h LOW-PRESSCRE TENSIMETER* BY K. C. D. HICKM.4N
No simple relation has been found between temperature and vapor pressure for all substances.' Many of the proposed formulas involve physical constants, other than vapor pressure, and these may not be available. Thus Clausiusz requires the molecular heat of evaporation, for the completion of his equations; and Nernst3 the critical pressure. I t is therefore not always safe to extrapolate pressure curves obtained a t one temperature range for use a t another range. Particularly is this true when the apparent vapor pressure of the substance is increased by decomposition or dissociation.p We have for some time been interested in certain high boiling organic liquids for use in high vacuum work.j Of particular importance were t'heir vapor pressures a t the temperature of the laboratory and a t that of the commoner refrigerating media such as ice, ice and salt, and solid carbon dioxide. When these high boiling liquids were heated sufficiently to give vapor pressures measurable in the usual apparatus6 they showed decomposition, and the logarithms of the pressures, plotted against the inverse of the absolute temperatures, as recommended by D u ~ h m a n d.id , ~ not fall on lines sufficiently straight for extrapolation. I t was decided therefore to measure the vapor pressures at the lowest pressures that could be read conveniently. Little direct work on small vapor pressures has been recorded. Ramsay and Young8 read to about one millimeter of mercury, and Hertz, who worked to a small fraction of a millimeter, did not obtain accurate results. A11 t'he recent investigators on low vapor pressuresg have used indirect means involving determinations of rate of diffusion, viscosity, ionic conductivity, and so forth, which require assumptions concerning the structure and degree of association of the vapor. The apparatus now presented does not provide a satisfactory means for determining low vapor pressures. I t does, however, yield determinations in the region between atmospheric pressure and onetwentieth of a millimeter of mercury, and thus extends the readings to a lower limit than ordinarily recorded. Furthermore, it is quick and convenient in use. A direct measurement of vapor pressure demands that the measuring instrument shall be in contact with the vapor and at the same temperature as the vapor; or that the vapor shall meet a column of a permanent gas, such as air, so that' the pressure of the gas may be measured outside the apparatus. Now, the McLeod gauge is valuable as a manometer because, at pressures comparable with that of mercury at room temperature ( I ~ I O O Omm.), the mean free path of the gas molecules is sufficiently large for the molecules to * Communication
S o . 391 from the Iiodak Research Laboratories.
628
K . C. D. HICKMAN
diffuse into the McLeod gauge unhindered by the stream of mercury vapor which may be passing in the opposite direction. It is manifestly impossible to measure the pressure of a tenuous vapor in one vessel by measuring the pressure of a permanent gas in another communicating vessel, if the pressure is low enough for the gas and the vapor to diffuse independently and unhindered by one another. The method is valuable only when the molecules are sufficiently crowded for the gas to bar the entrance of the vapor almost entirely during the time of experiment. The practical question a t issue is a t what pressure this buffering action breaks down.
TRAP
FIQI
FIQ.2
Consider the system shown diagrammatically in Fig. I. The substance providing the vapor is placed in the bulb, V, in sufficient quantity for a nongaseous phase always to be present. We will suppose that the evaporating surface in V is large compared with the capacity of the pump, P, so that a saturated vapor is maintained in V even during the operation of the pump. At the commencement of evacuation the pressure in the system is much higher than the vapor pressure of the substance and the pressure falls nearly uniformly throughout. When the pressure of the substance is nearly reached the pump will be concerned almost entirely with the removal of vapor. No more of the permanent gas will stream from B into V, although a little will diffuse through the vapor column. The gauge will indicate the pressure of the vapor as long as sufficient air remains undiffused.
629
LOW-PRESSURE TENSIMETER
There are cases, however, in which the vapor pressure is very small, where molecular collision occurs so seldom that the distinction between stream flow and diffusion vanishes and the permanent gas in B can pass through V to the pump as though no vapor were present. The pressure a t which hindered flow gives place to unhindered diffusion varies with the nature of the vapor, the size of the apparatus, and the duration of the experiment. It occurs a t about 0.10mm. with mercury and a t about 0.05 mm. with most heavy organic substances, in apparatus of the dimensions suggested here. The transition is extraordinarily sharp, for complete obstruction may exist a t 0.08 mm. and complete breakdown at 0.04 mm.
