June, 1956
MOLTENSALTMIXTURES
ture of the two gases could be determined quickly and accurately using an automatic-reading capillary tube viscosimeter to determine the viscosity. Acknowledgment.-The author expresses his appreciation for the cooperation and assistance
79 1
which he has received during the course of this work. Recognition is given to Dr. E. J. Barber for his direction of this work and to Mr. J. H. Junkins for his many helpful suggestions related to the design and operation of apparatus.
MOLTEN SALT MIXTURES. PART 2. THE REFRACTIVE INDEX OF MOLTEN NITRATE MIXTURES AND THEIR MOLAR REFRACTIVITIES BY H. BLOOMAND D. C. RHODES Department of Chemistry, Auckland University College, Auckland, New Zealand Received November 88, 1966
A method has been developed for the accurate measurement of refractive index of molten salts. Measurements of refractive index have been carried out over a temperature range of about 100' for pure molten NaNO,, NaNOa, KN02, KNOI and AgN03. The molar refractivity of each salt has been calculated. The refractive indices of molten mixtures of NaNOsKNOI, NaNO,-AgNOa and KNO3-AgNO3 have similarly been investigated and their molar refractivities calculated. The plot of molar refractivity versus mole fraction is for the NaN08-AgNOs and KN03-AgNOs systems linear within experimental error. For the NaNOa-KNOa system the deviations from the linear relation are slightly outside the limits of experimental error.
To explain the conductivity minima observed in isotherms of electrical conductivity of certain molten salt mixtures, such as the CdC12-KC1 system, Bloom and Heymann,' Bloom, et U Z . , ~ and Harrap and Heymann3 assumed that complex anions are present in such systems. Van Artsdalen and Yaffe4 have pointed out, however, that a conductivity minimum is observed in the system LiCl-KC1 in which the formation of complex ions is very unlikely. They drew attention to the danger of explaining such minima by the formation of complex ions, if there is no supporting evidence from other physical properties. I n certain systems, e.g., PbClrKCl and CdC12-KC1, the presence of complex ions has been qualitatively established by the large negative deviations from additivity, of electrical conductivity, together with a maximum value of the energy of activation for ionic migration, in the plot of property against composition. Such qualitative evidence, as well as that arising from considerable positive deviations of molar volume from additivity, does not yield quantitative information on the structure of the complex ions present. The investigation of refractive index and molar refractivity of molten salt mixtures was undertaken in an attempt to find a physical property which is likely to be affected considerably by the presence of complex ions and which can give quantitative information about such complexes. Refractivities of ions both in the free gaseous state and in infinitely dilute aqueous solutions have been obtained by F a j a n ~ . ~ Similar data on molten salts can be used to give information about the mutual influence of ions in a melt. I n order to minimize experimental difficulties, the investigation of relatively low melting salts was first undertaken. For the systems selected, (1) H. Bloom and E. Heymann, Proc. Roy. SOC. (London), 188A, 392 (1947). (2) H. Bloom, I. W. Knaggs, J. J. Molloy and D. Welch, Trans. Faraday Soc., 48, 1458 (1953). (3) B. S. Harrap and E. Heymann, ibid., 61, 259 (1955). (4) E. R. Van Artsdalen and I. S. Yaffe, THISJOURNAL,68, 118 (1955). (5) K.Fajans. 2. p h y s i k . Chem., 2 0 , 103 (1934).
