SOTES
April, 1963 tures in the range of interest. Th.e experimental vapor densities fall on a smooth curve which is best represented by the equation obtained by the method of least squares D g . / ~ m= . ~0.2072 - 6.182 X 10--4Tt 5.043 X 10-’T2
(OK.)
The probable error is &0.0007 g . / ~ m . ~ . A summary of t:he smoothed results, together with several derived quantities, is shown in Table I. The vapor pressures were calculated from Horiba’s experimental data,3extrapolated as necessary. TABLE I DENSITY,AT’OMIC VOLUME, AND COEFFICIENT O F CUBICAL EXPANSION OF LIQUIDARSENIC; VAPORDEKSITYAKD VAPOR PRESSURE
--D.
Liquid---
T, OK.
g./lcm.n
At. vol., cm.a
1090 m.p. 1150 1200 1250 1300 1320
5.22 5.19 5.16 5.13 5.10 5.09
14.35 14.44 14.52 14.59 14.67 14.70
Coeff. of cubical exp., T-1 X 100
Vapor density. g./cm.s
Vapor pressure, atm.
102.5 103.1 103.7 104.2 104.8 105.0
0.1326 .1632 .1916 .2224 ,2558 .269g
35.8 46.8 57.6 69.2 83.2 87.1
The experiments were carried out in a specially constructed furnace equipped with observation ports and three separate resistance windings. Temperature measurement was accomplished by three Chromel-Alumel thermocouples. The arsenic used was supplied by Penn Rare Metals, Inc., and had a purity of 99.99%. The major impurities were O . O O l ~ o Cu, O.OOl~oFe, 0.004% Sb, and 0.002% Si. No detectable reaction occurred between the arsenic and the Vycor glass. However, several tubes burst due to the high vapor pressures (about 90 atm. a t 1323’K.) and extension of the measurements to higher temperatures was not considered feasible with Vycor tubes. The design of the furnace incorporated several features to minimize the danger from bursting tubes. Arsenic is a very interesting element from the standpoint of allotropy. It exists in several solid modifications of which the gray, or metallic, form is stable at ordinary temperatures. On the basis of its physical properties Klemm4 has termed it a “Halbmetall.” Liquid arsenic is opaque but the vapor in equilibrium with it is yellow. Brewer and Kane5 examined arsenic vapor from a Langmuir type experiment with a mass spectrometer and found it to be predominantly Asd. Stull and Sinke6 state that according to the data of Brewer and Kane5 there is no appreciable concentration of the As2 species below 1000°K. The vapor densities obtained in this work are higher than those calculated from the ideal gas law assuming 100% As4 molecules. This corroborates the predominance of the As4 species up to 1323OK. JoliboisJ7who determined that liquid arsenic is opaque a t llOO°K., has suggested that liquid arsenic may change into a yellow transparent form (4) W . Kleinm, Angeu. Chem., 62, 133 (1950). ( 5 ) L. Brewer and J. 5.Kane, J . Phys. Chem., 59, 105 (1955). (6) D R. Stull and G. C. Sinke, “Thermodynamic Properties of the Elements,” -4dvances in Chemistry Series, No. 18,American Chemical Society, Washington, D. C., 1956. (7) P. Johbois, Compt. rend., 162, 1767 (1911).
925
AS^) a t some higher temperature. Rassow* has determined by direct observation that the critical temperature of arsenic is greater than 1400’ and presumably the liquid is still opaque a t this temperature since no information to the contrary is reported. At temperatures above 1700OK. a t atmospheric pressure, the As2 species predominates6 in the vapor but the pressure effect on the equilibrium is unknown. Dissociation to the monatomic species appears to be unfavorable even at 30OO0K. I n view of the fact that arsenic is a “Halbmetall,” it is not excluded that it may become nonmetallic a t higher temperatures. The critical temperature of arsenic is obviously higher than that of phosphorus, 993.8°K.,9 and probably less than that of antimony. The density of liquid antimony recently has been measured over a wide temperature rangelo and, in contrast to typical metals (whose density us. temperature behavior is in general linear up to the normal boiling point) shows significant curvature downward with increasing temperature. It is most likely that this deviation from linearity is due to structure changes in the liquid. I n view of their positions in the periodic table, it is reasonable to assume that arsenic will behave in a manner similar to that of antimony. That is, at higher temperatures the usual rectilinear diameter line mill begin to curve downward and significant structure changes may be expected. XOTEADDEDIN PROOF.--W~ have only recently become aware of previous measurements of the density of liquid arsenic. W. Klemm, H. Spitzer, and H. Niermann, Angew. Chem., 72, 985 (1960), report five points between 830 and 850”. These points show considerable scatter and it is not possible to establish a slope from them. H. Niermann, Dissertation, Miinster, 1961, reports six points between 771” (subcooled) and 960” which fall approximately on a straight line. Both sets of measurements are about 4% lower than our values. (8) €1. Rassow, Z. anorg. allgem. Chem., 114, 117 (1920). (9) W. Nlarckwald and K. Helmholtz, ibid. 124, 81 (1922). (10) &4.n. Kirshenbaum and J. $. Cahill, A m . SOC.Metals, Trans. Quart., 66, 849 (1962).
