X-RAY STUDIES ON T H E HYDROUS OXIDES 11. Stannic Oxide BY HARRY B. WEISER A S D Tv. 0. MILLIGAN
The question of the so-called stannic acids and the existence of definite hydrates of stannic oxide have been the subject of repeated investigations from the time of Berzelius to the present. The colloidal nature of the so-called stannic acids was investigated by van Bemmelen,’ and by Mecklenburg2 who concluded that the a and p acids were both hydrous oxides differing from each other in the size of the particles. This same conclusion was reached by one of us3 ten years ago. Investigations made a t that time showed that t’herewas no definite dividing line between the two preparations, and that the p oxide consisted of the larger particles. Gutbier, Hiittig, and Dobling4 concluded from pressure-temperature curves that no definite hydrates5 were formed. The x-ray diffraction patterns of their various samples as well as those of Mecklenburg which had aged for several years, showed that the original hydrous oxide was apparently amorphous and that the aged products exhibited a gradual formation of a crystal lattice identical with the mineral cassiterite, SnOz. X-radiograms made by Yamadd of several different prepar a t i o n ~ all , ~ revealed a structure identical with the anhydrous stannic oxide. Further x-radiograms of the so-called a and p stannic acids were made by Posnjak* who demonstrated clearly that the structure of both is identical wit’h the anhydrous oxide; and that the difference in the two oxides is due to a difference in particle size, the (3 being the larger. This is in accordance with the observations of Mecklenburg and of Weiser. Hydrous SnOzwas dried in a vacuum over P,06 by Forsterg who concluded from x-ray data that the products obtained had the struct,ure of cassiterite. X-radiograms of the fresh gel and the products resulting from the ageing and heating of colloidal SnOz were obtained by Bijhm.Io His published photographs show clearly the gradual increase in particle size from the original hydrous oxide to the anhydrous SnOn,the lattice in all cases remaining identical with that of cassiterite. In view of all the apparent’lyconclusive evidence that is available, it would appear that the question of the nature of the so-called stannic acids should be “Die Absorption,” 54 (1910). 207 (1912). 3 Weiser: J. Phys. Chem., 26, 6 j 4 (1922). Ber., 59, 1232 (1926). 5 Cf., however, Willstatter and Kraut: Ber., 59, 2541 (1926). J. Chem. SOC.Japan, 44, 210 (1923). 7 Rose: Pogg. Ann., 75, I (1848); Engel: Ann. Chim. Phys., (3) 12,463 (1844); Graham: Pogg. Ann., 123, 538 (1864); Schneider: 2. anorg. Chem., 5, 82 (1894); Zsigmondy: Ann., 301, 361 (1898); Collins and Wood: J. Chem. SOC.,121, 441 (1922). * J. Phys. Chem., 30,1073 (1926). Physik. Z., 28, I j r (1927). Kolloid-Z., 42, 283 (1927). 2
Z. anorg. Chem., 74,
X-RAY STUDIES ON THE HYDROUS OXIDES
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considered a settled one; but such is not the case. I n a recent investigation Thiessen and Koerner‘ claim that a stannic oxide gel prepared by the slow hydrolysis of stannic ethylate gives pressure-temperature and compositiontemperature curves that indicate the formation of a series of hydrates. From pressure-composition data the following hydrates are claimed : z Sn02.5Hz0; Sn02.2H20, 4Sn02.7H20and z SnOz.3Ht0and from temperature-composition curves; Sn02.H20 and zSnOz.HzO. X-radiograms of the several preparations were made, but no definite results were obtainable by this method. It was claimed that the lattice undergoes a slight expansion as the water content decreases;2 but that the “strong diffuse blackening” of the film renders impossible the making of exact measurements.