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FIG.3 We have experimented with a tensimeter utilizing the pressure region before breakdown. The essentials are shown diagrammatically in Fig. 2 . A ?\IcLeod gauge, protected by refrigerant, measures the pressure in the bulb, A, which rests at room temperature. A larger bulb, B, which is immersed in an electrically-warmed oil bath, communicates with A. The bottom of the bulb, B, opens into a flat spiral tube, Y, which is half filled with the liquid under examination. At the other end of the spiral there is a two-way connection by capillary tubes with a small condenser which is itself connected by a third pipe to a vacuum pump. The upper capillary contains a magnetically operated ball valve and it is this device which controls the passage of vapor to the condenser and thus the exhaustion of the bulbs. The lower capillary is a return tube for condensed material.
630
IC. C. D. H I C K M A S
The use of the apparatus raised some interesting points. It was the general practice to heat the bath until the liquid developed a vapor pressure of about 4 mm. The vacuum pump was started and the fall of pressure was plotted against time until no further change n-as recorded. A curve such as ( a ) in Fig. 3 would be obtained.* The inability to reduce the pressure below 4 mm. suggested that the capacity of the capillary was used completely in conveying vapor to the condenser and that the capillary, T,was providing a region where the air flow from B was checked by the vapor from the spiral. The magnetic valve was now closed so that further evacuation of the system ceased. To our satisfaction no increase in pressure was recorded a t the gauge. The supply of vapor and the consequent loss of heat all occurred at one end of the spiral while at the other end vapor a t the true temperature of the bath barred the entrance. It’ was therefore not essential to include a stop valve in the capillary of any later apparatus. The dimensions of the spirals and the capillary tubes were varied many times to find \Thether the vapor pressures recorded at different temperatures varied with the design. If the spiral was too small or was replaced by a bulb, and the capillary was of large diameter, then no steady back pressure was recorded. IYhen the spiral was longer than a certain minimum in rclation to the capillary, increasing the length gave no additional buffering action. The vapor pressures were independent of the shape of the apparatus. The physical event being measured was, of course, not the vapor pressure, but the rate of fall of pressure in the system. The vapor pressure was assumed to be that pressure at n-hich the rate of exhaust became indefinitely small. I t was essential that the exhaust pump should be able to maintain a comparatively perfect vacuum in the condenser and, secondly, that t’he fall of pressure against time should be recorded in well balanced units. The constant vacuum was secured by inserting between the apparatusand the “Hyvac” pump a small air-cooled condensation pump actuated by butyl phthalate.l0 The choice of co-ordinate dimensions in plotting was made to give the maximum change of slope (on semi-log paper) a t the region approaching vapor pressure. The vapor pressure would be reached when the initial rate of exhaustion had decreased more than fifty-fold. The apparatus had one valuable characteristic. Cyclic distillation was occurring all the time and dissolved gases were sucked away so rapidly that they exerted no measurable pressure. Organic liquids a t 2 0 O C . dissolve, on an average, SYc by volume of air.5 If this solution has occurred a t atmospheric pressure, the volume liberated at 0.05 mm. is some twelve hundred times that of the liquid. The gas may take hours for complete removal during which time, in any static system, or system of intermittent evacuation, its influence will be felt. In our apparatus the effect of dissolved air is not noticeable after the first few minutes providing the high temperatures and pressures are recorded first. * Exhaustion was always so rapid a t this pressure t h a t the equilibrium position was reached before the first measurement and the curve became a straight horizontal line.
