1186
R. B. ELLIS,J. E. SMITH, W. S. WILCOXAXD E. H. CROOK
Vol. G5
THE SYSTEM POTASSIUM BROMIDE-CADMIU&I CHLORIDE. I. SURFACE TEXSIOW BY R. B. ELLIS,J. E. SMITH, W. S. WILCOXAND E. H. CROOK? Southern Research Institute, Birmingham, Alabama John Harrison Laboratwy of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvan,ia Received January 9, 1961
Studies of the surface tension of the fused-salt system CdC12-KBr a t temperatures from near the nielting point up to 900” show that mixtures of these salts are decidedly non-ideal. Deviations from ideality reach maxima at 40 to 60 mole yo KBr in the stmatedtemperature range. Also there is a pronounced maximum in the “surface heat of mixing” a t about 60 mole yo KBr, which may be taken as an indication of the formation of complex ions. Observations of Raman spectra of melts at 67 and 40 mole 70KBr show evidence of complex ion formation. The observed Raman bands may be at.tributed to the species CdC14‘ :md CdBr4-.
Introduction Our work on the surface tension of fused saltsa has led us into a study of the KBr-CdC12 system, a system in which the formation of ionic aggregates or complex ions is to be expected. The Gibbs absorption rule leads one to expect the surface tension of a solution to be quite sensitive to small changes in solute species. Surface tension measurements should, t,herefore, give some indication of changes in the ionic constitution of liquids. We have measured the surface tension of the KBr-CdCh system at 10 compositioiis ranging from 0 to 100 mole yo KBr. and we have found this system to be far from ideal. The existence of complex ions in fused salt systems has bee-ti the subject of much investigation and discussion, and a large part of this attent’ion has been direc t,ed toward syst’ems involving cadmium halides,. Several reviews have covered the subject.++ The cadmium halide complexes are of t,he type Cd Xn-(n-z),where n = 3, 4 or 6, as disThe tetra-halo cussed by Barton arid complex, CdX4c2,appears to be the most likely one in most cases; see, for example, Bredig and Van Artsdalen.lo Most of the work has been done on chloride systems; nolie has been reported in which more than one halogen mas involved. Boardman, Palmer and Heymann’l have reported surface! t’ension measurements on a number of binary salt mixtures containing three ions, that is, t8woc.at8ion:swith the same anion or two anions with the same cation. They found that the sur(1) This work was supported in part b y the Atomic Energy Comniission under Contrac: KO.AT-(40-1)-2073 and in part b y Southern Research Institute. (2) Rohrn a n d Haas Research Laboratories, Bristol, Pa. ( 3 ) R. B. Ellis, .J. E. Smith, a n d E. B. Baker, J . Phys. Chem., 62, 766 f19.58). i 4 ) H. Bloom a n d J . 0’11. Bockris, in “Modern Aspects of Electroclipniistry So. 2,” Ed. b y J. O’M. Bockris, Academic Press, New l - o r k . S . \-,, 19.iH. lCh. 3 . );f G . .I. Jan/.. C Solonions a n d H. J. Gardner, Chem. Reus., 58, 461 ( 19.58). R. Van Artsdalen. J. Phus. Chem., 60, 172 (19.561. R. Van Artsdalen, in “ T h e Structure of Electrolytic Solutions.” Ed. b y W. .J Hamer, John Wiley BE Sons, Inc., hTeu P o r k , N. Y.,
1958. ( 8 ) G . E . Blomgren a n d E. R. Van Artsdalen, in “.4nnual Review of Physical Chemistry,” Vol. 11, Ed. by H. Eyring. Annual Reviews, Inc., Palo Alto, Calif., 1980. (9, .I. I,. Rarton :and H. Blooin. T r a m Fnrudaii S o c . . 65. 1792 (1959). ( I O ) 11. A . Bredig ani1 E. It. Tan Artstlalen, J . Cirem. l’/i&~., 24, 478 i19X).
(11) S . IT. Boardnian, A . I?. Palmrr and Faraday S o c . , 51, 277 (1955).