electrical conductivity and density studies have shown that their mixtures exhibit only minor departures from ideality. No measurements of refractive index of molten salt mixtures have yet been published but there have been some determinations of the refractive index of pure fused hydroxides, nitrates and other fairly low melting salts. Meyer and Hecks determined the refractive indices of KNOI, NaN03, KOH and NaOH and calculated their molar refractivities. Their method was to make cross-wires and their image coincide Using an autocollimator. A beam of light illuminating the cross-wires was directed into the melt and made to strike an immersed inclined mirror. The angle of the mirror was adjusted so that the cross-wires and their real inverted image coincided. From the angle of the mirror and that of the measuring telescope, the refractive index could be evaluated. Wagner' modified the apparatus slightly and determined the refractive index of a number of alkali and silver nitrates and other oxy-salts. His results did not agree very well with those of Meyer and Heck. Experimental The chemicals used were all of analytical reagent quality and gave colorless clear melts. Silver nitrate was kept out of light to prevent photolysis. Mixtures of salts were made up by weighing lumps of the solidified previously fused salts, directly into the container, Method.-Two sets of fine platinum cross-wires were rigidly supported a t a distance of 2.5 inches apart by means of Pyrex glass rods. They were fixed firmly in a furnace which consisted of a stainless steel tuhe ( 3 inch diameter, 24 inches long) heated by a Nichrome V element fitted in an insulated case. The cross-wires were illuminated by light of a sodium vapor lamp just above the furnace tube and were viewed by means of a transit theodolite which was mounted about three feet from the furnace in such a way as to allow three dimensional adjustment. By alternately focussing the telescope on the upper and lower cross-wires and making suitable adjustments to the angle of inclination of the theodolite telescope, the angle of inclination of the cross-wires could be determined accurately. The melt which was contained in a Pyrex tube (2 inch diameter, 7 inches long) could be raised and lowered in the furnace. The surface of (6) G. Meyer and A. Heck, ibid., 100, 316 (1922). (7) 0.H. Wagner, ibid., 131, 400 (1928).
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H. BLOOMAND D. C.RHODES
the melt was initiaIly situated below the lower cross-wire to enable the angle of inclination of the two sets of crossWires to be determined in air. After this was done, the tube containing the melt was raised so that both sets of crossWires were immersed. Owing to the refraction of light by the melt, the angle of inclination of the theodolite telescope had to be altered to bring both sets of cross-wires into focus in the line of the telescope. The angle of refraction was thus measured and found reproducible to better than 1' in 15O (fO.l%). During the measuring process the temperature of the melt was measured by a chromel-alumel thermocouple and pyrometer (calibrated at the melting points of vanow metals and salts). Temperature was controlled by means of a "Simmerstat" to f 1". The measurements were repeated at 20" intervals from about 20" above the melting point of the salt or li uidus temperature of a mixture to approximately 100" hiher.
Results and Discussion The refractive index, n, is defined by the equation n = - sin - - -(angle of incident beam)
sin (angle of refracted beam)
The angles of incidence and refraction were determined directly so the refractive index at various temperatures could readily be calculated. I n all cases the refractive index of pure salts and mixtures are linear functions of temperature (t"C.) so that results can be expressed in the form n = a - bt, where a and b are constants. All values of n are at the wave length of the sodium D line. Molar refractivity, R, is calculated from the Lorentz-Loren2 equation
Vol. 60
TABLE I1 REFRACTIVE INDEX, DENSITYO AND MOLARREFRACTIVITY (IN cc.) OF FUSEDMIXTURES OF NaNOs A N D KNO3 Mole
Temp.
NAOi
a
20 40 60 80
1.472 1.465 1.462 1.459
300400 280-400 300-400 300-400
10'
Density
1.7 2.126-7.44XlO-'t 1.5 2.136-7.60XlO-'t 1.4 2.132-7.40X10-4t 1 . 2 2.132-7.20X10-4t
R R (exp.) (add.)
13.07 13.16 12.63 12.76 12.18 12.35 11.80 11.94
TABLE I11 REFRACTIVE INDEX, DENSITY'4 AND MOLARREFRACTIVITY (IN cc.) OF FUSEDMIXTURESOF AgN03 AND NaNOa Mole
Temp.
AgNOi
range, 'C.
%
20 40 60 80
310-380 290-360 270-350 235-330
a
1.504 1.535 1.576 1.632
*
104
Density
1.0 2.530-8.OXlO-'t 0 . 7 2.906-8.3XlO-'t 0.6 3.310-9.0X10-4i 0 . 8 3.734-9.7X10-4t
R R (exp.) (add.)
12.52 13.47 14.42 15.33
12.48 13.42 14.35 15.29
TABLEIV REFRACTIVE INDEX,DENSITY~~ A N D MOLARREFRACTIVITY (IN cc.) OF FUSEDMIXTURES OF AgNOs A N D KNOI Mole
Temp.
A$Oi
ratF
a
30 55 58 70
250-350 150-300 190-300 170-300
1.538 1.588 1.593 1.614
b X
10'
2.0 1.9 1.9 1.4
Density x 104 1
2.669- 9.4X10-'t 3.194-10.7X10-4t 3.259-10.8X10-4t 3.518-11.0X10-4t
R
R
(exp.)
(add.)
