pot still. The p u t still provided a continuous feed for the tensimeter, which was operated a t a constant rate of collection of distillate, the temperature being raised continuously as required. Accuracy and reproducibility were of the order of 1yo, but shortcominge were the complexity of apparatus, the large holdup, necessitating a 1- to 2-liter charge, and the slrill and conscientiousness required for operation (no lunch periods or telephone eel IS). Theory of Open Path Evaporation
Lysing their flow tensimeter technique, ‘l’revoy and Hickman (19.53 review) had shox-n that 2-ethylhesyl phthalate ( E H P or Octoil) and 2-ethylhesyl sebacate (ETIS or Octoil S) each evaporated at 100% of the natural rate deduced from the KnudsenLangmuir equation at Ion. saturation pressures of 1to 10 microns of mercury but fell to SO% at 150 microns of nicrcary and proportionately lower percentages at higher pressures. Was the falling off cauFed by depletion of the liquid surface or congestion in the surrounding vapor? Burrows (8) elected to answer this question, harking back to Rondd Frazer’s calculations (1939) of the fraction of molecules likely to he returned froiii the vapor to the evaporator in the unrest’rictcd path still. IIe then defined or adopted three values: F , which is the ratio of the condenwr area to the total area of nio!ecular escape ( P was deduced as 0.8 for the flow tensimeter); K , which is the distance betn.een evaporator and condenser, divided by the niean free path An in the vapor a t equilibrium, so that’ K = d / E X h ~ andf, ; which is the escapc coefficicnt postulated by Hickman and Trevoy, being the net fraction of distillate actually captured by the conclenser. Fraser’a return factor is t,hus ( 1 - f ) ; and Burrow’s equation takes the form of f = F (1 - P ) ( 2 e - R - e -ZK). dpplied by Burrows to the puhli.sbed experimental data on 2-ethylhes~-lphthalate, a remarkable agreement between theory and reality is shown in Table I. Thus, the evaporation coefficient appears t o remain a t unity for high rates for a surface momentarily exposed, the fall in the rate of collecting distillate being cniised by back diffusion of vapor. In a concluding endeavor to Iny the ghost of the anonialow evaporation coefficient, Trevoy approached the problmi of glyerol(60) and IIickman of lyater (30j, tTvo substances of widely different character and vapor pressure, neither of which had previously yielded coefficientsgreater than E = 0.04. Experimental dificulties ryere the viscoyity of the former and the high enthalpy sntl tendency to freeze of the latter. Trevoy built an overviee fion- teneiinet er with two circulnt ing pumps in series and means Em extracting the frictional heat of AOW. IIickman built a minia-
-
K. C . D. HICKMAN received his Ph.D. in photographic chemistry from London University in 1924. In 1928 he switched to chemical engineering in the dual fields of waste product silver recovery and high vacuum distillation. He is the inventor of centrifugal molecular stills for distilling vitamins A and E from natural oils and has done work on the theory of short path distillation. Hickman currently is engaged in developing a new vapor recompression still for the cheaper production of fresh water from sea water. March 1956
ture, high velocity flow tensimeter and operated it continually over a 12-month period until four runs totaling 8 minutes of icefree operation were secured. Results, glycerol E = 1.0, unequivocally, and water E = 0.28, approaching unity after corrections forfand the probable temperature of the water skin.
