Murray 1975 Hexano-Benceno

8/16/2019 Murray 1975 Hexano-Benceno

1/8

J. Chem. Thermodynamics1975,7, 839-846

A continuous-dilution device for themeasurement of static vapour pressures ofbinary liquid mixtures

R. S. MURRAY and M. L. MARTIN

Department of Physical and Inorganic Chemistry,University of Adelaide, Adelaide, South Australia

Received 0 March 2975)

A continuous-dilution evice or the rapid measurement f static vaponr pressures fbinary liquid mixtures as a function of composition s described. hese measurementstogether with computed xcess ibbs ree energies re reported or benzene n-hexaneat 298.15 K and compared with the results f Dunlop et al. “)

1. Introduction

In recent years continuous-dilution dilatometry(*) and isothermal-dilution calori-metry(3S4) have yielded highly accurate excess volumes and excess enthalpies ofbinary liquid mixtures. This paper describes the construction and operation of acontinuous-dilution device for making rapid and accurate determinations of staticvapour pressures and excess Gibbs free energies of binary liquid mixtures. The entirecomposition range is covered in two experimental runs. The method has been usedto determine the excess Gibbs free energies for benzene + n-hexane at 298.15 K.

2. Experimental

A diagram of the vapour-pressure apparatus is shown in figure 1. A Pyrex-glassvacuum line, interposed with Nupro bellows valves and Kovar glass-to-metal seals,is mounted in a rigid brass framework to which is fitted an adjustable three-pointsuspension for levelling purposes. A calibrated measuring burette MB of volumeabout 18 cm3 consists of two sections of Veridia precision-bore (4 mm, 8 mm) glasstubing with three fiducial marks along its length. A bulb B at the lower end of theburette branches down to a mercury reservoir and out to valve 8 through which theburette is filed with liquid from the degassing apparatus. The mercury reservoir isconnected via pulley-operated valve 6 either to a vacuum or to a nitrogen supplyso that the level of the mercury in the burette can be lowered or raised. A glass bowlsurrounding the bulb is fYled with ice or liquid nitrogen during distillation of liquid

into the burette. A 2 mm bore tube connects the upper part of the burette to pulley-operated valve 4 through which additions of liquid are made to the vapour-pressure


2/8

840 R. S. MURRAY AND M. L. MARTIN

cell C. For very fine control and avoidance of side thrusts on the valve 4, a brass barof adjustable length is interposed between the valve spindle and the pulley shaft.The cell contains a coiled platinum stirrer mounted below a small glass-enclosedmagnet which is activated by a solenoid, and is connected to the manometer con-structed of Veridia precision-bore (20 mm) tubing and mounted over a mercuryreservoir. Mercury levels in the manometer are adjusted as described for the burette

FIGURE 1. Vapour-pressure apparatus. Manometer M constructed from 20 mm Veridia precision-bore tubing with fiducial marks F4, F5; cell C, capacity 50 cm3 containing liquid of known massintroduced through valve 2; platinum coil stirrer PS connected to a sealed glass tube containingsoft iron and lifted upwards by solenoid S; 12.7 mm Nupro stainless-steel bellows valves, 0, 1 to 3;and 6.35 mm Nupro stainless-steel bellows valves , 0,4 to 8, with connexions to the valves throughKovar glass-to-metal seals using zytel or teflon ferrules and Swagelok fitt ings; pulleys Pl to P3to operate valves 4 to 6; mercury reservoirs Rl and R2; Dewar D raised with rod R to surroundcell, ice water added through Dewar funnel DF; calibrated measuring burette MB constructedof 4 mm and 8 mm Veridia precision-bore tubing with fiducial marks Fl to F3 ; bulb B surroundedby glass bowl GB, liquid introduced from degassing apparatus through valve 8.

Calibration (by nitrogen compression) of the volume of the cell between fiducialmark F4 and valve 4, about 70 cm3, enables corrections to be made for vapourspace. When necessary a small Dewar can be raised with a rod to cool the cell andits contents.

