It is not po3sihle to compare their results precisely with the present results, but they appear to differ qualitatil-ely in two respects. (1) Fig. 1 of Delahay and E k e appears to show a linear initial dependei'ce of adsorption on time, although it is well knowi and recognized explicitly by them that the initial adsorption must vary as t''?. (2) Neglecting evaporation and using the present notation, velahay and Fike present r/i' as a function of L', D1'rm and t (ie., one variable and two parameters); 11-e present r/T, (and -could equally well preqent r/iT as a function of U and z = 2Co/rm d D f i (one parameter and one reduced variable). Since our treatment involves one less parameter than the trratmeiit of Delahay and Fike, our treatment requires milch less extensive computation aiid tahilation for presentation of equivalent information. *In implication of their treatment 111th which we do not agree, however, is the fol-
lowing: fo_ra given t, D1/I', the ratio r/F depends only on U = aCo, and not on a or Co separately. This statement is true for the linearized isotherm, but does not appear to be true for the Langmuir isotherm; since Delahay and Fike tabulate only the product aCoit is not possible to compare their work with the present work further. It may be concluded that a diffusion-controlled adsorption mechanism represents rather well the approach of boundary tensions of aqueous solutions of aliphatic acids and alcohols to equilibrium, and the steady-state boundary tension ohserved when simultaneous evaporation of solute occurs. It does not represent a t all well the initial dependence of boundary tension on time. Acknowledgment.-The author wiqhes to acknowledge a number of stimulating discussions with Prof. K. Ruedenberg on diffusion theory, which did much to inspire this work.
THE COSDUCTIVITY OF DILUTE SODIUM CHLORIDE SOLtTTIONSUXDER SUPERCRITICAI, COXDITIOn'S' BYJAMES F. CORWIN,ROBERT C. BAYLESS A N D G. E. OWEN Contribution from the Deparlment of Chemistry, Antioch College, Yellow Springs, Ohio Received November 18, 1959
'Thp conductivity of aqueous sodium chloride solutions above the critical temperature for the solution differs by a considerable amount when the electrode system is located a t the top of the container from that when the electrodes are a t the bottom. The difference is a function of the degree of filling (steam density) and is minimized by increasing the amount of solution in the container. A concentration gradient in the solution is proposed to explain the difference and comparisons are made with existing data which were made under conditions where homogeneity waa artificially maintained or ignored by equipment design.
Introduction The variiition in conductivity measurements in salt solutions under supercritical conditions was first reported by Swinnerton, et aLz in 1949. The measurements made a t that time using similar equipment to that described in this publication showed an almost constant bottom conductivity while the conductivity a t the top varied over a wide range. Equipment difficulties and lack of financial support made confirmation of the phenomena impossible until the present time. These present measurements confirm the existence of the difference in conductivity between the top and the bottom of the autoclavc but show that the constant bottom conductivity reported in 1049 is in error and that the bottom conductivity varies with steam density as does the top conductivity. The presence of a two phase system suggested by the constant conductivity measurements does not explain the present results as vel1 as the suggestion that a single phase system exists with a conductivity gra(1) This research was eupportrd in part by the L-iiifed States Air Poice through l h e I i r Force Ofice ot bcientific Research a n d Development Command, under Contract No. Al' 18(600)1490. Additional support was iereired f r o m the U. S. Army Signal Corps (Cont r a r t No. D 4 36-039 SC-64605) through its bignal Corps Engineering La1)oratories a t Fort hlonmouth, New Jersey. Reproduction in whole or in part 18 permitted for a n y purpose of t h e United States Go rernment. ( 2 ) A. C . Swinnerton. G E O u e n and J F. Corwin, Disc. Faraday Sac., 6, 172 (1949).
