Dec., 1963
VAPORPRESSURES OF BIXARY ALLOYS
2. Nonideail Surface Solutions.-Mixtures of 1octadecanol arid stearic acid for example, shorn positive deviations from ideal behavior. 3. Ideal or Nonideal Behavior with Complex Formation or Rearrangement at Specific Compositions.-Mixed monolayers of 1-hexadecanol and 1hexadecyl sulfate are of this typeag We point out that the behavior tihown in Fig. 2, ref. 9, is by no means unique. Robbins and La Mer [Fig. 11, ref. 71 found that pressure-area isotherms for octadecanol as the excess of spreading solvent, benzene, diffuses into the subphase, showed the same type of progressive discontinuities-which some have arbitrarily called phase transitions. The evaporation resistance technique is most useful in distinguishing between these types of surface behavior of mixed monolayers, provided precautions are taken to ensurc purity of reagents and of the water-air interface. At low surface pressures, both Rosano and La n!fer5 and Barnes and La Mer2z6observed a sharp rise in the evaporation resistancle of the pure alcohols between P = 8 dynes em.-1 and P = 15 dynes cm.-I. Barnes and La Mer suggested that this sharp angle could be associated with the transition from the liquid condensed to the solid state which occurs a t the same pressure. However, in nieasurirlg the evaporation resistance of mixtures of these and other alcohols in the low surface pressure region, these authors obtained values much higher than those of either pure compound. The higher resistance values obtained for these materials in the low surface pressure range indicates clearly that the sharp change in slope (curve E, Fig. 1) observed by the previous 1% orkers resulted either from contamination of the monolayer from external sources or the presence of impurities in the spreading material. We interpret the “kink” in specific resistance-surface (9) B. M. Fowkes, J. Phys. Chem., 67, 1982 (1963); p. 1983, Fig. 2.
2795
pressure isotherms (curve E, Fig. 1) to the process where the decrease in area results in squeezing out of impurities rather than to increase in pressure [ref. 5 , Fig. 7, and ref. 1, p. XV]. The extreme sensitivity of the evaporation resistance of surface monolayers to the presence of minute quantities of impurities or contamination, particularly a t low surface pressures, is further emphasized by these results. I n practice it was found that only by using freshly prepared solutions and by rigorous acid and steam cleaning of the still for preparation of the subphase water, that these higher resistance values could be reproduced a t low surface pressures. Frequently, solutions left overnight, while yielding similar values a t higher surface pressures, were found to produce much lower values in the low surface pressure region and to exhibit the sharp change in evaporation resistance observed by previous workers. A similar deterioration in evaporation resistance at lower surface pressures resulted from even the slightest deterioration in the quality of the water used in the subphase. The isotherm of specific resistance-surface pressure for the Czo alcohol did have a slight sigmoidal discontinuity a t very IOU- pressures (. This undoubtedly is due to trace impurities in this longer chain material and accounts for the fact that in the low pressure region for curves A and B of Fig. 1 some disagreement between theory and experiment is observed. It may be noted that the Czoalcohol, and indeed all alcohols used in this investigation showed only one peak in gas chromatography analysis. This finding highlights the importance of the evaporation resistance technique in studying architecture of monolayers; the “probe” in such measurements is a water molecule. Acknowledgment.-This work was supported by a grant from the 0. S. Bureau of Reclamation No. 1-1-06D-4018.
A RADIOACTIVE TRACER DEW POINT METHOD FOR MEASURING VAPOR PRESSURES OF BINARY ALLOYS. THE ZINC-ALUMINUM SYSTEM’ BY GEORGE J . LUTZAND ADOLFF. VOIGT~ Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa Received J u l y 22, 1965 A modification of the dew point method for the determination of partial pressures of a component metal in an alloy has been developed based on the detection of the y-radiation from a radioisotope of the condensing metal. The zinc vapor pressure was measured over a number of alloys in the aluminum-zinc system in the temperature range 550-750’. Thermodynamic properties, activities and free energies, enthalpies, and entropies of mixing, were calculated across the system for selected temperatures in this range.