Experimental I n view of the theoretical significance of the existence or non-existence of a series of Sn02 hydrates, it is the purpose of the present paper to consider ( I ) the dehydration of stannic oxides a t temperatures which are said t o give definite hydrates; and (2) the examination of these reputed hydrates by x-ray diffraction methods. I . The Thermal Dehydration of Hydrous Stannic Oxide A. Stannic Oxide prepared by Thiessen and Koerner’s Method Preparation of Oxide. The stannic oxide was prepared according to the procedure outlined in detail by Thiessen and Koerner. Sodium ethylate made by adding metallic sodium to absolute alcohol was allowed to react with an anhydrous SnClr solution in absolute alcohol in accord with the following equation: SnCL 4NaOCzH6+ Sn(OCzHs)a 4NaCI. All reactions were carried out with extra precautions to prevent the entrance of water vapor into the apparatus or solutions. The resulting mixture of stannic ethylate and NaCl was digested on a water bath, and finally the NaCl was filtered off. An amount of the alcoholic solution of ethylate equivalent to 25 g of anhydrous SnOz was added to 5 liters of absolute alcohol. The resulting solution was allowed to stand with occasional stirring in a flask with a neck 5 om in diameter. Moisture from the air hydrolyzed the stannic ethylate, the process being complete in 26 days. The gelatinous precipitate of SnOz was filtered off, washed with I liter of 96% alcohol in 100 cc portions, dried with suction on a filter paper, and kept in a glass-stoppered bottle until used. Rate of Dehydration. The SnO2 gel made by the above method consists of Sn02 alcohol, and water. Thiessen and Koerner obtained a compositiontemperature curve by heating a sample a t definite temperatures for 1 5 minutes over CaC12. The resulting curve is shown in Fig. I. The breaks in the curve are indicative of the formation of the definite hydrates Sn02.H20 and zSnOz.H20. It would not be expected that an equilibrium state would result from heating the samples for only I S minutes, and Thiessen and Koerner give no data to
+
2. anorg. Chem., 195, 83 (1931). 28, 151 (1927).
* Cf. Forster: Physik. Z.,
+
303 2
HARRT B . WEISER AND W. 0. MILLIGAN
Tern p e r a t u r e ,
Degrees
e.
FIG. I
Temperature-Composition Curve of Stannic Oxide (after Thiessen and Koerner)
show whether or not this is the case. It was thought desirable to determine the t h e required for equilibrium a t a given temperature before proceeding to obtain the composition-temperature curve. To do this a sample was placed in an electric oven with the temperature adjusted to 5oaC. The sample was removed a t intervals, allowed to cool, weighed, and the heating continued. The resulting curve showing the relation between composition and time of heating, is given in Fig. 2. It will be observed that about 8 hours is required to attain equilibrium a t the temperature stated. As would be expected, equilibrium is established more rapidly a t higher temperatures; thus a t 160°, 2 or 3 hours is sufficient.
T i m e , Hours
FIG.2 Rate of Dehydration of Stannic Oxide at jo"C
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3033
The form of the time-composition curve is what would be expected from a material containing water and a little alcohol: the rate of drying is quite rapid a t first and then falls off, decreasing to a relatively low value before the composition becomes constant. This means that, at first, alcohol and some of the water comes off followed by the greater portion of the water leaving the last trace of water which, as is well known, is very difficult to remove. The Composition-Temperature Curve. I n the light of the above experiments a compopition-temperature curve was obtained, taking care to heat the sample at each temperature for a t least 12 hours to ensure the establishment of equilibrium. Two samples were taken: the first was allowed to cake into a lump as the heating proceeded, and the second was pre-dried enough to allow it to be powdered. The samples were placed in weighing bottles, and heated to constant weight over calcium chloride at a definite temperature, after which the temperature was raised and the heating continued as before. The dehydration was carried out in a thermo-regulated electric oven, the temperature of
TABLE I The Thermal Dehydration of Thiessen and Koerner’s SnOn Tempfrature C
Not powdered Weight of SnOz sample % g
39.6 40.6 48.4 56.3 60.2 63.6 67.8 69.8 74.5 78.6 81.6 85.7 90.0 95.4 100.4 104.7 110.0
116.0 120.0
123 . 5 132.3 143.2 154.0 202
.o
0.9073 0.9069 0.8997 0.8933 0.8899 0.8857 0.8753 0.8735 0.8707 0.8690 0.8674 0.8671 0.8654 0.8637 0.8598 0.8575 0.8549 0.8540 0.8536 0.8531 0.8510 0.8465 0.8423 0.8283
Powdered Weight of SnOa Sample % g
83.25 83.29 83.95 84.56 84.88 85.28
86.29 86.47 86.75 86.92 87.08 87.11 87.28 87.45 87.85 88.09 88.35 88.45 88.49 88.54 88.76 89.23 89.68 91.19
0.3996 0.3989 0.3971 0.3964 0,3953 0.3950 0.3943 0.3935 0.3916 0.3908 0.3894 0.3889 0.3888 0.3887 0.3870 0.3847 0.3833 0.3780
86.13 86.28 86.67 86.82 87.06 87. I3 87.29 87.46 87.89 88.07 88.38 88.50
88.52 88.54 88.93 89.46 89.79 91 .os
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which remained constant within 0.5'. For temperatures below 100' a thermometer was used that had been calibrated against a standard thermometer checked by the Bureau of Standards. Above 100' an accurate thermometer was employed which had been checked a t points below 100' against the standard thermometer. The results obtained are given in Table I and in Fig. 3.