dient. The present view is coincident with that - ~ H. I). found by 0. hlaass and c o - ~ o r k e r s ~and Baehr6 mho have worked primarily with pure substances. Mayer7-'0 and eo-workers have proposed an anomalous region above the critical point which most closely resembles that found in this work. Various authors have objected to this proposal and have made measurements in various ways to refute the existence of such an anomalous region. h complete review of the controversy is presented by IIirschfelder, Curtis and Bird." The experimental evidence presented here shows a continuous and reproducible conductivity difference between the top and bottom of a high pressure vessel conhining sodium chloride solutions of several concentrations and steam densities at 390' which is a temperature well above the critical temperatures of such solutions. An average between (3) J. S. T a p p , E. R. W. Steacie and 0. Maass, Can. J. Res., 9, 217 (1933). (4) C. A . Winkler and 0 . Maass. ibid.. 9 , 613 (19331. (i D. )B. Pall, J. W. Broughton and 0 . Maass, i b z d . , 16B,230 (19381. (0) H. D. Baehr, 2. Electrockrm.. 68, 416 (1954). (7) J. E. hlayer, J . C h e n . Phps., 6, 67 (1937). ( 8 ) J. E. hlayer a n d P. G. Ackerman, ibid.. 6, 74 (1937). (9) J. E. Mayer a n d S. F. Harrison, ibid., 6, 87 (1938). (10) 5. F. Harrison and J. E. hlayer. ibid.. 6, 101 (1938). I l l ) J. D. Hirschfelder, C . F. Curtis8 and R. B. Bird, ";\~lolecular Theory of Gases a n d Liquids," John Wiley and Sons, Inc., New York, N. Y . , 1954, pp. 357-390.
642 To Conductivity Recorder
The1 m oc oup le I
hl Fig. l.-A2utuclave assembly.
6 s 10 12 Time, hours Fig. 2.--.0.025 S KCI, steam density = 0.30.
0
2
.4
these top :uid bottom coiiductivities compare very well with those obtained in homogeneous systems. Apparatus.-.h stainless steel autoclave containing an internal volume of 17.6 cc. and a spark plug, designed and built by t'he AC Spark Plug Division of Genera,l LIotori: Corporation were used. The autoclave was lined with platinum, 0.5 mm. thickness, extending from the base of thcs spark plug to the bott,om of the autoclave. The spark plug surface inside thc system (ooiiductirig arca) w t s covcJrtd nith gold foil, 0.002 in. thickness. Thwe linings prevcntcd cscessivc corrosion of the metal of 1)oth thc. spark plug and t lir autoi~l:tvr. P'igiiri; I shoLvs t h o :nitoc,l:ivc,-P1,:I.rl.r plug itsternbly . A cy1iritlrii~:tl furii:iw whic,li \v:i,s hriilt during t,ho initi:t,l stages of this investigation, and is thr result of several trial models, w ~ usrd. s It was constructed by using a steel pipr, 3.5 cm. ciianieter, 36 em. long, surrounded by a foamglass, insulating matrrial, 9.0 cm. thick, 22.0 cm. diameter, and 40 0 cm. long. Nichronic resistance wire, 24 B & S gage, was n'oiii 1.1 1.4 ti:rits on the uppc'r half' of tlir. pipe liiier and an
equal number on the lower half. The M hole furnace was mounted on a swivel so that the furnace and autoclave could be inverted without disturbing the connections. The resistance of the system was measured by an automatic recording Speedomax G Elertrolytic Conductivity Recorder, Model No. 1330175, built by Leeds and Northrup Instrument Company. The recorder, provided with :t manual temperature compensator, was set a t the value corresponding t o the temperature of the electrolvte being measured a t room temperature and was not changed during a measurement cycle. The recorder operated on a log sralr for recording the resistance with limitations from 10 to 20,OOOQ. Most of the solutions used showed resistance far above the upper limit of the instrument during a cyrlc when extremely small steam densities were measured, and when the electrode was not submerged. To rorrect for this condition, a resistance decade, Model Dr-1, built by thc Heath Company, Benton Harbor, Michigan, was introtluccd into the measuring circuit, connected in parallel to the cell circuit. The value on this decadr was set a t 19,000, enough to prevent the recorder from going off scale \%-henhigh resistance conditions eyisted. A Brown Eleetronik Indicating Proportioning Controller, Model No. 156 R 16 Ps-141, built by the Brown Instrumrnt Division of the Minneapolis-Honeyv-ell Regulator Companv, Philadelphia, Pennsylvania, was used t o control the ttsmperature input to the furnace. It proved quite satisfactory for controlling the temperature to within i 0 . 