63 1
LOW-PRESSURE TENSIhlETER
C ~ n ~ t r ~ c t i ~ n tensimeter d-A for routine service wmk has recently been built and has proved sufficiently satisfactory to remain in constant use. The glass work is grouped around two stalwart wood uprights connected at the top by a piece of soft pine. The uprights are fastened at the base to a small carriage on castors which is weighted down with a gasometer, pump, and
L FIG.j Tensimeter for the determination of low vapor pressures
cylinder. Approximately halfway up the woodwork is a crosspiece supporting a rising table. The arrangement is best understood by reference to the side elevation in Fig. 4 and the photographs, Fig. 7-10. The vapor unit and the stopcock mechanisms are detailed in Figs. 5 and 6. Referring to Figs. 4, 5 , and 7 , A is the spiral and B the diffusion bulb. C is the filling tip, D and E are the capillaries, and F and 11 are condensers
FIG. I"
'Ibe
CWYW for B i i u ~ i b f rof suhst,nnew :tu' sirown i i i Figs. 1 1 , 1 2 , and I,?. Xo a t t e m p t s have hem ~iraih:t o test t,hii curves by t i l e formulas of < 'lamius;' Riot,' or Ilankint:" because in t h i s work t l ~ ev x ~ mprcssurcu, in thc: region before brrakdown, were found t o lie so riearly on straight lines that it. seeemed mare aeeurate 1,o extrapolate these where nrwssary than to caleolate the latent heats step by Step from tlre sairir nurncrical dat,a that
sirpplicd the OUTYCS. Special attention ha? k e n paid to the phthalie esters, which are recorded separatrly in Yig. 1 3 . So perfeet,ly pardlcl do t,lir C U ~ Y C S lie that a knowledge oi the boiling point at any one pressiire of a member of
63 j
LOW-PRESSURE TESSIMETER
FIG. I I
a'
2s
I'
E50 2% I10
190
TEMPERATURE
170 iMi50 140 I50 I20 110
100
90 8 0
70
60
IN D E G R E E S CENTIORADE
FIG.1 2
30
44
SO
IO
IO
0
636
K. C . D. HICKMAN
the series enables its boiling point a t any other pressure t o be predicted with certainty. It is pefhaps superfluous to remark that all samples to be used in the apparatus must be most carefully fractionated. R e have found slow sublimationGin a vacuum most advantageous for the final purification.
FIG. I3
The apparatus is recommended especially for use with gasolines on the one hand, and for heavy lubricating oils on the other. -1complete determination, including cleaning, refilling, and warming, takes less than three hours. I t is necessary that some portion of the tube leading from the vapor bulb t o the McLeod gauge shall be more than 80°C. lower than the temperature in the bulbs. Therefore when working with heavy lubricating oils no refrigerant is needed at the traps; but with gasolines and the more ordinary organic liquids, solid carbon dioxide or liquid air should be used.
References Preston: "Theory of Iieat," 3rd Ed.. p. 393 (1919). * H a m , Churchill and Iivder: J. Franklin lnst., 186, 1 j ii918>. Kernst, W , : " T h e o r e t i d Chemistry," Ed. j , Londo;, SIacmillan & Co., Ltd., p. 814 (1923'. 4 I T e have found the heavy paraffin hydrorarbons evolve products of cracking at temperaturps below z o o T . 5 J. Phys. Chem., 34, 637 1930'. Regnault: ?f&moiresde l'.kcademie, 26, j r ( 1 8 6 2 ) . Dushman: High Vacua," General Electric Review ;1922). ,J. Chein. S o c . , 47, 42 (1885). Egerton: Phil. Llag., 3 3 , 3 3 (1917); &ran and SIack: J. Am. Chem. Soc., 4 7 , 2 1 2 I 1925); Langmuir: J. Am. Chem. For., 35, 107 (1913); Phys. Rev., 2, 239 (1913);Pirani: L'erh. deu:sch. pliysik. Ges., 4 , 686 (19061. Rev. Sei. Instruments, 1, 140, March, 1930, 51 Griffin: "Sliscellaneous Scientific Papers," (1881). Rocheder, .