Introduction The dew point method of determining vapor pressures uses the equilibrium between a condensed phase and its vapor in a unique manner. When the vapor of one component in equilibrium with a multicomponent condensed system is cooled, condensation occurs a t the “dew point”, L e . , the temperature a t which the vapor pressure of the pure condensable component is equal to its partial pressure over the multicomponent system. (1) Contribution No. 1359. Work was performed in the Ames Laboratory of the U. 6. Atomic Energy Commission. (21 Correspondence should be directed to this author.
If the dew point can be observed and the vapor pressure us. temperature curve of the pure component is known, the partial pressure above the mixture can be determined. The method was first used with aqueous solutions by Lescoeur a t the end of the last century and was first applied to metal systems by Hargreaves3 in his classical work on the vapor pressure of zinc over brasses. The technique which Hargreaves used to observe the dew point was to place an evacuated quartz tube, containing the sample at one end, in a tubular furnace with (3) R. J. Hargreaves, . I Inst. . MetaEs, 64, 119
(1939).
< '
I
I
Fig. 1.-Dew point q q x m t u s : A, furnace windings; B, thermocouples; C, cooling g:m inlet; D, condensat,ion bubble; E, quar1.z den point cell; F, stainless steel sleeve; (:, tmtalum crucible; H, alloy sample; I, refna:t,ory block.
two independently heated sections. The furnace was brought to the desired temperatnre, and the end away from the brass sample was slowly cooled, the coolest portion of the quartz cell being lined up with a hole through the furnace for viewing the condensation of the zinc vapor. A t its dew point, zinc condensed in small droplets. Re-entrant tubes sealed into both ends of the quartz cell carried thermocouples for measuring the sample and dew point temperatures. Refinements of this method were made by I'rost and Naskrey,' who employed a photoelectric cell and determined the dew point temperature by a decrease in the light transmittancy in the region of condensation. Burgan, Hall, and Hehemanns sealed tungsten probes into the cooled area so that when the dew point was reached, the condensing metal closed an electrical circuit. This paper describes a modified method of deteimining the dew point based on the detection of the y-radiation from a radioisotope of the condensing metal. The results of the measurements on the alnminum-zinc system are presented. Experimental Materials and Apparatus.--Zinc-65 nativity W I L ~obtained by irradiating I h n k e r Hill slab zinc in the CPd ieiwt~orat the 14) R. R. T. Frost and J. T. Msskrey. Great Britiin Atomic Enemy Re~rarctiFahblishni~nfRpport AERE hf/R 1898, Hnrrrll. Bcrka. Endand. 1956. ( 5 ) B. R. B a r ~ a n R. , C. Ilnll. and R. E. Hebemann, J . In.1. Melala, 80. 413 (1952).
Argonne Niit,ion:d 1 , : h m t w y t r i :m :wtivit,y r i f : h u t , 3fl inc. pcr g . of zinc. With ita n i n d r -,-my ( I 2 \Iw.I :xnd ils long h:df-Iife U 5 t l daw). Zn'5 is a vel.? cmvcnient activity t u use i n I l w apparatus. The aluminum metal used in t h e experiments u'u.i not,ch-hnr ingot uhtainrd f n m t h e Aliiiiiiniiiii ( b .of .4nwriw. It, w:u1 spei:ilied by t.lir m:iiuifartuwr 11) IH. !I!I.!I I-";diiiuiniin). r, 1 he nujor impurity w ~ qi i i m w t i n i : i t v t l 11, IN? pwwnl i n thr :mount uf 2Olt-5110 p.p.in. The experimental sct,upi ~ r dto niwmre the zinc vnpm prcssur,~ is shown srhernxticnlly in Vig. 1. The experiments wew performed in B quartz cell 1 in. in diameter and 11 in. lung. T h c t top of the cell w:u provided with two small reentrant tubes. One extended to the hottom of t,he cell and contained a thermor.r,iiplc for measuring the sample temperature. The other terminxtecl in a 0.375-in. diameter eondcns:Ltim bubble at thc top nf the cell and served m an inlet for c d i n g gas well as .z thermonnilh well for continuous monitoring of the bubble temperature. The furnace had three separate sets of windings to insure good temperature control in all regions. In addition, it was fitted with a stsinless steel sleeve which