5 " nccurncy of the control point. Two variacs connected betneen the controllers and the furnace allowed the heating elements to be controlled for heat input. An 8-point Speedomax Tl-pe G Recorder, Model X o 1203223, built by Leeds and Northrup Company, Philadelphia, Pennsylvania, was installed t o record the temperatures throughout the hydrothermal system, and provided a continuous check so that the temperatuie could be kept constant. Experimental Procedure -Techniques common to the measurement of conductivitv were used in handling the solutions and the autocrave filling except that a i r e a t deal of care was used in maintaining an oxygen-frer atmosphere and oxygen-free solutions. Thr presence of air caused a great amount of variation ill results. A measured amount of solution was added t o the autoclave using a buret while a nitrogen atmosphere was maintained and the autoclave sealed with the spark plug as tightly as possible by hand. The autoclave \vas then transferred to a vise where the spark plug was tightened against a copper gasket using 130 to 140 foot pounds of torque. The autoclave and solution were n cxighctl to within an accuracy of 10 mg. on an ana1ytic.d balancc whirh had one pan substituted by a sling m:de to accommodate the container. The autoclave was placed in thr fi1rn:tc.e in an npright position (spark plug a t top) and the re~istanccof the solution measured by inverting the funi.rce which Fubmerged thc electrode. Thr fiirri:ire was then brought t o the control temperature of 300" usuallv nith the elrctrode at the top. Both upright and inverted Dositions wrre used without changes in results. After reaching the control temprratuie, t,hr: furn:tce cwuld he invrrted or uscd upright in order t o comptrr resistanccs at' the top and hottom. Figure 2 wprestInt,s a complctc run for 11 hours with rontinuous mr:tsuremcnt of rcsistance, and temperature of it solution of 0.025 S KC1. ilt the completion of a set of mcasurrmcxnts, the f u r n n w was cooled to room t'emperature and tlic rcsistnncc: :igain measured t,o make sure that, no, or only minimum, ch:ingee in thc solution verr made by the heating. the autoclave was weighed again to check for I( Thc degree of filling of the autoclaw dc strani drnsity and the prc'ssiirc. of thc syrtrni. Sincr tlic prrssurv can he wtiniatctl from stcain 1 :il)lt, n-crix not nic:tsiircd anti iinlcss the ti.nipcr:it iirc, M tlie prcssiirr \\-as ronst,:tnt for :illy p:&c,iil:!r solutioii nit':^ ureti. The degrw of filling of the c1ontaiiit.r rail Iw tixiis. latrd directly into stc:nn tit~iisityif one iissiinii's i1i:it the, coiitnincr is f:ilI :tbove t lie critical tcnipc~rntiirc.
Results Figure 2 is representative of mniiy experinwnt,s
May, 1960 TABLE I 390" Cell constant 0.073 =t0.003 cm.
SODIlJM CHLORIDE SOLUTIONS .4T S t r a m D ,g . / c - . C'oncn., n!oles,t
0.1 .01 ,0025 ,001 .000s 0.1 .01 .0025 ,001 ,0001 0.1
.01 ,0025 ,001 .0001 'j:,,111
L).
-0.50
'r
0.1
.01 .0025 .001 ,0001 0.1 .01 ,0025 ,001 ,000 I
T
18.0
76.0 135.0
45.6 178.0 178. 0 1080.0 5030.0
40.5 Iii6 , 0 460.0 900.0
1700.0
Ecjuiv. conductance at 390" 35.0 40.5 138.0 166.0 308.0 3'30.0 608 . 0 770.0 3480.0 4700.0
0.32
70.0 !IS.0 180.0
300. 0 470.0 10.4 -
1 . 1
4.05 2.44 1.55 10.3 77.0 162.0 244.0 1s50.0
(12) J. K. Fopo, S. W. Benson and C 212 ( 1 9 5 4 )
T
--"
Specific conductance at 390" ( L X 57.0 4 , 17 31.7 13.3 2.6 6.95 3.18 6.ti4 1.2s 5.2 0.943 3.80 2.56 0.486 1.30 cia. 0 133,o 266.0 520.0 2560.0
Equiv. conductance at 390" 4.2 31.7 26.0 ti9.5 52.0 127.0 98.0 372.0 486.0 1300.0
S. Copelnnd, J. Chem. P h y s ,
0.36
24.0 74.0 120.0 150.0 320.0
B
18 50 95 105 215
0 0 0 0 0
40.6 14.6 7.7 ti. 9