T e m p e r a t u r e , Degrees
c.
FIG.3 Temperature-Composition Curve of Stannic Oxide prepared by Hydrolysis of Stannic Etliylate
B. Alpha and Beta Stannic Oxides For purposes of comparison dehydration curves were obtained for samples of the so-called CY and p stannic oxides. A sample of CY oxide was prepared by the addition of a slight excess of ammonium hydroxide to a solution of SnC14. The resulting gelatinous precipitate was washed by centrifuging, and finally TABLE I1 The Thermal Dehydration of Alpha and Beta SnOz Temperature "C
Alpha Weight of Loss in sample weight
Beta Weight of Loss in sample weight
g
CI
/O
g
70
1.8620
0.0
0.0
49.0
1.7980
59,4 69,I 80.3 89.6 99.6
1.7267 1.6944 1.6698 1.6511 1.6317
4.33 7.27 8.95 10.32 11.33 12.42
3.7966 3.7395 3.6398 3.5863 3 . 5422 3.5166 3.4938
39,6
1.50
4.13 5.54 6.70 7.38 7.98
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air-dried until it could be powdered. The p oxide was prepared by treating pure metallic tin with concentrated "03, washing and drying the resulting product in the air. The dehydration of both samples was carried out according to the procedure given above. The results are given in Table I1 and in Fig. 4. The percentage loss in weight of Thiessen and Koerner's stannic oxide was calculated a t each temperature and the results shown in the third curve of Fig. 4. The significance of the curves obtained from the various preparations will be discussed after a consideration of the results found by an x-ray examination of the alleged hydrates of Thiessen and Koerner.
T e m p e r a t u r e , Degrees C. FIG.4 Temperature-Composition Curves of the So-called Alpha and Beta Stannic Oxides
11. X-Ray Examination of Thiessen and Koerner's Stannic Oxide. Separate samples of SnOzprepared by the method of Thiessen and KGerner were heated to temperatures that should give definite hydrates according to the composition-pressure and composition-temperature curves of these investiTABLE I11 X-Ray Diffraction Data for Dehydrated SnOz I
3
2
- -25'- _ _ 36'_ D I D I 3 . 3 5 10 3.37 8
2.65
IO
2.65
IO
1.78 1.43
IO
1.78 1.44
IO
5
5
3.40 2.64 1.77 1.43
5
4 90°
50'
D
I 8 IO IO
5
D
3.35 2.63
1.78 1.43
160'
3.35
I 9
IO
2.65
IO
IO
1.77
IO
3.40 2.67 2.35 1.77
5
1.43
7
1.32
2
1.21
5
4
D
6 Cassiterite D I
I 8
1.44
10
IO
5 IO
1.16
2
1.10 1.05
4 I
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HARRY B. WEISER AND W. 0. MILLIGAN
gators. The following temperatures were chosen: 25', 3 6 O , 50°, goo, and 160'. Specimens of the heated samples were sealed in tubes of "nonex" glass (a special glass made of materials of low atomic weight) and x-radiograms made by the powder method in the usual manner, using a General Electric x-ray diffraction apparatus. In each case the pattern of NaCl was obtained on the same film for purposes of calibration. The interplanar spacings and the visually estimated relative intenzities are given in Table I11 and in chart form in Fig. 5. The spacings are in Angstrom units and the intensities on such a scale that the most intense line is designated as IO.