14.40 14.94 14.96 15.30
14.37 15.01 15.06 15.41
sity. The discrepancy between density measurements reported by different investigators may be as where M = molecular weight, defined in the case much as 3=0.5%. Thus the possible error in refractivity due to density errors is estimated to be of of mixtures as the order of 10.5%. M = Mlzl + M e a ( E = mole fraction) In all cases the molar refractivity calculated from The density, d, of fused salts and mixtures was the refractive index and density data were constant taken from published values; NaN02, NaN03 and within 0.5% over the temperature ranges investiKN03 by Bloom, et aLj2andAgN03from Boardman, gated. This compares favorably with the greater Dorman and Heymann.* As there are no reliable variation of values of R obtained by Meyer and published results for KN02, density determinations Heck and Wagner. Our values of refractive index were carried out, it being found by a sinker method agree better with Meyer and Heck than with Wagthat ner, whose results for NaN03, KN03 and AgNOa, d = 1.976 - 6.2 X lO-'t ( 1 in "C.) investigated also by us, are from 1 to 2y0 higher. Results of refractive index, molar refractivity and The disagreement between the results of Meyer and Heck and those of Wagner is not systematic and is density are given in Tables I, 11,111and IV. thus most likely due to random errors inherent in The maximum error involved in the measurement their experimental method. In their method which of refractive index is +O.l% (being that involved reflection of light from an inclined mirror in the measurement of angle by the theodolite). involved immersed in a melt, unpredictable errors due to surI n the determination of molar refractivity, there is face imperfections the platinum mirror are posa more serious error, i.e., that associated with den- sible. I n addition,oftheir investigations required the measurement of small angles ( 1 4 " ) , in which an TABLE I error of only 1' would lead to an error of the order of REFRACTIVE INDEX, DENSITY AND MOLARREFRACTIVITY 1% in refractive index. Neither Meyer and Heck (IN cc.) OF PURE FUSED SALTS nor Wagner investigated any mixtures. n = a - bt (t in "C.) The refractivity difference R K N O~ RKNO,= Temp. 1.90 cc. while R N ~ N O ~ R N a N O , = 1.91 cc. Hence range. b X R, Salt OC. a 104 Density cc. 1.905 represents the difference between the refrac9.63 tivities of 02-in NOa- and (e-)z in NOz-. NaNOl 315-400 1.476 2.0 2.022-7.46XlO-'t It can NaNOIl 320-460 1.495 2 . 0 2.134 -7.03 X 10-'t 11.54 also be seen that R K N O~ R N ~ N O , 2.03 = cc. while 11.67 RKNO,- R N a N O t = 2.04 CC., hence R K +- R N a + = KNOz 440-500 1.461 1.75 1.976-6.2XlO-'t 13.57 2.035. This value is identical (within experimental KNOI 345-480 1.473 1.7 2.116-7.29XlO-'t 16.20 error) with the value of R K N OAgNO, 260-365 1.706 1.55 4.190-10.8X10-'t ~ R N ~ N for O ~infin(8) N. IC. Boardman, F. R. Dorman and E. Heymann, THIS JOURNAL, 68, 375 (1949).
(9) H. M. Goodwin and R. D. Mailey, Phys. Rev., 46, 489 (1907). (IO) H. Bloom,to be published.
ADSORPTIONON GLASSAT HIGHRELATIVE PRESSURES
,June, 1956
itely dilute solutions: 13.25-11.21, determined by Fajans and Luhdemann.“ Hence the effect of ionic interactions in molten KNO3, KN02, NaN03 and NaNOz is either negligible or cancels out for the four salts. Molar Refractivities of Mixtures of Molten Salts.-The measured values of molar refractivity ( R cxp.) of mixtures of NaN03-KN03, NaNOr AgN03 and KN03-AgNO3 are given in Tables 11, I11 and IV, respectively. These tables also give the calculated values ( R add.) of molar refractivity from the usual additivity relation,12 i.e. R (add.) = Rlxl
+ RZXZ
It can be seen from Tables 11,111 and I V that the differences between R(exp.) and &!(add.) are small. The difference R(exp.) R(add.) is inside the limits of experimental uncertainty for the NaN03AgN03and KNO8-AgNO3 systems, but slightly outside these limits for the system KNO3--NaNO3. The electrical conductivities of mixtures of
-
(11) K. Fajans and R. LUhdemann, 2. physik. Chsm., BBn, 150 (1935). (12) K. Fajans, in Weissberger “Physical Methods of Organic Chemistry,” 2nd ed., Vol. 1, Interscience Publishers, New York, N. Y., 1949, p. 1170.
