VAPORPRESSURE OF MERCURY FROM 250 TO 360”
July, 1855
58 1
THE VAPOR PRESSURE OF MERCURY A T 250-360O BY F. H. SPEDDING AND .J. L. DYE Contribution No. 375 f r o m the Institute f o r Atomic Research and Department of Chemistry,’ Iowa State College, Aines, Iowa Rewired September 9 , 1964
The vapor pressure of mercury was measured with a new-design isoteniscope in the region 250 to 3GO”. The vapor pressure fits the equation log,, P = 10.59901 3335.027/8 - 0.865372 log10 e ( l ) ,in which e = 273.160 t , with t in “C. (Int.)
+
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This Laboratory is currently engaged in the determination of some thermodynamic properties of rare earth-metal solutions. In the course of this study it was desired to measure the vapor pressure of mercury. The necessary apparatus was coiistructed for this purpose, arid included a new type of isoteniscope, arid a thermostat hath which controlled the temperature of a molten salt bath to f0.005° between 225 and 400 In order t o determine the accuracy attainable with this apparatus, the vapor pressure of pure mercury was measured a t a. number of temperatures between 250 and 360”. The design of the isoteniscope used in this research is being modified a t present to make tlie instrument more rugged. The values obtained for the vapor pressure of mercury were more self-consistent than any data previously reported, but the least-squares expression agrees very well with the values found by other iiivestiga t om. Apparatus.-The isoteniscope, Fig. 1, was constructed of Pyrex glass. The pressure-sensitive element was a 0.2 mm. thick glass diaphragm made in a glass-blower’s latha from a section of 45 mm. tubing. The platinum contacts were connected thi ough a relay system to solenoid valves which automatically regulated the pressure above the diaphragm to withiti about 5 mm. of the piessure in the system. The pressure of the system was determined more accurately by nianually adjusting the prepsure above thc diaphragm until the “break” point wa5 just reached. The external pressure -was then measured with a manometer and cathetometer system accurate to f 0 . 0 5 mm. The isoteniscope was constructed so that even with a vacuum in the system it required several millimeters of pressure above the diaphragm to reach the “break” point. This TO PRESSURE REGULATOR AND MANOMETER A C E
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CTAINLESS STEEL C P R I l l G CCNTACT
LIP
THIN GLASS . DIAPHRAGM SLIDING CON TACT
ISOTENISCOPE JACKET
PLliTINUEA
T O H i FLASK
Fig. 1.-Isoteniscope. (1) Work was perfortlied i n the Alnes Laboratory of the Atomic Enorgy Cotrtiiiission.
called thc zero correction a n d wts subtract,ed from all readings. The zero correction was determined a t various temperatures by a calibration run prior to the actual run with mercury. The correction changed by about 1 mm. from room t’emperatures to 300”. il 25-ml. flask was connected to t.he isoteniscope and also to a vacuum system. A number of sealed glass tips which could be broken by “m:Lgnctic hammers” were included to connect, the Hg flask with the vacuum system. This is especially important when the vapor pressure of an amalgam instead of pure Hg is being measured, because the composition of the amalgam can be changed by distilling Hg out, of the system without opening the system to the atmosphere. The thermostat bath and control circuit are shown diagrammatically in Fig. 2 . Two elements, each about 100 ft,. in length, were mound together around the steel core, one of #lS “Chrome1 A” wire for the heating current, and the other of #20 “Hytempco” wire for a resistance element. The windings were separated from each other and the steel core by 3 / / 1 6 in. porcelain fish spine insulators strung on the wires. \nts
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.. 5 0 OHM VARIABLE RESISTOR
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POWER
WINDING1
IliSULATION
‘:’GALVANOMETER
‘RESISTANCE
_ WINDING _ ~ P’ig. 2.--Thcrmostatic: h t h and ooiitrol circuit. -.