The method for degassing liquids has been described by Dunlop et ~1.c~) Usingthe apparatus shown in figure 2, liquid is distilled from the storage flask SF intoflask .F which is cooled with liquid nitrogen, With continuous pumping, the frozenliquid is slowly sublimed on to the liquid-nitrogen-cooled cold finger. Although onesublimation completely degasses most liquids the procedure is always repeated.

The measuring burette is filled with liquid as follows : with liquid in the flask belowclosed valve 9 the degassing apparatus is connected to the vapour pressure apparatusthrough valve 8 and both sections are pumped down overnight on the bench to a


3/8

CONTINUOUS-DILUTION VAPOUR-PRESSURE APPARATUS 841

pressure of less than 1O-3 Pa. After closing valve 8 about 25 cm3 of liquid 1 arethoroughly degassed and then distilled through re-opened valve 8 into the bulbunder the burette with the mercury level slightly below the glass bowl which is filledwith suitable coolant. Valves 4 and 8 are closed and, after thawing, liquid 1 is forcedinto the burette by raising the mercury level so that it is finally contained underpositive pressure between valve 4 and the mercury meniscus which, ideally, isjust

to valve 8

“=?l

highvacuum

FIGURE 2. Degassing apparatus. Storage flask SF containing liquid and drying agent, sealedwith Nupro valve; 6.35 mm Nupro stainless-steel bellows valves , 0, to 13; cold-fingerCFto holdliquid nitrogen; flask F to contain liquid distilled from SF: Pirani gauge head P: tran T to removevapour used in flushing the apparatus; glass T-tubing connects valves-10 and 11 to-valve 8 of thevapour pressure apparatus, replaced by glass U-tubing when degassing the liquid to be distilled intoa weighed ampoule or flask; weighed break-seal ampoule A with 6.35 mm Kovar glass-to-metalseal for attaching to valve 13. (For connexion to valve 2 of the vapour-pressure apparatus the 6.35 mmglass-to-metal seal is removed from the reweighed filled ampoule and a 12.7 mm Kovar glass-to-metal seal is joined at the break-seal end of the ampoule.)

below fiducial mark F2. Any excess liquid is bled off through valve 8 before thedegassing apparatus is detached. The vapour-pressure apparatus is then transferredto a thermostatted bath to test the effectiveness of the degassing procedure. Afterre-connecting the apparatus to the pumps with flexible stainless-steel tubing andre-evacuating, the mercury is raised to suitable levels in the manometer arms and asmall volume of liquid 1 is admitted to the cell until the mercury meniscus in theburette rises close to the fiducial mark F2. The vapour pressure of liquid 1 is measuredas described below. The apparatus is then removed from the bath.

The following procedure is used to admit pure liquid 2 of known mass to the vapour

pressure cell: with valves 10 and 11 connected with glass tubing, about 25 cm3 ofliquid 2 of known mass are degassed and then sublimed into, and finally sealed in,


4/8


either a small weighed flask with attached Nupro valve or a weighed break-sealampoule attached to valve 13. The mass of liquid 2 is determined from accurateweighings, to +O.OOOl g, corrected to vacua. After attaching the ampoule or flaskto valve 2 of the vapour pressure apparatus and thoroughly evacuating, the liquid isdistilled into the cell cooled with liquid nitrogen. The frozen liquid is pumped onbriefly before raising the mercury level into the manometer arms and then allowingthawing to proceed.

The loaded vapour-pressure apparatus is placed in a vibration-free thermostattedbath, reconnected to the pumps, and levelled. After temperature equilibration thepositions of Fl and F4 and all mercury menisci are measured with a 1 m cathetometer(Precision Tool & Instrument Co.). To make an addition of liquid 1 the cell is firstcooled by adding ice + water to the suitably positioned Dewar and then a smallquantity of liquid is admitted from the burette through valve 4. The ice waterremains in position in the Dewar due to its density and by cooling the cell contentsobviates large pressure changes if the added liquid has a vapour pressure very differentfrom that of the liquid in the cell. (This procedure is also used to bring back into thecell any liquid which has condensed on the mercury surface in the manometer, aproblem which occurs with pure liquids and sometimes with mixtures.) After tem-perature equilibration the mercury level in the burette is recorded and the levels inthe manometer are read off until, with periodic stirring, the difference is constant.These three readings together with the known mass of liquid in the cell give thecomposition of the cell contents to a precision of better than +0.0002 in the molefraction and the vapour pressure to within 54 Pa. Further additions of liquid aremade to complete the remaining half of the mole-fraction range. At the conclusionof the run the contents of the cell are distilled into a weighed flask with attachedNupro valve to test for quantitative transfer of liquid 2 and accuracy of buretteadditions. The second half of the mole-fraction range is covered by a second series ofmeasurements with the positions of the liquids reversed.