surrounded most of the dew point cell for the purpose of leveling out temperature gradients. It w ~ neewary s to provide B window in the fumnee easement for pmitiuning the dew point rell prior to taking data. I n some experiments this opening was stopped with it firehrick, quartz plug or a metal radiation shield or fitted with R set of furnace windinga. These precmtinns appeared to have no eRect on the data obtained. The counting apparatus consisted of B Nuclear Chicago DS-5 standard scintillation detector probe with a I-in. NsI (TI) crysthl connected to x Xuclear Chicago hIlRGPsrnlingunit. The C r P tal was fitted with II water-ewled lead jacket and eollimnt,irr, and the detector pmhe was positioned ronrially with the olloning in the furnnre urnenlent and placed so that the crystal was nhmt 14 in. away from the condensation bubble. The crystal wits shielded from ?-radiation from the alloy by its jnrket, and additional lead shielding was provided hetween tho hottom of the quartz cell and the rr,mtal. Procedure.-Approrimntely 1 g. of zinc rontaining Zn" and an sppmprintr qu:rntit,y of xluminom were put, into i i tantalum erurihlo and plnred in the quartz cell. The top of the cell was sealcd on and the assembly wm evacuated to :*IO p thruugh a quart%tube eonled int,o the cell. The system w'm then flushed with helium. This prmednre WIIIR repeated two or three times and followed hy B find evsrunt,ion :niter r h i r h the tuhe was sealed oR. The furnxre m e m b l y was heated t o above the melting point of aluminum and held at this temperature tu allow the zinc and aluminum metals t,o nlloy. The furnrire was then cooled to :L suitahle temperature for taking data. When the system had come to equilihrium, n crmtnilled stream of air was allowed tn pass into the gas inlet tuhe. This wm adjusted so that, BR the dew point temperature was approached, the condensation huhhle was being cooled at the rate of I - P I min. or less. Simultaneotlsly the counting rate was recorded in 30-see. intervals. When the dew point wils reurhed, an increme in counting rate of 30% or more was observed. Indusion of a rate meter or recorder in the counting system did not inmeme the precision or sensitivity of the method. The apparatus was first tested with pure zinc. The dew point was observed to orcur randomly 0.5-1.75' below t,he sample temperature. The temperature of the sample iind the initial difference between the sample and bubhle temperatures had no apparent eflert on the temperature dillwenre when condensation occurred. The pressure range over which me:rnsurements were made was from l a 6 than 1 to above 100 mm. MeRsurements rere generally made by starting at a low sample temperature and proreeding to higher temperature. However, as ?heck on the attainment of equilibrium, the sample temper;iture w : oec:minnnlly ~ Iiwered for one or two measurements.
Results In the dew point method, only two sets of measurements were made, the temperature of the sample and the dew point temperature. The dew point temperature data were converted to logarithm of pressure by use of Kelley's eqnation for the vapor pressure of pure 7IillC.l
VAPORPRESSURES OF BINARY ALLOYS
Dec., 1963 log P i , + , , )
=-
9.843 - 6754.;/T -- 1.318 log T -
6.011
x
10-5T (I)
Chiotti and Gill7ha\-e compared this expression with the following one derived from heat capacity data revised by Kelley8 and data of Stull and Sinkeg for the enthalpy and free energy of vaporimtion and sublimation of zinc. log P i a i m )
-6678
= ___ -
T
1.274 log T
+ 9.568
(2)
These two squations agree very closely over the temperature range of interest. Data given for particular temperatures in the recent compilation by Hultgren, et al., lolead to vapor pressures 3-47, lower than those from eq. 2. However, since the method used in this work involves measurements of r’elative quantities the error introduced by the use of slightly different equations would be much less than this percentage. Hence the explicitly stated equation of Kelley, eq. 1, was used. These vapor pressure data were then expressed as empirical eq,uations relating the logarithm of the partial pressure to the temperature of the sample. Two-constant equations of the form log P