Table I PO2
t
T
100.7 110.8 120.9 130.9 140.7 151.0 161.5
374 384 39.4
Ka
,--2K
,-K
re __
f b
rm
1.oo 1.00 0.99 0.96 0.90 0.83 0.80
1.01 1.08 0.98 0.95 0.92 0.80 0.79
Ethylhexyl phthalate 1.2 2.8 6.9 15.0 31.0 65.0
136,O
0.051 0.117
0.280 0.594 1.199 2.453
401 414 424 434
5.008
-
0.950 0.890 0.756 0.552 0.301 0,08G 0.0067
0.902 0.792 0.571
0,305
0.091 0.0074 0.0000
a K = d s - - 3o 16 X p-m- ( min microns) k X AE T = 0.80 0.20(2e-R e-*R) b/ 2 experimental rate, Trevoy and Hickman _____ Tm theoretical rate
+
-
~
Experiments with relatively stagnant liquids have also been contmucd. “Pure” distilled water, resting in [t half-filled N i t e r flssk and exposed to high vacuum, was found to evaporate a t about 0.0025 times the Langmuir-Knudsen rate, just befoi e freezing, and vapor emerged chiefly from visibly working crateis (31). In other expcrimentu, schizoid surfaces of 2-ethylhexj 1 phthalate or heavy white petroleum oil were stirred under high vacuum with a broad, partly immersed paddle and shown to emit leas vapor in front of the paddlc than behind because the torpid aiea would be pushed in front, leaving a momentarily clean woi king surface bchind (29). The most surprising fact emerging from thi. 11 ork was that, T.I hile the torpidogenic agent is often a nonvolatile surface seehing solute, it is far more often a volatile nioietJ- adsorbed fiom the vapor phase. Th(A nature of thiq moiety - there is a suggcstion that it may be a silicon derivative. volatile at least from a two-dimensional layer -and whether it iniieascs the weight of a single surface molceule or whether it bridges and thus joins tvto or mole smface molecules are quccitions that await further investigation. At least, we can interpret plauiibly the well known contradiction that most vacuuni pot still distillations quicken as they proceed, with lowering of tempeiature, even though the tine boiling point is knox n to be iising The volatile torpidogenic agent has gradually been removed from the still 17 ith the condensates. If it is too early to deduce laws from the few examples that have been colltcted over the past 5 yea1 q, thc following rules can be applied with sonic certainty: The evaporation cocflicients of all newly exposed (>0.01 second old liquid surfaces i u unity, irrespcrtive of such factors as cheniicnl composition and polarity. Stagnant liquids acquire surfaces with an evaporation eoefficient less than unity and less than can be accounted for by ratea of self-diffusion. The obstructive surface is gcnerally due to adsorbed impui ity. The most prevalent adsorbed iinpur ities come from thp vapor above the liquid. Elimination Curve
The classic mathematical treatment ~ J YK. Embrec (193T) of stepwise molecular distillation has recently been extended by the Spanish school, who have also I e-exaniincd and redefined the original methods and postulates of the Rochester group, including the cyclic batch still and constant yield oil. The first paper (39) discards the stepwise mathematics in favor of a continuouj progression. Part 3 (41) defines a theoretical calculus of the
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UNIT OPERATIONS REVIEW
CJJRTESY
AT.I’.TIC
Twenty-stage molecular still
elimination curve, while Part 2 (40) is devoted to a recalculation of Gray and Cawley’s data (1940) on the elimination maxima of certain fatty acids and to establishment of standard conditions for comparison with data gathered by other distillation techniques. Part 4 (@) is concerned with the exact minutiae for operating the still, with emphasis placed on regulation of flow and Part 5 ( 4 3 ) with the preparation of mineral constant yield oils. Part 6 ( 4 4 ) is an exercise in reconciling calculated and found elimination maxima of the butyl esters of some of the more common heavy fatty acids. Reserved for Part 7 ( 4 5 ) is an attempt to integrate the special theory of the elimination curve R ith the general concepts of physical chemistry. l m o n g a number of conclusions, there may be included: A specific constant-ofelimination can be defined for each pure substance; another constant will interrelate the maxima of compounds in a homologous series, and latent heat of evaporation can be calculated from elimination data. The seven papers have bcen translated by their authors and the composite is being arranged in a single article by, J. L). Trevoy for publication, by invitation, in Chemical Reviews. Miscellaneous Stills and Studies The need for disturbing the surface of the distilland diiring distillation is