Bath-temperature control to better than + 0.002 K is achieved by use of a thermistorbridge and the feedback from a sensitive chart recorder to activate a thyratron relayand a 100 W blackened light globe. The absolute bath temperature is measured toabout f 0.003 K with a pressure-insensitive bomb-calorimeter thermometer calibratedcarefully and frequently against a Leeds and Northrup platinum resistancethermometer.

3. Materials

“Univar” A.R. grade benzene and Merck “Uvasol” spectroscopic grade n-hexanewere purified by methods previously describedc5, @ and stored over clean sodiumwire. Impurities of not more than 0.002 and 0.02 mole per cent for the respectiveliquids were revealed by gas-chromatographic analyses using 10 m columns packedwith 10 mass per cent of didecyl phthalate or squalane on AW-DMCS Chromosorb Wat 333 K in an Fl 1 Perkin-Elmer gas chromatograph with flame-ionization detection.

Samples were degassed and stored in flasks SF with attached Nupro valves (figure 2)over B.D.H. type 4A molecular sieves which had been previously outgassed and dried


5/8

CONTINUOUS-DILUTION VAPOUR-PRESSURE APPARAT US a43

at high temperature in vacua. These sieves were chosen for their extreme affinity forwater coupled with their ability to be degassed, a property not possessed by sodiumwire. Both liquids were degassed, as described above, just prior to use.

The condition of a liquid to be used for vapour pressure measurements was evalu-ated by measuring the equilibrium vapour pressure in the cell, then sweeping awaythe vapour into a cold trap and retesting the remaining liquid. (The manometer levelswere lowered with liquid in the cell by first ballasting the high side of the manometerwith vapour from the same liquid contained in a flask attached to valve 2.) Thisprocedure produced no observable changes in benzene but successively reduced thevapour pressure of n-hexane to a constant value indicating the presence of traces ofeither air or volatile impurity in the sample. Samples of these liquids of similar purityhave previously been used for related work in this laboratory at which time theirdensities were determined accurately and are in accord with those in reference 1 towithin &2x low5 g cme3. The masses of benzene and n-hexane injected from theburette during a run were computed from these densities.

The vapour pressures of benzene (12.683 kPa) and n-hexane (20.153 kPa) at298.15 K are in good agreement with literature values corrected to IPTS-68.‘l’

4. Results and discussion

Excess Gibbs free energies GE(~l) were calculated from measured vapour pressuresusing the method of Barker. (‘) A vapour phase correction was applied in order toobtain the true composition of the liquid phase. A computer program was usedto calculate GE&) and yl, the vapour phase composition, from an initial estimateof x1 which ignored the vapour phase entirely. This value of y1 was then used tocompute a new x1 and the iterative cycle continued until convergence was obtainedin both modes, i.e. until successive values of x1 showed negligible changes while thesum of the squares of the pressure residuals was minimized to give a least-squaredfunction of the form:

GE(xl)/RT =~1x2 I;i Ui(Xl-~z)i. (1)

Addition of extra terms ai and the introduction of a skewing parameter K in equa-tion (2) due to Myers and Scott,(*)

GE(xl)/RT = ~1 x2 Z, Ui(Xl -~z)i/{ 1-K(X, -x2)}, (2)

produced no further significant reduction in the sum of the squares of the pressureresiduals.