=
A/T
4-B
evaluate accurately the errors in the calculated activities. For this reason and because the partial molal quantities of the aluminum would in the usual treatment have to be evaluated by a graphical integration of the Gibbs-Duhem equation, it was felt desirable to fit the data to an analytical expression relating activity and mole fraction. Such an expressioii has been proposed for “regular” solutions’l l 2 consisting of two molecules of similar size. In view of the fact that the molar volumes of zinc and aluminum differ by less than 9%, it was decided to fit the data to the regular solution model. By this definition a regular solution is one obeying the relation In y1
Z I N C VAPOR I’RESSURE AS
FOR
VAl%IOT:S COMPOSITIOXS IS THE h L V M I N t 3 - ~ 1 N C SYSTEM
+
Mole 70 zinc
9.3 21.1 33.5 33.8 44.2 49.1 56.0 56.7 61.7 70.0
log P = A / T B log pressure in atmospheres Temperature in OK.----A €I
-7889 -6667 -6571 -6167 -6533 .-6308 -6000 -6188 -6394 .-6089
f 95 f 64 i 100 f 161 f 63 f 60 i 312 f 92 f 34 zt 49
*
6.146 0.094 5.239 5 ,067 5.271 i ,106 4.907 3: ,165 5.317 f ,068 5.141 f .062 4.881 4, ,349 5.063 f ,116 5.289 f ,036 5.026 zlz ,052
Temp. range, OK.
960-1065 782-1079 896-1051 929-1055 853- 995 894-1071 867- 952 823-1000 824-1043 855-1036
Thermodynamic Calculations.-Thermodynamic activities of zinc in solutions of various compositions were calculated over the temperature range 900--1050°K. from the vapor pressure d a h as expressed by the equat,ion given in Table I and eq. l,. It is difficult t o (6) K. K. Kelley, U. 9. Bur. Mines Bull. 383, 1935. (7) P. Chiotti and K. J. Gill, Trans. Met. SOC.4 I M E , 221, 573 (1061). (8) K. K. Kelley, U. S.Bur. Mines Bull. 584, 1960. (9) D. R. Stull and G. C. Sinke, “Thermodynamic Properties of the Elements,” American Chemical Society, Washington, D. C., 1956. (10) R. Hultgren, R. L. Orr. P. D. Anderson, and K. K. Kelley, ”Selected Values of Thermodynamic Properties of Metals and Alloys,” John Wiley and Sons, Ino., New York, N. Y., 1963.
=
aXZ2
(3)
in which a is a characteristic parameter reflecting the deviation from Raoult’s law. Values of a! were calculated a t 25’ intervals from 950 to 1050’K. for each of the ten alloys measured, using the relation y1 =
P/POX,
eq. 3, and obtaining P from the data listed in Table I. The average value of a was determined at each temperature as was its standard deviation with the results shown in Table 11.
were utilized since the accuracy of the data did not warrant a more complex treatment. Zinc-aluminum alloys of ten different compositions varying from 9.3 to 70.0 atom yo zinc were studied with the apparatus. All measurements were made in the liquid region. Samples of sufficient size were utilized and a suitable temperature range for measurements selected so that there was generally no concern over depletion of zinc in the condensed phase. A summary of the results obtained is shown in Table I. TABLE I .4 FCNCTION OB TEMPERATURE
2797
VALUESOF
CY
TABLE I1 BETWEEX 900 ALII 1050°K
T e m p , OK
900 925 950 975 1000 1026 1050
a
Oa
0 427 461 497 546 570 626 666
0 089 070 053 038 030 026 032
For a given alloy the curves of 01 us. 1/T were straight lines over this temperature range. Howerer, there were large variations in the slopes of these lines for different alloys. In order to obtain an average value for this slope, da/d(llT), which could be applied to the system as a whole within known limits of precision, the equations of Table I and eq. 1 were used. Since from eq. 3 P
da!
x*2-
d(l/T)
d In P d In Po - ___ - ___ d(l/T)
d(l/T)
Thus da
q$Fj
x22
== 2.303(A
- 6754.5
+ 6.011 X lO-5TS) + 1.318T
The terms in T and T 2do not introduce a change in dol/ d(l/T) that is appreciable in comparison with other errors, so that an average temperature could be used for the evaluation of da/d(l/T), resulting in a value of - 1530 i 684 (OK). for this quantity. (11) E. A. Guggenheim, “Mixtures,” Clarendon Press, Oxford, 1952. (12) G. N. Lenis and hl. Randall, “Thermodynamics,” 2nd Ed,, revised, McGraw-Hill Book Co., Inc., N e w York, N. Y., 1961.