GASEOUS FLUORIDES OF XESO1C” BY M.4RTIN H.
~ T U D I E R4 N D
Chemistry Dzeision, Argonne .\-atzonal
ERICN. SLOTH
Laboratory, Argonna, Illinozs
Received September 88, 1962
The existence of xenon tetrafluoride, first reported by Claassen, Selig, and Malm of this Laboratory,2has been verified further by observation of gaseous XeF4 in the Bendix Time-of-Flight mass spectrometer. Their suggestion of the existence of a lower fluoride of xenon has been confirmed by the observation of the difluoride of xenon as an independent species. I n addition, a number of oxyfluorides of xenon were seen. The masses of the observed ions correspond to the formulas Xe +
Xe++
XeO +
XeF +
XeF + +
XeOF +
XeF2+
XeFz++
XeOF2+
(1) Based on work performed under the auspices of the U. S. Atomic Energy Commission. (2) H. €1. Claassen, H. selig, and J. G. Malm, J . Am. Chem. SOC.,84, 3593 (1962). (3) D. B. Harrington, “Encyclopedia of Spectroscopy,” Reinhold Publ. Corp., New York, N. Y.. 1960, pp. 628-617.
NOTES
926 XcFa+
XeFII++
Vol. 67
XeOFa+ __
The underlinrd ions prohahly arise from indrpendent neutral species whereas the others are primarily fragmentation product,s.
A
Fig. 2.--hlaqs
A
Xe*
spurlla of xriwn and rnrnwy.
vw
Xe+ Fig. X-MW
XeFl+ XeF2+
XeF3+ Hg* I Xe F4*
spectra d xenon fluorides. XC+ spectra dietortd bemuse of its very lnrgc sire.
XeF+
XeF2+
Fig. 4.--Xlars
spectra of XoF and XeF,.
A 300mg. sample of solid xenon tetrafluoride in a nickel weighing can was attached to the gas inlet system of the spectrometer. Ions produced by electron bombardment of the vapors were identified by their masses.and the characteristic xenon isotopic abundance pattern (Fig. 1). Since fluorine has only one stable isotope, this pattern is preserved in fluorides of xenon. The isotopes of xenon and mercury (Fig. 2) as well as hydrocarbon peaks (which occur at every mass from impurities in the spectrometer) were used to determine precise masses. The mass spectrum of gaseous xenon fluoride ions from the sample at room temperature is given in Fig. 3. It will be noted from Fig. 3 that the abundant Xe'23F4(mass 203) peak occurs one mass unit higher than that of HgzU4, the highest stable isotope of that element. With the nickel weighing can at a temperature slightly above -80" only the species XeF+ and XeF2+ were observed (Fig. 4). We conclude that XeFz exists as an independent species of greatc volatility than XeF4. The XeF, is probably a primary gaseous product since it has the highest mass of any fluoride observed. I n addition, variation of intensity ratios with electron energy indi. cates that XeF+ and XeFa+ were observed primarily as fragmentation products. The residue from which the bulk of a XeF, sample had been removed by vacuum distillation was analyzed in a similar manner with variations in temperature, vapor pressure, and electron energy. I n addition to the fluorides observed in the fint sample a number of oxyfluorides were present in the spectrum. All of the species listed above were observed. Although XeO+, XeOl"+, and XeOFt+ are formed by fragmentation of the higher oxyfluoridrs, some fractionation of XeOFJ+ and XeOV4+with respect to each other suggests that both XeOl"J and XeOFl may have an independent existence. The presence of XeOF, suggests the existence of XeF6. Acknowledgment.-We gratefully acknowledge the cooperation of C. 1,. Chcrnick, H. H. Claassen, H. H. Hyman, J. G . Malm, and H. Sclig, who supplied all samples.