Table 1 lists the experimental vapour pressures for two series of experiments atcalculated mole fractions x1 and y1 of n-hexane for the liquid and the vapour phases,as well as the activity coefficients of n-hexane and benzene in the liquid phase andexcess Gibbs free energies at each mole fraction. The second virial coefficients of thevapours used in calculating GE are those contained in reference 1.

In table 2 a comparison of the values of GE computed from the values of ni for bothruns in this work is made both against the originally reported values of Harris andDunlop(‘) and, more directly, against the values calculated from their data fitted to


6/8


TAB LE 1. Experimental results for n-hexane (1) + benzene (2) at 298.15 K

Sp = p(expt) - p(ca lc.)

Xl Yl plkpa Sp/kPa fi h P/J mol-1

Run 1

00.00390.01070.02120.03120.03980.06290.10790.14020.16430.19530.22470.25990.28530.31010.3320

*0.407s0.43240.45990.49470.53320.56670.6076

0.65020.70460.75930.82810.89780.93470.95910.98351

-0.01320.03520.06630.09360.11520.16700.24730.29360.32380.35850.38830.42060.44220.46240.47900.53510.55220.57150.59490.62110.64410.6721

0.70180.74110.78240.83790.89830.93350.95770.9827

-

12.68312.80713.01813.31113.58413.80714.34815.24415.78016.13716.55916.90417.28817.53717.76117.96318.50918.67818.83419.04619.24419.40019.576

19.73719.90620.04120.15620.22420.12020.19420.17320.153

- -0.003 2.16810.008 2.13270.002 2.08090.004 2.03370.006 1.99520.001 1.8992

-0.003 1.7407-0.007 1.6459-0.006 1.5838

0.000 1.51250.005 1.45340.005 1.39130.003 1.35180.001 1.31680.017 1.2885

-0.003 1.20700.004 1.1851

-0.009 1.16280.004 1.13760.001 1.1132

-0.003 1.0946-0.001 1.0749

0.000 1.0575-0.003 1.0392-0.004 1.0249-0.005 1.0120

0.013 1.00400.002 1.0016

-0.001 1.00060.000 1.0001-

a, = 0.61990 al = -0.11672, az = 0.03304, a3 = -0.01375

E(SP,)~ = 0.050 kPa2

Run 2

0 - 12.683 -0.0133 0.0430 13.079 -0.0030.0287 0.0866 13.507 0.0000.0463 0.1304 13.958 0.0030.0872 0.2128 14.849 0.0010.1404 0.2940 15.784 - 0.0020.1966 0.3603 16.571 -0.0030.2590 0.4201 17.283 0.0030.3243 0.4739 17.880 -0.0030.3736 0.5106 18.274 0.0030.4094 0.5362 18.530 0.0060.4562 0.5688 18.825 0.002

*0.4569 0.5695 18.821 -0.0050.5271 0.6172 19.212 -0.0030.6277 0.6867 19.650 -0.009

-

l.1.00011.00051.00121.00191 SW451.01271.02081.02781.03821.04931.06411.07561.08761.09881.14171.15701.17471.19851.22621.25171.2845

1.32061.36981.42261.49361.56991.61151.63931.6674

-

2.10512.03451.95931.80711.64591.51131.39421.29901.24141.20571.16591.16531.11731.0672

-1.00021.00091.00241.00821.02031.03811.06301.09441.12121.14241.17211.17261.22131.2999

-7.5

20.539.857.772.5

110.6176.2217.0244.1275.2300.8326.7342.3355.2364.7384.,387.1387.6384.9377.8368.2352.3

331.2297.6256.6195.4123.0

80.951.621.1-

-25.152.882.9

146+216.3275.7325.6361.3377.9384.9387.6387.6379.3343.3


7/8

CONTINUOUS-DILUTION VAFOUR-PRESSUR E APPA RAT US

TABLE l-cuntinued

845

Xl

0.69110.74400.81350.86440.92240.97050.98961

Yl PI@ Gp/kPa A A2 GE/J mol-l

0.7314 19.885 0.007 1.0445 1.3552 307.30.7708 20.028 0.006 1.0297 1.4050 269.80.8260 20.152 -0.003 1.0153 1.4763 210.*0.8690 20.206 -0.003 1.0080 1.5333 160.10.9217 20.221 -0.003 1.0026 1.6038 96.70.9685 20.202 0.009 1.0004 1.6674 38.30.9888 20.169 O.OOG 1.oooo 1.6941 13.8