recognized by Kretchmar in a rotating, multipleplate still. In the same vein, two laboratory stills have appeared, one employing a rotating flask ( 6 4 ) and the other a heated rotating cylinder held within an evacuated horizontal glass condenser (66). A commercial centrifugal still has been patented by G. Burrows (Brit. Patent) which has means for residue feed-back; and another commercial, stationary still for use in a somewhat higher pressure range has also been patented (64). A three-stage, falling film still has been constructed in a South African laboratory (66). Two papers in fields periphcral to the present review deserve study by those concerned with the rigid evaluation of still efficiencies. Double withdrawal and the gambler’s ruin discusses multiple batch fractionation (18), and a discussion of properties and lubricant application of aliphatic esters (27‘) describes a large number of phlegmatic liquids, including homologous esters. As E. S. Perry has pointed out (1955 Separations Conference, Colby
498
Junior CollegP, N. € I , ) , the pair 2-ethylhexylphthalate-2-ethylhexyl sebacate is unique in its constancy of relative volatility, yet a is too large for comparing stills of more than 5 effective plates. Kanted: endowment of a research project that will discover an equally reliable pair(s) with smaller alpha(s). Two separations that have been made with the molecular still are noteworthy: neorctinine ( 2 1 ) from a crude synthesis mixture and fluoranthrene from chrysene (14) by means of a 5-inch centrifugal still. Vapor pressures (50) of many heavy hydrocarbons have been measured and correlated. There has been considerable activity in examining the vapor pressures of metals and refractory compounds, generally by the effusion method. A miniature crucible of graphite or other infusible material is provided with a small hole through which vapor escapes. The test material is placed in the crucible and the loss of weight noted after heating in the vacuum furnace. Vapor pressures of silver (is),silver and gold (47), copper and gold (56), sodium (38), gallium (69), lithium hydroxide (ZS),titanium REF h11.0 C Q and copper (26),and calcium ($4)were measured and also the vapor pressure and molecular weight of tin and tin vapor (67). Vapor pressurea of the fluorides of plutonium and americium ( 1 6 ) were measured and the rate of evaporation (7) of potassium chloride was studied theoretically ( 1 7 ) . An ingenious combination of the effusion method with molecular beam techniques allowed the determination of the vapor pressure of carbon from a graphite crucible (23). Rates of evaporation of metals mere estimated from carry-over in a nitrogen stream (86)by a Russian nmker. The formation of titanium carbide under high vacuum conditions (61)has been studied, as well as the carry-over of silica by eteam (15). VACUUM FURNACE
Large European furnaces and machinery for degassing metals on a production scale vicre diqplayed a t the Achema Exhibition a t Frankfort-ani-3lain (1) .4 furnace has been described rhich melts and casts a 400-pound charge of steel in 2 hours with an electrical input of 250 kw. (65). There are smaller furnaces for heat-treating metals (19) and melting metals in the laboratory (66). Of the miniature variety for analytical usc, there are a fusion furnace for determining oxygen in chromium (33) and another for analysis of various gases in titanium, zirconium, and molybdenum (48). THIN FILMS
BY
EVAPORATION
For papers on vacuum deposition, see bibliographies appearing approximately quarterly in Vacuum. In the article on reactive spluttering (Sd), the authors treat the interaction of carrier gas with the material to he spluttered. MANOMETERS AND
GAGES
These have not been included in this review, through lack of relevant material.
New Vacuum Techniques Although not of immediate concern to the vacuum chemist, the probable future impact makes it desirable to note three highly significant advances in the production of very high vacuums. Alpert (d), of Westinghouse laboratories, from a study first cen-
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HIGH VACUUM DISTILLATION
“Recent laboratory developments have shown that molecular stills of 20 to 50 stages are practical and can be operated successfully. Although it is difficult to see how high vacuum distillation as a unit process can be widely justified a t mesent, the increased efficiencies made possible here may pave the way to more general utilization. ” H. W. FIELD, Vice President The Atlantic Refining Co.
