THE COLOR AND CRYSTAL STRUCTURE OF PRECIPITATED CADM IUhI SULFIDE W. 0. MILLIGAN Depurtment of ChemzstrzJ,The Rice Institute, Houston, Texas Received December 16, 19%
Cadmium sulfide may be either yellow or red, depending on the conditions of formation. Earlier investigators (3) attributed this difference in color either to the presence of impurities or to the existence of yellow and red allotropic modifications of the compound. The possibility that the difference in color was due to allotropy was questioned by Allen and Crenshaw (I), who prepared crystals of the compound by a variety of methods and found them to be identical in structure with the mineral greenockite, irrespective of the method of formation and the color. It was concluded therefore that the differences in color were due to differences in the physical character, such as particle size and the nature of the surface of the particles, rather than to allotropy. The yellow form is usually obtained by precipitation from alkaline or cold solutions, and the orange or red from acid or hot solutions. Heating the yellow material gives an orange to red colored product, the change being more or less reversible on cooling (6). In the absence of x-ray diffraction methods of examination, Allen and Crenshaw (1) were unable to determine the structure of the extremely minute crystals of the freshly precipitated salt and were forced to grow them to a size that could be observed optically, either by heating the dry powder or by digesting the mass under pressure at high temperatures. This always yielded hexagonal crystals like greenockite, but Bohm and Niclassen (2) showed by x-ray diffraction methods that the yellow precipitate thrown down from cadmium sulfate solution was cubic, being similar to the cubic zinc blende. This was confirmed by Ulrich and Zachariasen (5), who showed that cubic or @ cadmium sulfide (aa = 5.820 d.)was precipitated from a saturated cadmium sulfate solution by hydrogen sulfide and dried a t 70°C. On heating this yellow cubic sulfide to 700-800°C. in the presence of sulfur vapor, hexagonal or a cadmium sulfide resulted. There is therefore no doubt of the existence of two polymorphic forms of the sulfide, but since both the cubic and the hexagonal forms may be yellow, it is obvious that the color differences are not due to polymorphism. Within the current year NIuller and Loffler (4) precipitated cadmium sulfate solutions containing varying amounts of sulfuric acid with hydrogen 797
798
W. 0. MILLIGAN
sulfide, and obtained precipitates which varied in color from yellow to red, the samples being more red the higher the concentration of acid. Since all of these preparations gave a cubic x-ray diffraction pattern, the differences in color were attributed to variation in particle size. It would appear TABLE 1 Color und crystalline form of precipitated c a d m i u m sulfide
1
CADMIUX
1I
SALT U S E D
0.1 N
I
-I-
W l T I i O U T THE A D D I T I O N OF ACID
At 30°C. Color
1
I
At 100°C.
~
Crystalline form
j
I N T H E PRESEXCE OF ADDED ACID
Coior
-1-1
I
At 30°C.
Crystalline form
Crystalline
Color
~ Yellow Yellow Yellow Orange Orange
* Trace of
At
_______
~
Cubict Cubict I-Iexagonal Hexagonal Hexagonal
_
Red Red Red
form
_
Cubic Cubic Hexagonal
cubic p cadmium aulfide.
t Trace of hexagonal a cadmium sulfide.
TABLE 2 X-ray difraction data (I C.4DMIUM SULFIDE PRECIPITATED AT 100°C. A R O J ~C~ d C h l C I D
dhbl
3.55 3.34 3.14 2.43 2.06 1.888 1.750
I
dhkl
1.567 1.390 1.320 1.248 1.187 1.150 1,125
9 8 10 4 10 8 9 1 1 2 1 2 1 1 1
1 066 0,982
1 1
1.668
6 CADMIUM S U L P I D E P R E ~ I P I T A T E D AT 100°C. FROM CdSOa, ACID
3.36 2.90 2.06 1.756 I . 683 1,456 1 335 1.302 1,189 1,121 1.026 0,982
I
10 3 10 9
from the x-ray studies of the gel that precipitated cadmium sulfide is always cubic, the hexagonal form resulting only after subjecting the gelatinous mass to high temperatures. Such is not the case, as the following experiments show.