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The resistance windiiig was balaiicetl a t any desired bath tempeimatur: in a Wheatstone bridge circuit, thermostnted in oil a t 25 . Any unbalance of the bridge resulting from heating or cooling of the resistance winding of the furnace was detected by a galvanometer whose beam nctivated a photocell relay circuit. When closed, this relay circuit shunted out IL variable resistor in the power circuit, thus increasing the power input to the bath. Although the temperature of the air gap in t,he thermostat may have fluctuated because of the cycling, the large heat cn.pacity of t8he salt-bath compared to the air gap prevented the s:dt-bnth froni eshi1)iting vari:ttions in temperature due to the cycling. As n result of t,he stable control circuit and the large heat capacit.y of the salt-bath, the temperature oE the hath was constant, nithin =k0.005” for several hours and 10.01” for much longer periods. The thermostat was niounted on :tn elevator which permitted it to be raised and lowered. The isoteniwopn system remained fixed in position, with the source of heat 1,eing raised t,o it. The inner container of tlie thermost,at was made of nickel sheet, and contained a salt-bath mixture of NaNOa and Ihand Menzies employing a cubic equation for the t#herrnometer resistance taking the boiling point of mercury (4) A . Stnitli a n d A. K'. C. RIenzies, J . . A i i ~ . C h e i n . ,S'oc., 32, 1434 (1910). ( 5 ) .4. W. C. Menzies, 2. p h y s i k . Chein., 130,R O (1927). (6) T. B. Douglas, A . F. Ball and D. C. Ginnings, J . Reseaxi' NatI. Bur. Standards, 46, 331 (1951).
July? 1955
683
IlE.\CTIO.U BET\VEEN 0 - C H L O R O N I T R O B E N Z E N E .iND E T H A S O L . \ M I h ' E
fouiicl by Smith aiid Rlenzies ils a foiirt'h fixed point for thermometer calibratioii. I11 addition these authors have calculated the vapor pressure of mercury from thermal measurements on the liquid and estimates of the second virial coefficient of the vapor. Their results are given by their equation 35. Epstein and Powers' obtained an expression for the vapor pressure of mercury utilizing all available t.hermodyiiamic measurements which is intended to be valid from the triple poiiit to the critical point. Beattie, Blaisdell and Kaminsky3 measured the vapor pressure of mercury very accurately in the range 660 to 8GO mm. and with these measiiremeiitls fixed the boiling point as 356.580 O. The vapor pressures calculated from the expressions of the five investigators ment,ioned have been tabulated for rounded values of temperature from 250 to 360" in Table 11. The values of Smit,h and hlenzies are for the correct,ed temperature given by Douglas, Ball and Giiinings.6 It is seen that the agreement between 1-arious investigators is quite good. ,411 expi,essions must of course give the same value a t the normal boiling ( 7 ) L. F. Ellstein and 11. D. Powers, AECU-1640, Seiit. 1951.
TABLE TI THEVAPORPRESSURE OF MERCURY I N >IN. AT ROUNDED TEhIPERATIrRES ACCORDING TO \TARIOUS I SI'ESTIGATORS t
(OC.)
S a n d D.
250 200 270 280 290 300 310 320
74.41
330 340 350 360
90.40 123.00 157.17 108.02 247.11 3013.68 377.32 460.94 559.30 674.28 870.95
B., B. and Ierized.~-'Using larger excesses of ethanolamine and a temperature range of 57-82', the described product could be obtained i n 95% yield, indicating t,he suit8abilitlyof these conditions for the kinetic study. The course of this invest'igat.ion was patterlied after Swain's study of the displacement reaction of trityl chloride i n which kinetic evidence for tthe termolecular mechaiiism vas established.8 Experimental Starting Materials.-o-Chloionitrohmzene ( E . K. \\.liiLe Label grade) was recrystallized from pctvoleum ether twice to n pale yellow solid, m.p. 31.5'. Ethanolamine, ethylamine, n-l)ut,ylaniine, pyridine, methanol, et,hnnol, isopropyl alcohol, n-hesanol, 2-butosyethanol arid n-butyl etjher wei'e ( 5 ) P. Karrer, E. Sclilittler, I