- 20.153 - - - -

a, = 0.61993, al =-0.11567, a2 = 0.03563, a3 =-0.00392

Z @ = 0.025 kFa2

TAB LE 2. Comparison of excess Gibbs free energies GE for n-hexane (1) + benzene (2)with the results of Harris and Dunlop(l)

Run 1 Run2 Harris and Dunlop”)

observed cake., equa tion (1)

Xl GE/J mol-1

0.09787 e 162.5 161.5 162.2 162.00.17675 257.1 256.3 257.6 257.9

0.29029 345.1 344.7 346.6 347.20.36452 375.7 375.5 377.a 377.80.37531 378.5 378.4 380.3 380.60.48754 385.7 385.7 387.0 386.50.59742 356.7 357.0 356.8 355.90.69838 301.8 302.6 300.8 300.60.79959 222.0 223.5 220.6 221.60.90882 110.7 112.4 109.8 111.7

a Misprinted in the original paper.(l)

a four-parameter equation (1) at their mole fractions. The agreement is within experi-mental error which, from an analysis of the equations : ( ’ )

GE = x&+x,& (3)

P? = RTln(~~i/~ix,)+(Vi-Bii)(Pi-~)+~6ijYj”, (4)

is approximately +2 J mol-l taking into account the approximation 6,, = 0 whichis widely used and is discussed in reference 1. The external consistency is shown bythe measurements around the overlap region (marked by asterisks in table 1).

It is believed that this apparatus offers some advantages over continuous-dilutionmethods previously described. ~‘,~r) The relative simplicity of the cell ensures speedyequilibration after an addition is made and, because of its extremely small leak rate(less than 2 x 10V4 Pa mm-l), the vapour pressure of an enclosed liquid remainsstatic within experimental error for long periods. The “mercury piston” type buretteallows the introduction of rigorously degassed liquids and their subsequent storage inthat condition for indefinite periods.


8/8


As transfer and testing of degassed liquids in the apparatus require 1 to 2 d, whilemeasurements of vapour pressures for each half of the composition range take afurther day, a complete mixture can normally be studied in 5 to 6 d.

We are grateful to Mr J. Netting and Mr G. Duthie for the construction of theapparatus and to Mr M. A. Yabsley for assistance with the computing program.This work was supported by a grant from the Australian Research Grants Committee.

REFERENCES1. Harris, K. R.; Dunlop, P. J. J. Chem. Thermodynamics 1970,2, 805.2. Stokes, R. H.; Levien, B. J.; Marsh, K. N. J. Chem. Thermodynamics1970,2, 43.3. Stokes, R. H.; Marsh, K. N.; Tomlins, R. P. J. Chem. Thermodynamics1969, 1, 211.4. Ewing, M. B.; Marsh, K. N.; Stokes, R. H.; Tuxford, C. W. J. Chem. Thermodynamics1970,

2, 751.5. Bell, T. N.; Cussler, E. L.; Harris, K. R.; Pepela, C. N.; Dunlop, P. J. J. Phys. Chem.1968,72,4693.

6. Shepard, A. F. ; Henne, A. L.; Midgely, Jr., T. J. Amer. Chem. Sot.1931, 3, 1948.7. Barker, J. A. Aust. J. Chem.1953, 6, 207.8. Myers, D. B.; Scott, R. L. Ind. Eng. Chem.1963, 5,43.9. Scatchard, G.; Raymond, C. L. J. Amer. Chem. Sot. 1938, 60, 1278.

10. Tomlins, R. P. Paper presented at the Fifth National Convention of the Royal Australian

11.Chemical Institute, Canberra, 1974.

Gibbs, R. E.; Van Ness, H. C. Ind. Eng. Chem.1972, 1, 410.

Documents

Murray 1975 Hexano-Benceno