_
COLOR AND CRYSTAL S T R U C T U R E O F C A D M l U M S U L F I D E
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EXPERIMENTAL
Hydrogen sulfide was passed into approximately 0.1 N solutions of cadmium sulfate, nitrate, chloride, bromide, and iodide at both 30°C. and 100°C. I n a second series of experiments the solutions were made acid by the addition of 1 cc. of the corresponding “concentrated” acid to 50-cc. portions of the various cadmium salt solutions. The twenty samples were washed and dried a t room temperature. Some of the samples from the bromide and iodide solutions formed very stable sols, and were precipitated by the addition of ether or alcohol. The various samples were examined by the x-ray diffraction method, using the General Electric diffraction apparatus. The observations of crystal form and color are listed in table 1. Typical patterns are shown in chart form in figure 1.
O-CDS,YELLOW FROM CDSQ B - C D S , RED FROM
CDso,,
ACID
W - C D S , YELLOW FROM CDCL,
K-CDS,RED FROM CDCL,, ACID FIG. 1.
a AND
p
CADMIUM SULFIDE PRECIPITATED .4T
100°C.
Typical diffraction data are recorded in table 2; the values for the lattice constants of various samples agree within experimental error with the values of Ulrich and Zachariasen ( 5 ) . CONCLUSIONS
Inspection of the results in table 1and figure 1shows that either the cubic or hexagonal modifications of cadmium sulfide may be yellow or red (or orange), depending on the conditions of formation and treatment. All samples appear more or less orange upon grinding the dry aggregrates to a powder. The cubic form is usually obtained from the sulfate or nitrate solution, especially from hot or acid solutions. The hexagonal modification is usually obtained from the chloride, bromide, or iodide solutions. Small amounts of the cubic form sometimes occur in essentially hexagonal precipitates; this is probably more or less accidental, depending upon slight
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W. 0. M I L L I G A N
variations in the conditions of precipitation. The difference in color must be attributed to difference in the physical character of the precipitate, such as particle size, the nature of the surface of the particles, and the state of aggregration. Difference in particle size alone will not suffice t o explain all the known facts, since large crystals of greenockite are yellow, although red (or orange) particles are usually larger and more granular than the yellow ones. The view that color differences are essentially due to differences in physical character of the samples is in agreement with the observation that the red particles obtained from hot acid solutions possess less adsorptive capacity than the yellow particles formed in the cold (7). SUMMARY
The following is a brief summary of the results of this investigation: 1. Either cubic p cadmium sulfide or hexagonal 01 cadmium sulfide may be yellow or red, depending upon the physical character of the precipitate. 2. The p cadmium sulfide tends to be precipitated from cadmium sulfate solutions, and under some conditions from the nitrate solutions, especially if the latter are hot and distinctly acid in reaction. 3. The 01 cadmium sulfide tends to be precipitated from the chloride, bromide, and iodide solutions, but these precipitates may also contain some of the p modification. 4. The lattice constants found for the two modifications of cadmium sulfide agree within experimental error with those of Ulrich and Zachariasen. REFERENCES (1) ALLENA N D CRENSHAW: Am. J. Sci. 34, 341 (1912). (2) BOHMAND NICLASSEN: Z. anorg. allgem. Chem. 132, 7 (1923). (3) BUCHNER:Chem. Ztg. 11, 1087, 1107 (1887). Cf. KLOBUKOW: J. prakt. Chem. [2] 39, 412 (1889); cf. reference 6. (4) MULLERAND LOFFLER: Z. angew. Chem. 46,538 (1933). (5) ULRICHAND ZACHARIASEN: Z. Krist. 62, 260 (1925). (6) Cf. WEISER: The Colloidal Salts. Mcgraw-Hill Book Company, New York (1928). (7) WEISERAND DURHAM:J. Phys. Chem. 32, 1061 (1928).