E. Heymann, T r a n s .
face tension of mixtures showed negative deviations from simple additivity that increased with the difference in size of the replacing ions. In some systems the deviations were of sufficient magnitude to imply the presence of complex ions, one of these systems being KC1-CdC12. However they did not consider a case in which there mere two cations and two anions.
Experimental A. Surface Tension. Method.-Measurements were made by the maximum bubble pressure method and surface tensions were calculated from the data by means of the Schrodinger equation .12 Apparatus.-The surface tension apparatus was a conventional maximum-bubble-pressure apparatusl3 set up in a specially designed dry box. The capillary consisted of a short length of 19-gauge hypodermic tubing, made of 89% platinum-ll% ruthenium alloy, swaged into a heavy platinum-rhodium shank 12 in. long. The precious metal shank was in turn welded to a nickel extension tube which gave an over-all length of 24 in. The capillary bore was measured across eight diameters, 45’ apart, by means of a microscope fitted with a filar micrometer. Measurements were made to =tO.OOOl cm., and corrections were applied for thermal expansion. Bubbles were formed with helium, and a helium atmosphere was provided within the dry box. Sample temperatures were determined by use of a calibrated platinum-platinum-lO% rhodium thermocouple, encased in a platinum-rhodium sheath and immersed in the salt mixture. Chemicals.-In all cases, Baker and Adamson reagent grade salts were used. KBr and CdClz were oven dried at 200°, mixed in the proper proportions, and ground in a ball mill. Drying of the mixtures was then completed by heating i n z’acuo a t 400’. If possible, they xere filtered through a Pyrex frit before use. The melting points of mixtures containing more than 0.6 mole fraction KBr prevented their filtration in a Pyrex apparatus. These KBr-rich mixtures were fused in Vycor and reground before use. In all cases, the mixtures were handled in an inert atmosphere after the drying in vacuo. Procedure.-For a typical run, three samples of a given mixture, contained in platinum crucibles within sealed jars, were placed in the dry box of the surface tension apparatus, the box was thoroughly purged, and measurements were made on each sample, over the desired temperature range, on succeeding days. Bubble pressures were measured with a dibutyl phthalate manometer, read with a cathetometer. Immersion of the capillary to a known depth and bubble formation were accomplished by accepted trchniq~es.1~ The density of the mixture, needed for surface tension calculations, was determined in a separate apparatus by the buoyancy method. Sample compositions were determined by analysis after the run-cadmium by dircct EDTA titra(12) E. Schroedinger, Ann. Physzk. 46, 413 (1917). (13) See for example; F. M. Jaeger, Z. anorg. allgem Cbem., 101, 1 (1917), a n d “Optical Activity a n d High Temperature Measurements,”
McGrav-Hill Book Co., New. York, N. Y., 1930; a n d J. I,. Dah1 and F. R. Duke, J . Phus. Chem., 62, 1498 (1858).
July, 1961
~ U R F A C ETENSION OF THE
y =
SYSTEM POTASSIUM BROMIDE-CADMIUM CHLORIDE
1187
TABLE I SURFACE TENSION OF KBr-CdCI? MIXTURES a - bt ( y in dynes/cm. and t in degrees centigrade) No. of
RIole % KBr
a
0 7.8 30.0
101.4 101 .o 113.5 11T 116.4 110 124.1 127.9 130.9 139,6
40.4 49.9 60. 4
T0.1 80.3 90.3 100
b
0.0280 .0288 ,0484 ,056 ,0516 .042 ,0590 .os79 ,0581 .0668
Standard dev
.
0.6 .7 .3 1.6 0.9 1.2 0.7 .8 .8 .8
individual points involved
Temp. range, "C.
14 26 30 51 25 31 22 31 25 11
580-921 569-769 479-736 484 - 702 466-642 479-684 571-812 622-926 688-91 2 803-9T2
-
(Hs/a)d (Ha/a)m
Hsis
dynes/om.
dynedcrn.
