NOTES
August>,1961 TABLE I1 CONDUCTANCE OF BulKPi 50% Benzene-DCB -64.74'--
44.260-
7 -
I
104~
0.6420 0.8218 1.099 2.231 3.114
19.54 9.949 5.046 1.014 0.5089
lO*C
19.86 9.808 4.882 0.9980 0,4952
A
0.8245 1.050 1.386 2.766 3.751
Bromobenzene 7-350lodc
2.50
c
A
104C
9.940 4.891 2.483 0,9987 0,5001
r-25'---
10%
A
8.420 7.073 6.061 5.187 4.349 3.025 2.014 1.321 0.5534
1.520 1.647 1.765 1.897 2.057 2.417 2.881 3.449 4.902
0.1847 .2415 .3230 .4899 .6737
A
1.982 2.187 2.384 2.677 3.074 3.200 4.511 6.754
7.580 6.145 5.103 3.972 2.945 2.278 1.248 0.4634
-44.58"-
-65"-
104~
A
9.762 4.807 2.439 0.9810 .4913
0.2704 .3549 .4756 .7201 ,9927
104~
A
9.576 4.715 2.392 0.9621 .4818
0.3823 .5041 .6760 1.026 1.414
50 mole % benzene-DCB solvent, we can only observe values of Ao2R, the reciprocal slope of a Shedlovsky since measurements were confined to 44 and 65". In bromobenzene, low concentration data were obtained a t 25 and 35" and values of Ao, as well as K , appear in Table I11 for these two temperatures. TABLE I11 DERIVEDCONSTANTS FOR BurNPi Solvent
50 mole % benzeneDCB Bromobenzene
Temp., "C.
253 44.26 64.74 25 35 44.58 65
10'do*K
Ao
10'K
30
2.74
1467
35", there is little justification in discussing the significance of this value. It is probably best concluded that a study of the temperature coefficient of an ion pair dissociation constant will not allow much to be said concerning the values of the molecular parameters that determine the coefficient. T H E SYSTEMS T I N T S L U M PEKTACHLORIDE-FERRIC CHLORIDE AND NIOBIUM PENTACHLORIDE-FERRIC CHLORIDE BY CHARLESM. COOK,JR.,AND ROBERTB. HAND E. I. du Ponl de Yemours & Company. Pzgments Department, Walmzngton,DeEawarc Receaued Mag 5,1961
During a study of the properties of ferric chloride the solid-liquid equilibria of the system T a c k I:ezCl6and NbCI6--Fe2Cl6were reinvestigated. Experimental Ferric chloride (Fisher Scientific Company-purifiedsublimed) was purified by slow resublimation in a stream of oxygen-free chlorine. TaC15 was prepared by reaction of T a metal with oxygen-free chlorine. NbClj was prepared hv chlorination of NbzOj followed by passing the product with Cle over carbon a t 600" to remove SbOC13. Chlorides were stored and manipulated under inert atmospheres. Mixtures of FeC13 and TaC15 or NbClb, sealed, under ca. 0.1 atm. Clz, in a cylindrical Pyrex vessel of 25 mm. I) X 80 mm. L, were melted within an insulated cavity enclosed bv an externally heated steel jacket. During cooling a constant temperature difference was maintained between jacket and sample, the jacket power being regulated by a differential thermocouple input to a variable reluctance furnace controller. Sample temperatures, measured by a Pt-lO% Rh thermocouple in a Silicone oil-filled thermowell extending into the sample, were plotted as cooling curves on a Leeds & i'iorthrup recorder. St intervals this recorder was standardized by observing the indicated signal when a known voltage, from a Leeds & Northrup t y e K bridge, was impressed upon it. Agitation was provic&by a Burrell wristaction shaker to which sample and jacket mere attached.
Results The TaC15-FezC16 system, presented in Fig. 1,
4.85 8.20
___....
13 18
.
1.53 1.01
___.-.
0.443
____I.-..__... b . b
0.902
Discussion In bromobenzene, the A0 values a t the lower temperatures are increasing faster with t.emperature than one would expect if the Walden product, A o ~ a were , a constant. Further, the value a t 25" 60 80 20 is much lower than is observed in high dielectric 0 MOLE % FeZCls. solvent^.^ This result is another indication that system TaC15-FeZCls: 0, this work; 0 , Aoqo decreases with decreasing solvent dielectric Fig. 1.-The Morozov. constant.8 The uncert,ainty in A0 is of the order of *1 equivalent conductance unit. We are thus left with an uncertainty of 14% in the value of K shows a single eutectic at 14.5 mole % Fe2C16and a t 25". This is the most unfavorable case. Since 203" in good agreement with Morozov,1 who lo' and 200". there is only a 40% change in K between 25 and cated the eutectic a t 13.9 mole % Freezing points of 215 and 307" were observed for 40
(7)
T.Shedlovsky, J . Franklin Inst., 226, 739 (1938).