GEORGEJ. LUTZAND ADOLFF. T'OIGT
2798 1.01
I
I
I
I
I
I
I
I
Vol. 67
TABLE IIr THERMODYNAMIC ACTIVITIESAND FREEENERGY, EKTHALPY, AKD ENTROPY OF MIXINGIN
THE
ALUMINUM-ZINC SYSTEM AT 950OK. Asmix,
XZIl
0.1 .2
.3
.4 .5 .6 CI
. I
.8
XZn Fig. 2.-Zinc activity in Zn-A1 alloys a t 1000°K.: - - - -,ideal solution; 0,activities from vapor pressures of individual alloys; , activities calculated from regular solution model.
The values of a from Table I1 and this value for da/ d( 1/T) were used to calculate the thermodynamic quantities a t 0.1-mole fraction intervals across the system. The activities of both components were determined from the values of a and eq. 3. The results of these calculations for the system at 1000°K. are shown in Fig. 2. The points represent the activities calculated directly from the vapor pressures while the solid line is based on the regular solution model with a at 1000°K. = 0.570. The free energy of mixing can be defined as that of an ideal solution, AF,id = R T ( X 1In XI 4-XZIn X,), plus the excess free energy, A F m ex = RTa(XlX2). From the relation
.9
aAl
0.904 1,001 .816 f ,002 .732 f ,003 .650 f .005 .567 1 ,007 ,479 f .008 .383 f ,009
,276 f ,011
.150 f ,006
cal./mole
cal./mole
-529 1 9 - 793 f16 -955 f20 ,- 1043 f23 - 1073 f24 - 1043 1 23 - 956 1 20 - 793 116 - 529 f9
-273 f122 - 486 f217 - 638 1285 - 730 f326 - 760 f340 - 730 f326 -638 f285 -486 f217 - 273 1122
cal./deg.mole
0.27 f .13 .32 1 .23 .33 f .30 .33 f .34 .33 k .36 .33 f .34 .33 f .30 .32 f .23 .27 f .l3
Asmix,
XZn
W n
aAl
0.1
0.171 f ,004 ,306 f ,006 .416 f ,007
0.907 f ,001 ,822 f .001 ,743 f ,002 ,668 f .004 ,591 f ,005
.2 .3 .4
1509 f ,006
.5
.59l f .005 .668 1 ,004 .743 f ,002 ,822 ,001 .907 f .001
.7
.8
x2'
0.150 f .006 ,276 f ,011 ,383 f ,009 .479 f .008 '567 f ,007 ,650 ?C ,005 ,732 f ,003 ,816 f ,002 ,904 f .001
4Hmixv
TABLE IV THERMODYNAMIC ACTIVITIES AXD FREE ENERGY, ENTHALPY, AND EKTROPY OF MIXINGIN THE ALUMINUM-ZINC SYSTEM AT 1050°K.
.6
it follows that AH, = RXIXz da/d(l/T). Calculated values for the activities and free energies, enthalpies, and entropies of mixing a t 950 and 1050'K. are given in Tables I11 and IV. The standard deviations were determined from the relations UAP, = R T X ~ X ~ U , , U A H , = R X l X z U ( d a / d ( l / T ) ) , and Q h'l Ul = UCi. It is inherent in the model that the values are highly symmetrical. Also, the fact that the system is quite close to ideal results in low values of the thermodynamic changes which occur on mixing so that the uncertainties are relatively large. As mentioned before, in lieu of a reproducible temperature dependence for da/d( 1/T) this was taken as constant over this temperature range, resulting in the same values for AH, at both temperatures. The results of these calculations can be compared to those tabulated by Anderson in Hultgren, et U Z . , ~ ~but the comparison is not particularly meaningful since the temperature used for those calculations is 750°K., considerably below the range of the present measurements.