109 109
0 -4 +3 +3 -3 - 18 -3
127 133
130 121 140 144 147
-4 -6
0
138
tion, bromide and chloride by potentiometric titration with Agn'O,, and potassium by difference. B. Raman Spectra. Apparatus.-A Cary Model 81 recording Raman spctrometer was used with the modification that a resistance -xire was wrapped around the sample tube and cooling wat,er mas circulated through the containFr for the filter solution. The mercury blue line a t 4358 A . was used for excitation. Procedure:--A filled sample tube was heated in a flame to melt down the sainple and then was placed in the instrument where the resistance winding was used to maintain the desired tempera.ture. Exposures next were made by the normal procedure for this instrument.
Results A. Surface Tension.-The coefficients of linear equations derived by t'he method of least squares from the surface tension data obtained on the KBrCdClz system are listed in Table I. Surface tension-composition isotherms, calculated from these equations at three temperatures, are shown in Fig. 1. Our data for KBr are 2% higher than those reported by Bloa'm, Davis and Jarnes,'4 and our CdCI2 data are 2% higher than the values given by Boardman." The difference in the CdClz data is not surprising since Boardman determined the surface tension of this salt relative to tap water and our measurement's were made by an absolute technique. The details of Bloom's work are not available to us a t the present time and hence no compa,rison between our data and his is justified. It should be not'ed, however, that the KBr and CdC12dat'a presented in this paper have about the same temperature coefficients as the data given by the other investigators. The precision of t'he points plotted in Fig. 1 is about +0.5 dyne/cm. in surface t,ensiori and +O.Z mole % in composit'ion. Alt,hough the experimental errors are of the same order of magnitiide as the difference between the points from 0 t>o60 mole 7c KBr, in the 600' isot'herm, the points were connected as shoFm in order to emphasize the miiiiina at 40 and 60 mole o/o, since the subsequent discussion supports the reality of these minima. B. Raman Spectra.-Spectra on samples of two composition?, 40 and 67 mole yc KBr, were obtained at tempcratures varying from 300 to 500". In each caw two peaks were seen, at 251-252 and 168-169 crn.-'. Ahestimate of line intensities was iiot niadc, siiwc thc experimental t8ecahniques $ 1 4 ) H. Rlooni, I'. 66. 1170 19CtO)
(;.
1)avis and D. K. James, T r a m . F a m d n v Sor...
70
I 0
CdCL Fig. 1.-Surface
I
20
I
I
40 60 Mole tension-composition CdC1, system.
sc
I
so
100
KBr isotherms in KBr-
for obtaining the spectra were designed to give only qualitative results. Discussion Our measurements on the KBr-CdCI2 system indicate an extreme deviation from additivity at 40 mole % KBr and a secondary deviation at 60 mole yoKBr. The temperature dependence of the magnitude of the deviation is of particular interest. At 40 mole yo KBr the deviation from additivity increases with increasing temperature while at 60 mole % KBr the reverse is true. The same types of variations appear in Boardman's KCl-CdC1, data. Noteworthy is the fact that, in both sets of data, the composition of the extreme deviation shifts from 60 to 40y0 with increasing temperature. Bloomi4 has sumehted that the surface heat cwiteiit per unit ai;ci, H , = y - T(dr/d7'), be
used to compare different salts. The quantity Hela is a constant, independent of temperature,
ture of the surface of fused mixtures of KBr and CdC12, but there are some assumptions that are reasonable and consistent with existing data. 1. The species present will be principally determined by these equilibria, assuming that KBr is completely ionized