FIG.j X-Ray Diffraction Patterns for Stannic Oxides: Thiessen and Koerner's Preparations dehydrated at ( I ) zj"C, (2) 36%, (3) 50°C, (4) 90°C, and ( 5 ) 160"C, (6) The Mineral SnOz.
Discussion of Results Composition-temperature curves of hydrous stannic oxide have previously been obtained by Carnelley and Walker' and by van Bemmelen.2 The smooth curves obtained by these investigators were quite similar to those for the a! and /3 stannic oxides, shown in Fig. 4. The curve obtained from the preparation of Thiessen and Hoerner is similar except for a change in its direction a t about 60-70'. The reason for this is obvious when it is recalled that the material consists of Sn02, water, and a little alcohol. The odor of alcohol is still apparent in a sample that has been heated to 40' and there is no question but that traces of alcohol persist a t least up to the boiling point. As has been seen, samples of SnO2 containing only water give a smooth, continuous curve. Samples containing alcohol but no water would be expected to give a curve of 2
J. Chem. SOC., 53, 83 (1888). "Die Absorption," 54 (1910).
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the same type but would reach the maximum sooner. I n the case under consideration, in which both loss of alcohol and loss of water are superimposed on the same curve, the two effects added together algebraically would be expected to give a curve of the shape found experimentally to hold. Since Thiessen and Koerner did not allow sufficient time for equilibrium to be established, the breaks in their curves are without significance so far as hydrate formation is concerned. This point has already been adequately discussed by Posnjak in connection with similar results of Willstatter and Kraut. Since the crystal structure of all the products formed a t various temperatures is identical with the mineral cassiterite, SnO,, with respect to the position and relative intensity of the lines on the x-radiograms, no definite hydrates of SnOz are actually formed. The only difference in the diffraction effects is that the size of the particle, as indicated by the width of the lines in the products obtained by dehydration, are smaller than those which compose the mineral cassiterite. It should be noted from Table I11 and Fig. 5 , that the interplanar spacings do not change (within the expected experimental error for such wide lines) with varying amounts of water, as would be the case if the SnOz and water formed a solid solution. This shows that the water must be adsorbed on the surface of the finely-divided crystals. Hydrous SnOz, whether in the so-called a or p form or as prepared by the method of Thiessen, consists of very small crystals of SnOzwhich adsorb water or both water and alcohol in the Thiessen preparation. When the latter material is dehydrated, a t temperatures up to about 60-70' most of the alcohol and some of the water is driven off; at temperatures above this point where little alcohol is left, the dehydration proceeds in a uniform manner as in the case of the familiar a and /3 preparations. Thiessen and Koerner found that the general blackening of the film upon which the x-ray diffraction patterns were recorded precluded accurate measurements, and concluded that the method was not suitable for distinguishing the presence or absence of hydrate formation. No such difficulties are encountered, however, when the proper exposure is made, and a special glass of low atomic weight is used. It is true, of course, that the broad, diffuse lines cannot be read with the accuracy obtainable on sharper lines; nevertheless all linea recorded in Table I11 are clearly visible on the original negatives. summary
The results of this investigation are as follows: I. Hydrous stannic oxide has been prepared by the hydrolysis of stannic ethylate according to the method of Thiessen and Koerner, who claim that a series of definite hydrates of SnOzresults from the thermal dehydration of this material. 2. Composition-temperature curves have been obtained under conditions that ensure the establishment of equilibrium. The curves are smooth indicating the absence of hydrate formation. Breaks in the curves of Thiessen and
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HARRY B . WEISER AND W. 0 . MILLIGAN
Koerner were due to their failure to allow sufficient time for equilibrium to be established under a given set of conditions. 3. X-radiograms of preparations obtained by dehydration at definite temperatures, show that no definite hydrates are actually formed, the several products consisting of SnOz of varying particle size with varying amounts of adsorbed water. This confirms the observations of Gutbier, Hiittig, and Dobling; Posnjak; Yamada; Weiser; and others. 4. The claim of Thiessen and Koerner, that the x-ray diffraction method is not suitable for determining the presence or absence of hydrates, has been shown not to be valid; on the contrary, when the proper precautions are taken the resulcs are definite and conclusive. The Rice Institute, Houston, Texas.