(9) R . AI. Fuoan, P r o ? . S a t i . Amd S e i ) , 46, 807 (1958);
(1)
I. S.Morozov. Zhur. A7eovu. Khim., 1, 2792 (la.%)
100
1468
COMMUNICATIONS TO THE EDITOR
TaC152 and for FeC13, respectively. Schafer and Bayer3 record the melting point of FeC4 as 307.5" and point out that the apparent melting point of FeC&when measured in a CLdeficient atmosphere becomes depressed by contamination with FeC12. Their dat,a indicate that 7 mole yo FeCh in FeCh lowers the apparent melting point to 303", the FeC13m.p. reported by Moroxov. FeCL if present should depress the observed ferric chloride liquidus temperatures, and Morozov's values consistently lie below the liquidus calculated using d In X F ~ ~ C I ~ / dT-l = -AHr/R with AHr = 20.6 kcal./mole and represented by the dotted curves in the figures. 1 0 The NbC15-Fe2C16 system, presented in Fig. 2, shows a eutectic a t 9.5 mole yo FezCls and 191". Fig. 2.-The The NbC15has :m.p. 205".* During cooling of com(2) H. Schiifer and C. Pietruck. Z. anorg. Chem., 261, 174 (1951), report Tach m.p. 216.5'; NbCIs m.p. 201.7'. J. B. Ainscough, R. Holt and F. Trowse, J . Chem. Soc., 1034 (1957), report TaCls n1.p. 216.9'. NbCls n1.p. 203.4'. ( 3 ) 13. Schzifer and L. Bayer, Z . a n o ~ g .Chen., 271, 338 (1963).
20
Vol. 65
40
MOLE
60
%
80
I
100
FezCI,j.
system NbCla-FepCla: Morozov.
0, this work;
0,
positions containing > 0.1 a weak evolution of heat, which occurred reproducibly and which was independent of rate of cooling, was observed at 193" before crystallization of the eutectic a t 191".
COMMUNICATIONS TO THE EDITOR DETECTION OF STRUCTURAL DIFFERENCES IN POLYMERS IN A DENSITY GBADIENT ESTABLISHED BY ULTIRACENTRIFUGATION Sir: W7ehave developed a method of separating polymers based on differences in partial specific volume. We have applied the method to separating highly branched material from a previously described copolymer' and Lo the separation of atactic polystyrene from stereoregular polystyrene. It is seen easily that in the vicinity of a branch point of a polymer molecule, the "density" of the molecule will be slightly greater than in the linear portion of the molecule. Likewise, if the polymer consists of stereo regular sequences the volume that the molecule occupies in solution mill vary with the amount of :stereoregularity. We have adapted the density gradient method first introduced by Meselson, Stahl and Vinograd2 to synthetic polymers in the analytical ultracentrifuge. Essentially, a system of two solvents is used to set up a density gradient; one solvent is much more dense and has a higher molecular weight than the other solvent. The concentration of the more dense solvent and the speed of ultracentrifugation are chosen so 1,hat in the vicinity of the middle of the cell the qliaiitity (1 - 6 p ) equals zero. Here d is the partial specific volume of the polymer, and p is the density of the solution. If differences in partial specific volume of the various components of the bulk polymer do exist, then each component will collect in its own region of (1 - V p ) = 0. If (1) L. H. I'eebles. Jr.. J . A m . Chem. Soc., 80, 5603 (1958). (2) hl. Meselson, IF. 1%'. Stahl and J. Vinograd, Proceed. Nut. Acad. Scz., 43, 581 (1957).
care is taken to use very small density gradients within the cell, quite small differences in 8 can be measured. A 1 E.A. solution of sample 6,' dissolved in dimethylflormamide containing 135.6 g./l. of bromoform was spun in the ultracentrifuge for 150 hours at 33,450 rpm. During this time the material clearly separated into three discrete bands, each band being located at a different position in the region of the center of the cell. I n another experiment, 85 hours at 19,180 rpm., the branched material clearly separated into two distinct bands with a partial specific volume difference of 6.1 X lo-* ml./g. This difference is too small to be detected by standard means. Under these conditions, the linear polymer did not sediment. In order to see whether atactic polystyrene could be separated from stereoregular polystyrene, a mixture of 380 g./L of bromoform in benzene containing 0.32 g./l. of atactic polystyrene of 5 X lo6 molecular weight and 0.32 g./l. of stereoregular polystyrene of 20 X lo5 molecular weight was spun for 56 hours at 33,460 rpm. The latter material is largely stereoregular since only 2% of it is soluble in hot methyl ethyl ketone. The different molecular weights were deliberately chosen so that after separation into the component parts, the fractions could be identified since the breadth of the band depends in part upon the molecular weight. The molecular weights of these polymers are sufficiently high so that the molecular weight dependence of the partial specific volume is negligible. Again, the two materials separated into distinct bands. Comparison of these results with samples run under identical conditions except that the individual polymers were used instead of a mixture showed that the individual polymers collected a t the identical density value. The atactic polymer, however, apparently contained a small