azn
4Fmim
.9
*
,609
f ,006 .416 f ,007 ,306 f .006 ,171 f .004
4Frnixv
4Hmixt
cal./mole
cal./mole
cal./deg.mole
- 553 f 6 - 822 fll - 983 ?C 14 - 1071 517 - 1099 f17 -1071 f17 -983 f14 822 fll - 553 f 6
-273 fS22 - 486 f217 - 638 1285 - 730 1326 - 760 1340 - 730 f326 - 638 1285 -486 f217 - 273 1122
0.27 f .12 .32 f .21 .33 f .27 .32 1 .31 .32 f .32 .32 f .31 .33 f .27 .32 f .21 .27 f .12
-
Also he does not consider that the measurements on which his calculations are based have much accuracy. The activities listed by Anderson are higher by up to BOT0than the present values; his AF values are about half the magnitude of these. He does not list values for AH or A 8 for liquid alloys.
Conclusions The modified dew point method in which a radioisotope of the volatile metal is present in the alloy is SUCcessful in giving consistent results which are useful in calculating thermodynamic properties. The tracer involved, like Zn", must include in its decay process an energetic y-ray. However, ?-radiation with somewhat lower energies would make shielding to reduce background easier, without reducing the desired signal.
KIXETICSOF FLVORIKATION
Dee., 1963
The main advantage of the method over a visual technique would appear to be the objectivity in ascertaining the dew point. The sensitivity of the method-that is, the minimum amount of condensate that can be observed-is probably about the same as for a visual method. With other tracers, higher specific activity, and different geometrical arrangements it
2799
might be possible to increase the sensitivity beyond that achieved in this study. Acknowledgments.-The assistance of D. L. Haes, W. A. Stensland, and R. G. Clark in building and maintaining the apparatus is gratehliy acknowledged. Valuable discussions were had with Dr. P. Chiotti on the interpretation of the results.
KINETICS OF FLUORINATION. I. ADDITION OF FLUORINE TO 2,3-DICHLaROHEr;AFLUOROBUTEhT]E-2l BY ALANS.RODGERS Contribution No. $7’4 from She Central Research Laboratories, Minnesota Uining and Manujacturiiig Company, St. Paul, Minnesota 55119 Received July IW, 1963
+
The rate of the reaction, CF3-CCl=CCl-CFs Fz + CF3-CFCl-CFCl-CF3, was investigated in the gas phase a t temperatures between f15 and -20” with reactant concentrations between 1 and 10 X lo-* mole/l. The reaction was found to be homogeneous. Within this range of variables the rate of production of thn adduct was given by: c I [ C ~ F ~/dt C ~= ~ ]1.1 X 10” exp( -12,500/RT)[C,iF~C1~]1/~[F~]a/2 mole/l. see. The reaction mas found to be inhibited by oxygen. The experimental results are interpreted in terms of a chain reaction in which initiation occurs by the bimolecular reaction CF-CCL=CCl-CF3 Fz +CF8-CFClCC1-CF, F.