tered around the ion gage ( 3 ) has succeeded routinely in creating iTacuums in the range 10-9 t o lo-” mni. of mercury. IIe has discovered the curious I)ut important phenomenon that a mass of outgassed sheet copper placed between an oil diffusion pump and the point of high vacuum lisp prevent’s, completely and for many weeks, the back-diffu ion of volatiles, so that the pump can create “emptiness,” lini ted only b y the natural infiltration of atmospheric helium and the ability of gages t o measure (63). The second advance-------and there is no significance in the order presented-is the Kvapor-ion pump of Herb ( 2 7 ) and associates (20). This relies on the wtiporation? continuous or occasional, of a thin titanium wire frd from a reel against a source of heat, such as a tungsten arc. The evaporatcd titanium deposits on the walls of the containcr ( 6 6 ) , generally a pipe 10 to 20 inches in diameter, entmpping in a succession of monolayers all but the noble gases. Thus, a metal w1iic.h must be melted in bulk under high vacuum t,o remove gases (inring manufacture becomes the getter for those self-same gases in the Evapor-ion pump. A feTv grams of titanium Fire per day sufficw to keep a large “dry” Fystem under sustained high viiciiiiiii of 10 - 8 mm. of mercury. The suffix ion refers t o the electric field that can be imposed on the pumping vapors to aid in reinoval of noble gases (6). The third advance refers to the imminent superseding of liquid air by a new, inerprnsive, niiilti~tagerefrigeration systcm ( 3 4 ) developed by Kennedy and Smith for use as the cooled trap and largr, generally nuclear, vacuum systems. Acknowledgment
..ichnon-Iedgrnents are made to Benjamin Dayton and t o Donald Trevoy for conti il,utions to this review.
Bibliography (1) Acheina Exhibition, Frankfort-am-Main, May 14-22, 1955. (2) -4lpert, D., J . Appl. Phys. 24, 860-76 (1953). (3) Alpert, D . , Burits, R. S., Ibid.,25, 202-9 (1954). (4) .4xeleff, I., Petersen, E . C., Second Vacuum Symposium, Committee on Vacuum Techniques, 3lellon Institute, Pittsburgh, Pa., Oct. 15, 1955. (5) Booy, H., Altershoff, 77’. G., Langedijk, S. L., Phillippi, G. Th., Waterman, H. I., J. 1 n s L Petroleum 39, 688---94(1953). (6) Bradley, R. S., Proc. Roy. Soc. L o n d o n A211, 524-9 (1953). ( 7 ) Bradley, R. S . , Volans, P., Ibid., A217, 508-23 (1953). (8) Burrows, G., J . Appl. Chem. 4, 394400 (19%). (9) Burrows, G., Metropolitan T-ickers Guz. 25, (1953), (Leaflet 902/ 19-1). (10) Burrows, G., T r a n s . l n s t . Chem. Engrs. London 32, 23-34 (1954). (11) Burrows, G., Jackson, R., T’aci~um 2, 50-1 (1952). (12) Burrows, G., Prcece, F. If., b. A p p l . Chem. 3, 451-62 (1953). (13) Burrows, G., Preece, F. H . , T r a n s . Inst. C l w n . Engrs. L o n d o n 3 2 , 99-114 (1954). (14) Canton, .4.,Feldman, J., Orchin, 31., A n a l . Chem. 26, 13T4-7 (1954). (15) Carlson, E. T., Peppler, R. B., Wells, 1,. S., J . Research S a t l . Bur. Standards 51, 1T9--84 (1953). (16) Carnigha, S. c., Cunningliarti, B . B., Reo. Sci. I n s t r . 26, 485-8 (1955). (17) Cohen, G . , Xlurphy, C. AI., others, ISD.ENG.C H E X 45, 176675 (1953). (18) Compere, E. L.. Ryland. -4.L., Ibid., 46, 24--34 (1954). (19) DavidJon, H. W., B U I T O O ~T. , H., Engineering 177, 106-8 (1954).