for a rnolten salt. In the case of an ideal mixture, surface heat contents should be additive (Has)m" = X l ( l 1 S i S ) l XZIH,/*)Z where X is the mole fraction of the component indica;ed by the subscript. The difference beCdClz C d + + + 2 C1k = 0 0031 (1) tween the actual value of (Hs/a)mand the ideal CdClL + 2C1CdCla' k 4.4 (2) value, (H8/>JrnO, is, therefore, a measure of the CdBr2 C d + + + 2Brk = 0.00035 (3) departure 0" a system from ideal behavior. HSla for non-polar liquids is about 50 dyne em.-' and CdBra + 2BrCdBr4' (4) for highly ionic molten salts 150-200 dyne em.-'. Furthermow, as the degree of association of a Thc equilibrium constants were determined a t 300" molten salt increases, Hela decreases; e.g., the by Van Artsdalen.6 It is reasonable to assume that the equilibrium constant for (4)will be somevalue Tor MgCls is TT dyne cm.-' (Bloom14). The values of Hsia and of (Hs~a)rno - ( H , I & ) ~ ,what higher than that for (2). It is evident that calculated from our data, are given in Table I. there will be very little free Cd++. From the Except for the value at GO mole % KBr, the devia- relative magnitudes of the k's, Br- will tend to tions from additivity are not significant. The displace C1extremum a t 60 mole % KBr may be taken as CdCln + 2Br- I 'CdBr2 + 2C1- IC = 10 ( 5 ) evidence for the existence of a complex on the basis CdCk4Br4CdBr4- + 4C1- k > 1 (6) cf Blclom's argument. It should be emphasized that the composition at the extremum is not neces- and the concentration of free Br- will be low exsarily the compositron of the complex. cept for mixtures rich in KBr (>75 mole %). The variation of surface entropy with composi- Therefore the species present in major proportions tion, ilidical ed by the temperature coefficients in will be K+, CdC12, CdBrl, CdCL-, CdBr4= and Table I, has some significance. There is a nega- c1-. ' KBr, tive dmntion from additivity a t 60 mole % 2. The un-ionized molecules, CdC1, and CdBrz, which is consistent with the existelice of a complex, since :t complex would introduce more order into and the complex ions, CdC4- and CdBrr-, will the distribution of ions in the surface. On the tend to lower the surface tension of the mixtures other harid, there is a positive deviation near 40 and therefore will tend to concentrate in the surmole (GKBr, indicating an increase in disorder, face. 3. The large negative deviations in surface which may be explained in terms of mixing several tension from additivity, reaching an extremum a t ionic and molecular species of different sizes. Mlle. D e h ~ a u l l has e ~ ~reported an intense Raman 40 mole % KBr, are due to the presence in the surline for the CdBr4= complex a t 163 em.-' and one face of the several species. The fact that the for Cd CL= :it 250 em. - I , in aqueous and alcoholic deviations become more pronounced with increassoluticm. j!3ueslGfound a line a t 259 cm. -l in fused ing temperature may be explained on the basis of mixtures in the XC1-CdCl2 system. He originally the disorder arising from the variety of species of assigned the formula CdC13- to the chloro-complex, different sizes. but it is noiv generally accepted that the forniula 4. The secondary minimum in the surface tenshould be CdC14=.10 Crook" also has observed sion-composition isotherms at GO mole yo KBr the line at 238-250 cm.-' in fused KCl-CdC12 mix- may be associated with a maximum total concentures and cliie at 166-167 ern.-' in fused KBr- tration of complexes, CdC4- and CdBr,-. CdBrz niixtures. Therefore the spectra of our Acknowledgments.-The authors wish to acKBr-CdClz mixtures may be taken as evidence knowledge the assistance of Mr. A. C. Freeman for the existence of both CdBr4- and CdC14= com- and MI. Bobby C. Park in performing the laboraplexes in these mixtures. tory work. Also we thank Dr. J. O'ILI. Bockris for Conclusions permission to use the Raman spectrometer in his It is not possible to describe accurately the ria- laboratory and for helpful discussions. Furthermore me express our appreciation to Dr. Bloom for (15) R I . L. Dclxaulle, BdL. aoc. cham. France, 1294 (1955). making his paper'4 available to us in advance of (16) 7V. Rues, Z. anorg. allgem. Chem., 279, 104 (1955). publication. (17) E' H. Crook and J O'LI. Rockris, t o be published.
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+