+
Introduction The quantitative aspects of the kinetics and mechanism of the reaction of fluorine with organic compounds has received very little attention despite a sustained intereat in the synthesis of fluorocarbons by direct reaction with fluorine as well as by indirect methods.lb It is generally accepted that fluorine reacts with organic materials uiu a free radical mechanism. Such a mechanism consists of initiation, propagation, and termination reactions. Thus far, most attention has been centered upon the propagation reactions for substitution. The relative rates of hydrogen abstraction by fluorine atoms has been determined for several alkanes.2-6 Recently the absolute rate of reaction betwelen fluorine and carbon tetrachloride6 has been determined. The results indicated that the dissociation and recombination of fluorine mas the initiating and terminating reactions. Addition reactions, however, have thus far been ignored. This is, therefore, the first study which has been reported on the kinetics of addition of fluorine to carbon-carbon double bonds. Experimental The apparatus consisted of two parts; one part was a conventional glass vacuum system equipped with an oil diffusion pump for handling condensable organic materials; the second part was a vacuum system fabricated from 0.25 in 0.d. Monel tubing ,joined with Monel Swageloks and containing Hoke M-440 bellows valves where necessary. This part was equipped with a rough vacuum pump protected by a soda-lime trap and was used in all operations involving fluorine gas. These two systems were joined by means of a Swagelok fitting using Teflon ferrules. -__
(1) (a) This research was supported b y the Advanced Research Projects Agency under Contract NOrd 18688 and was monitored by the Bureau of Naval Weapons; (b) J. R I . Tedder, “Advances in Fluorine Chemistry,” Vol. 11, Butterwortha Publishing Co., London, 1961, pp. 104-138. ( 2 ) G. C. Fettis, J. H. Knox, and A. F. Trotman-Dickenson, J . C h e m Soc., 1064 (1960). (3) P. C. Anson, P. S. Fredricks (in part), 2nd J. M. Tedder, zbid., 918 (1959). (4) P. S. Fredricks and J. M. Tedder, ihid., 144 (1960). ( 5 ) P. D. Mercer and H. 0. Pritchard, J . Phys. Chem., 68, 1468 (1959). (6) D. T. Clark and J. M. Tedder, 2nd International Symposium on Fluorine Chemistry, July, 1962.
+
Similar fittings were used for all other metal-to-glass connections. After the metal parts were passivated with fluorine and the entire vacuum system outgassed, a pressure of 1 X mm. was readily obtained. The addition of fluorine to 2,3-dichl~rohexafluorobutene-2was followed by observing the pressure change during the reaction. This was accomplished by means of a pressure transducer (variable reluctance type) from the Pace Engineering Corp., Model CP-53. The output of the transducer, 0-5 v. over the range of 0-760 mm., was fed into a voltage divider and zero suspression circuit, then to a Sargent strip chart recorder, Model SR. The instrument was carefully calibrated with a cathetometer from 1 to 25 mm. and was accurate to =!=0.2 mm. in this range. From 50 to 700 mm. i t was accurate to i l mm. However, the slope of the curve, pressure v s . millivolts output, in this range did not vary by more than 1%. Therefore, pressure differences (i.e., rate measurements) were accurate to fO.l mm. ( A P measurements were of the order of 10 mni.). Temperature control was maintained by a constant temperature liquid bath regulated by a Bayley on-off regulator, Model 231. The reaction system consisted of two flasks, a reaction flask:and a storage flask, connected by a Hoke C-416K cam-operated diaphragm valve. The entire system was immersed in the bath liquid. Thereby, each reactant was maintained a t the reaction temperature. The reaction was started by pressure expansion from the storage flask t o the reaction flask. The dead space (approximately 8 cc.) from the reaction flask to the pressure transducer, which was a t room temperature, contained only the diluent gas. The volume of the storage flask (glass) was 505 cc. The volumes of the reaction flasks were Teflon-packed reactor 148 cc., Teflon-coated reactor 153 cc., and glass reactor 180 cc. The fluorine was obtained from Matheson Co. It was passed through a sodium fluoride scrubber and stored in a 2-1. Monel flask. The fluorine was analyzed in the following manner. To a 335432. flask containing 75 g. of Hg, 0.082 g. of gas to be analyzed was added; this corresponded to approximately 120 mm. of pressure. After the reaction with mercury was completed the residual pressure in the flask was about 0.5 mm. Thus the fluorine gas is a t least 99.5% pure. 2,3-Dichlorohexafluorobutene-2was obtained from Hooke Chemical Co., dried over PgOs,and distilled into a gas storage bulb. Gas-liquid chromatographic analysis on an 18 ft. X 0.25 in. 0.d. column packed with 30% Kel-F pentamer (Minnesota Mining and Manufacturing Co.) on firebrickat 65’ showed that the cis-trans isomer ratio was 1:9. The structure of the isomers was confirmed by infrared and n.m.r. analysis. IIexafluoroethane was obtained from E. I. duPont de Nemours and
