May 5, 1952
REFRACTIVE INDEX STUDY OF TIIE RhCl f Knr
RbKr f KCl SYST~M
2355
[CONTRIBUTION FROM THE CHEMISTRV DEPARTWENTOF ST. LOUIS UNIVERSITY]
Reactions Between Dry Inorganic Salts. VIII. A Refractive Index Study of Solid Solutions Formed by the Reciprocal System RbCl KBr e RbBr KCl’
+
+
BY LYMAN J. WOODAND L. J. BREITHAUPT, JR.~ For wme x e a s of the composition d i a m m s 01 the reciprocal system RbCl 4- KRr F! KbBr
T KCI the results of X-ray diffractionanalysis expected for a single ternary solid solution and for a co.existing pair of hinary solid solutions cannot be distinguished from eachother. I n theseareastherefractivc indexes calculated fur the two interpretations of the X - n y measurements are \TTV cliflcrent from each other and n definite choice between the two alternatives becomes oosiible. A l l observations are in complete accord with the view that. for each mixture, one solid solution is formed which contains all of the rubidium, potassium, bromide and chloride ions
Recently an X-ray diffraction study of the reciprocal system RbC1-I- KBr
RbBr
+ KCI
was made and it was then found that, after crystallization of any molten mixture of salts of this system, only one X-ray pattern is observed. The observed unit cell edge is always in good agreement with the cell edge calculated by the use of Vegard’s additity law on the assumption that all of the rubidium, potassium, bromide and chloride ions are contained in a single solid solution. Except for the intensities of the odd number index lines, e.&, 111, 311, etc., the single diffraction pattern corresponding to each ternary’ mixture, can be equally well accounted for on the basis of two co-existing binary solid solutions. The expected intensities for the odd number index lines of each member of a given binary pair are equal to each other but are always greater than the expected intensities for the corresponding ternary solid solution. A study Of this diagram indicates that in the central area the calculated difference in intensities hetween a Pair of binary solid SOl~tiOnSand the corresponding ternary is Of such a magnitude as to be readily measurable. For compositions indicated approximately by the shaded areas, this differentiation becomes difficult and after a time impossible as each comer is approached. Furtbermore the entire upper right hand side of Fig. 1 is equivalent, point for point, with the lower left hand half and corresponding points in these two halves of the diagram cannot be distinguished from each other by X-ray analysis. (For example compare points D-25 and D-24; D-8 and D-7; D-32 and D-27, and so on.) It has been found possible to use refractive index measurements to supplement the X-ray diffraction analysis in a very satisfactory manner. A study of the refractive indexes of a series of mixtures, whose compositions are shown in Fig. 1, has been made and the results obtained are described in the section on Experimental results.
Materials, Apparatus and Methods The refractive index was determined by the oil immersion method by observing the movement of the Becke line as the ( I ) Read bcforc the Ninth Pittsburzh Diffraction Confmnce. (2) ~ y m a nJ. Wood and L. 1. Breithaupt, 11..T n R JOuRN*Lt 74s 727 (1952). (3) The terms binary and ternary -e used in the sense Of the Ph.* rule. While all 01 the evidence indicates that thew mixtures nre made UP of ion3 it is necessary for PurWe9 of Caleulntion, to assume rn “Stoichiometric Molecule" which is invariably tnkeen as 811 io0 P i r e o m i t i o r Of 0 ° C CPtiO. and one mion.
s
. V’
2
Fig 1.-Electron ratios in alternate odd number index Experimental mixtures are Indicated by D-numhers; electron ratios for the ternary d l d solution are indicated by numbers in the direction of BB‘; electron ratios for the binary mhtures are indicated by numbers in theses in the direction M I . focus of the microscope was changed. For this work a set of certified inimersion liquids varying in steps of 0.002 unit was used. By comparison an oil was first selected whose refractive index was close to that of the crystal. The temperature of the oil was then carefully raised or lowered by means of water jackets‘ above and helow the microscope slide until the crystal disappeared into the oil. The temperature a t which the crystal disappeared into the oil was observed and the temperature a t which it reappeared was observed and the refractive index of the oil a t the middle of this temperature range was taken as the refractive index of the crystal. Under favorable conditions this temperature range was sometimes no greater than 0.5‘ and since the temperature coefficient of the diffraction liquids was about 0.0004, the refractive indexes could (under favorable conditions) be determined t o an accuracy to about i. 0,0001. Illumination was by sodium vapor light. In order to add 70 the infortkJioF%gained zbout the TPciprwal s p f e m KbCI T K H r RbRr T KCI by the Srav analvSis and bv the refractive index measurements. the milting boint of &h mixture was determined. A &all amount of each mixture was placed in a small melting tube made of high melting glass (vycor). This melting point tube was placed in a cylindrical melting point block which was then laced in a laree wcor test-tube. This test-tube was heated electrically in a d insulated tube furnace. The melting of the niixture was observed visudly by looking through a small Opening passing through the point block a t goo to the melting point tube, The temperature
=
(4) Lyman J. Wood and Albert Frank, A n d . Chmr., 28, 695 (1951).
,
560
plotted against molar com-
kB~Il.55981
-- -- - - - - - .- - - 0D-- 3011.55e31 -- - --T\,
- -
0-1911 116ll
.. -o
-. --
-position. The straight lines 0-21(1554TI .. -"O------______ 'I Wi? shown in the figure are drawn
on the assumption that the refractive indexes are addiA'* '\.. . y - , ; 9 1 1 14341 tive and the points plotted 11.539%-32/ 2 1,540represent experimental ob?. \. .. 0 servations. It is quite clear /. + 0 \ . - that the points are very a I530-'b!$p/' E nearly on the straight lines l w L 11 s z 3 e i D - ~ b ~ , ' and that the experimental 1520observations agree remarkLL /. 0 '\., ably well with the assumpI 1 50931 /. "6". \ -. . tion that the refractive in5n I 510