793
KN03-hraNO3 were measured by Goodwin and Mailey9J3 and those of NaN03-AgN03 by Byrne, Fleming and Wetmore.14 The surface tensions of the NaN03-AgN03 and KNOrAgNO3 systems have been measured by Bloom,’O while those of KN03-NaNOs were investigated by Boardman, Palmer and Heymann.16 In these systems, isotherms of electrical conductivity and surface tension have small negative deviations from additivity while molar volumes have almost ideal values. Such behavior indicates that in the systems KN03NaN03, NaN03-AgN03 and KN03-AgN03, little or no ionic interaction takes place. For these systems, the deviations from additivity of molar refractivity are very small. Further investigations, including those on mixtures of PbC12-KCl and CdClZ-KCl where strong interactions are expected, are at present being carried out in this Laboratory. The authors are pleased to acknowledge a grant from the University of New Zealand Research Fund for the purchase of the apparatus used. (13) H. M. Goodwin and R. D. Mailey, Phys. Rev., 96, 28 (1908). (14) J. Byrne, H. Fleming and F. E. W. Wetmore, Can. J . Chem., SO, 922 (1952). (15) N. K. Boardman, A. Palmer and E. Heymann, Trans. Faradai SOC.,61, 277 (1955).
R.
MULTILAYER ADSORPTION ON PLANE SURFACES BY CAPACITY MEASUREMENTS. I. ADSORPTION ON GLASS AT HIGH RELATIVE PRESSURES BY U. GARBATSKI AND M. FOLMAN Department of Chemistry, Technim, Israel Institute of Technology, Haifa, Israel Received December 1 , 1966
The adsorption of water vapor at relative pressures from 0.505 to 0.9976 and of isopropyl alco>ol from 0.2 to 0.993 on glass plates is measured by change in electrical capacity. Layer thicknesses up to hundreds of A. are found. Experimental details are given.
Introduction The physical adsorption of vapors is generally explored on powders or other systems of high specific surface area. In the evaluation of the resulting isotherms generally there appear two difficulties of which one is the uncertainty about the state of the surface and the other the entering of capillary condensation into the picture besides adsorption on the surface in its strict sense. For this reason, we sought a convenient method to use more defined plane surfaces of necessarily rather small areas of square centimeters instead of square meters as used generally (preliminary publication‘). Adsorptions on surfaces of small area have been measured in different ways among others by Frazer,6 Iredale,2 Cassel, * MacHaffie and Lenhe~-,~ (1) U. Garbatski and M. Folman, J . Chern. Phys., 9 9 , 2086 (1954). (2) T. Iredale, Phil. Mag., [e] 46, 1088 (1923); 48, 177 (1924); 49, 603 (1925). (3) H. Camel, Trans. Faraday SOC.,28, 177 (1932). (4) J. R. MaoHaffie and 9. Lenher, J . Chem. Soc., 127, 1559 (1925).
Smith,B Latham,’ Deryagin* and B o ~ d e n . ~ Iredale and Cassel used Gibbs’ isotherm to calculate adsorption on mercury from the change in its surface tension. MacHafTie and Lenher, Smith and Latham measured the change of pressure with temperature, of vapors included in a vessel of the adsorbing material (e.g., glass) and showed from the p-t curves obtained, that at cooling, condensation was preceded by multimolecular adsorption measurable up to 20 and even 200 layers. Frazer and later Deryagin used the change in polarization of plane polarized light reflected from glass covered with the adsorption layer. The method chosen by us consists in measuring the change of capacity in a condenser consisting of two plates of the adsorbent kept at a fixed small distance by metallic holdera cemented to their backs which constitute the plates of the condenser. (5) J. H. Fraaer, Phus. Rev., [21S8, 97 (1929). (6) . I . W . Smith, J . Chem. 800.. 2045 (1928). (7) Q. H. Latham, J . Am. Chsm. Soc.. 60,2987 (1928). (8) B. V. Deryagin, et al., C.A., 46, 4881 (1952). (9) F. P. Bowden and W. R. Throssel. Ndw4.160, 601 (1951).