March 1956
Davis, R. H.. Divatia, A. S.. Rev. Sci. Instr. 25, 1193-7 (1954). Dieterle, J. M., Robeson, C. D., Science 120, 210-20 (1954). Ditmars, W. E., Johnston, H . L., J . Am. Chem. SOC. 75, 146970 (1953). Doehaerd, Th., others, Bull. soc. chim. Belges 62, 498-544 (1953). Douglas, I. P. E., Proc. Roy. Soc. L o n d o n 67B, 783-6 (1954). Dulton, D. -4.. Chenlistry & I n d u s t r y 1953, 1383-5. Edwards. J. W., Herrick, IT. L., Ditmars, W.E., J . Am. Chem. SOC.75, 2467-70 (1453). Herb, R. C., University of Wisconsin. Madison, Wis., Physics Department Report, March 15, 1954. Ilickman, K., IND.ENG.CHEN.45, 44 (1953). Ibid., 46, 45-52 (1954). Ibid., pp. 1442-6. Hickman, K., Torpey, W.d.,Ibid., pp. 1446-50. Holland. I,., Siddal, G., T’acuum 3, 245--53 (July 1953) (published June 1955). Horton, W. S.,Brady, J., An.ul. Chem. 25, 1891-8 (1953). Kennedy, P. B., Smith, I€., Second Vacuum Symposium, AIellon Institute, Pittsburgh, Pa., Oct. 13-15, 1955. Kotov, E.I., I’esfnik. A k a d . Nazrk. Kazakh. S.S.R.6 (KO.1) (whole S o . 46), 37-51 (1949). Langmuir, I., l.’acuu?n 3 (KO.2), 113 (1954). l l a i r . B. J.. Pignocco, A. J., Rossini, F. D., A n d . Chem. 27, 170-4 (1955). llakansi, AI. hI., Muendel, C. H., Selke, W. A., J . Phys. Chem. 59, 40-1 (1955). JIarin G6rriz, -4.,Martin Garcia, D . , others, Anales real. soc. espaA. f i s y q u i m . .!fadrid 48B, 825-40 (1952). Ibid., pp. 841-50. Ibid., 49B, 19-22 (1953). Ibid., pp. 107-14. Ibid., pp. 519-28. Ibid., pp. 579-86. Ibid., 50B,711-12 (1954). 1IcCabe. C. I,., Birchnell, C. E., J . Metals 5, 707 (1953). McCabe, C . L., Shadel, H. XI., Birchnell, C. E., Ibid.. 5, 709 (19531 I XlcDonald, R. S.. Fazel, J. E., Balis, E:. W., A n a l . Chem. 27, 1652-6 (1955). (49) XIelpolder. F. W., TTashall, T . il., Alexander. A. J.. Ibid., 27, 974-6 (1955). (50) lleyers, H. 5.. Fenske, 31. R . , IKD. ESG. CHEX 47, 1652-8 (1955). (51) LIiereon, G. A,, others, Zhirr. Priklad. Khim. 25, 134-47 (1953). (52) Nagashima, T., Yamamoto. T., Haltori, S., 61/6 Rutszari 22, 293-6 (1953). (53) Paschke, R. F., Kerns, J. R., Wheeler, D. H., J . Am. Oil Chernists’ SOC.31, 5-7 (1954). 154) Pyle, C.. Lane, A , , U. S. Patent 2,609,334. (55) Rossman, M. G., Tarwood, J., Brit. J . Applied Phus. 5, 7-13 (1954). (56) Schwartz, J. C . , Second Vacuum Symposium, XIellon Institute, Pittsburgh. Pa.. Oct. 15, 1955. (57) Searey. A. W.. Freeinan. R. D., J . Am. Chem. Soc. 76, 5229-32 (1954). (58) Sims, R . P. A.. I’acuzam 2, No. 3, 245-56 (1952) (published .4ugust 1953). (59) Spieser, R., Johnst.on, H. L., J . Am. Chem. Soc. 75, 1469-70 (1953). (60) Trans.First I-a,eiiion S u m p . 1954 (Committee on Vacuurn Techniques. Box 1282, Boston, ;\h%.). (61) Trevoy, J. D., ISD. ESG. CHEM.45, 2366-9 (1953). Anal. Chem. 26, 492-4 (1954). (62) Trevoy, J. D., Torpey, 1V. (63) Varverin, L. J.. Carmichael, J. H., J . Appl. Phys. 26, 782-3 (1955). (64) Volk, AI, E., A n a l . Chem. 27, 1207 (1955). (65) Winkler, O., Congr. intern. fouderie, 21 &me. Florence, Italy, 1954. (66) Zaugg, €1. E., Shavel, J., Jr., A n d . Chem. 27, 1999 (1955).
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