X-RAY STUDIES ON THE HYDROUS OXIDES. IX SCANDIUM OXIDE MONOHYDRATE' HARRY B. WEISER
AKD
W. 0. MILLIGAN
Department of Chemistry, The Rice Institute, Houston, Texas Received March 21, 1938
The addition of alkalis to solutions of scandium salts gives a white hydrous precipitate. Crookes (2) represented the composition of the airdried gel by the formula Sc2O3.3Hz0,and Bohm and Niclassen (1) found that the gel gave a n x-ray diffraction pattern different from that of the anhydrous oxide. More recently 8tBrba-Bohm and Melichar (3) added freshly precipitated scandium oxide to boiling potassium hydroxide solutions and obtained from this mixture definite crystals which, after washing with alcohol, had a composition represented by the formula K & ~ C ( O H ) ~ . H ~3H20. O ] . This compound was hydrolyzed on washing with water, giving a product which was formulated as [2Sc(OH)J .HzO (= SczOa.4H20). The preparations analyzed by Crookes and by &t&rba-Bohmand hlelichar were not dried under equilibrium conditions, and therefore their analysis of the products is insufficient to establish the existence of the alleged hydrates of hydrous scandium oxide. The x-ray data of Bohm and Xiclassen indicate that the precipitated oxide is either some definite hydrate or a second modification of the oxide. This paper gives the isobaric dehydration and x-ray diffraction analysis of four samples of precipitated scandia. EXPERIMEKTAL
Preparation of samples Sample A . To 100 cc. of a 0.1 M solution of scandium chloride a t 25°C. was added a slight excess of ammonium hydroxide solution. The resulting white gelatinous precipitate was similar to hydrous alumina prepared in the same manner. Sample B was prepared like sample A except that the scandium chloride solution was at 100°C. This precipitate was somewhat less gelatinous in appearance and settled more rapidly than sample A. A preliminary report was presented at the Ninety-first Meeting of the American Chemical Society, held in Kansas City, Missouri, April, 1936. ,669
670
HARRY B. WEISER AND W. 0. MILLIGAN
Sample C was prepared in the same manner as sample A, but from a different supply of scandium chloride. Sample D. Scandium oxide gel precipitated a t 25°C. (cj. sample A) was washed rapidly, using the centrifuge, and added at once to a boiling solution of potassium hydroxide. Part of the gel dissolved and part was peptized,
TEMPERATURE - DECREES C
FIG.1. Dehydration isobar for hydrous Scz03.H20(sample A)
FIG. 2 FIG.3 FIG.2. Dehydration isobar for hydrous SczOa.HzO(sampleB) FIG. 3. Dehydration isobar for hydrous SczO3~Hz0(sarnple C)
giving a cloudy sol. After centrifuging the mixture at 2500 R.P.M. for 5 minutes to remove suspended material, it was allowed to stand 2 days until a considerable amount of a powdery precipitate had formed on the bottom and sides of the flask. Each of the above four samples was washed u i t h 50-cc. portions of water by the aid of a centrifuge until the wash water gave no test for chloride
671
SCANDIUM OXIDE MONOHYDRATE
(hydroxide in the case of sample D), after which they were air-dried 'at room temperature.
Dehydration isobars Samples A, B, and C were dehydrated in an apparatus already described (4, . , 6). a t a n aqueous vapor pressure of 23.6 mm. The resulting isobars are given in figures 1 to 3. It is obvious from these curves that
FIG.4. X-ray diffraction patterns SCZO, H
D
IOO'C
HLO 155' C SCZQ
Hz.0
c
200'
SCzOs'HzO 350' C
SCzOs
450' c
%rO3
900' sCz03
c HzO
SAMPLE D
a 4 Fro. 5. X-ray diffraction patterns
scandia precipitated a t 25°C. and at 100OC. is the monohydrate Sct03.Hz0, corresponding to precipitated alumina, which is r - A I 2 0 3 . H20. X-ruy examination X-ray diffraction patterns were obtained for saniples A and B with the General Electric apparatus. The samples in "nonex" glass tubes were examined with M o K, x-radiation; the exposure time was about 30 hours.
672
HARRY B. WEISER AND W. 0 . MILLIGAN
The films were standardized against pure sodium chloride in the usual way. The positions and intensities of the diffraction lines are represented in chart form in figure 4. The close resemblance between the diffraction .HzO and the corresponding yA1203.HzO ( 5 ) indicates patterns of Sc~03 a similarity in crystal structure. Portions of sample C which had been dehydrated a t temperatures of loo", 155", 200", 350*, 450°,and 900°C., and of sample D were examined using Cu K, x-radiation; the camera diameter was 57.6 mm. and the exposure time 30 to 60 minutes. To prevent gain or loss of mater during examination, the samples were sealed in tubes of Lindemann glass following the procedure previously described (4, 6). The several patterns are represented diagrammatically in figure 5 . They show (a) that the monohydrate lattice persists until the samples are almost anhydrous and ( b ) that but one modification of the anhydrous oxide exists between 450" and 900°C. Sample D, prepared by a procedure similar to that of &rba-Bohm and Melichar, gave the same x-ray diffraction as thc precipitated gel, Sc203.H20. It is probable, therefore, that the alleged compound, [ ~ S C ( O H.HzO ) ~ ] or Sc203.4&0, of St6rba-Bohm and Rilelichar is hydrous sCzo3.H20. SUMM.4RY
The following is a brief summary of the results of this investigation: 1. The addition of alkalis to solutions of scandium salts at 25°C. or 100°C. throws down a white, highly hydrous precipitate. 2. Isobaric dehydration studies show that the precipitated gel is hydrous ScsOa. HzO. 3. X-ray diffraction examination shows scandium oxide monohydrate t o have a crystalline structure distinct from that of the anhydrous oxide, but similar t o that of y-A1203*HzO. 4. There is no indication of a second modification of anhydrous ScnOs between temperatures of 450°C. and 900°C. 5 . The alleged compound [ ~ S C ( O H .HzO ) ~ ] of StBrba-Bohm and Melichar is probably hydrous SczOs.HzO. REFEREKCES (1) BOHM A N D NICL.ASSEIV: 2 anorg. allgem. Chem. 132, 1 (1924). Phil. Trans. 209A, 15 (1908). (2) CROOKES: H M MELICHAR: Collection Czechoslov. Chem. Commun. 7, 131 (3) ~ T ~ R B A - B OAXD (1935). (4) WEISER AND MILLIGAN: J. Phys. Chem. 38, 513, 1175 (1934); J. Am. Chem. SOC. 69, 1457 (1937). (5) REISER AND MILLIGAN: J. Phys. Chem. 42, 673 (1938). (6) WEISER, MILLIGAN, AND EKHOLM: J. Am. Chem. SOC.68, 1262 (1936).
