THE POLTllOKPHIS31 OF SODIUM SULFATE:11. THE D E S S I T I E S OF X?;HTDKOPS PODIL31 SrLFATE XT z j o
ni- F. c.
K R A C E K ASD R. E . GIBSOS
Introduction From observations of heating and cooling curves for anhydrous .odium sulfate crystallized by several methods Kracek' showed that this salt eshibits complex polyinorphisni over the temperature interval from 190" to zsooc'. Correlation of these observations with the results of microscopic examination, by lliigge? and Kyrouboffj3suppleincnted by his own microscopic studies, led him to the conclusion that S a 2 9 0 4can exist in five modifications, some of which exhibit pseudo-inonotropic behavior while others can be readily inverted in the dry state. The form of anhydrous sodium sulfate stable at ordinary temperature is known as thenardite. When thij is passed thru a complete cycle of inversions, the result'ing product exhibits optical properties which differ significantly from those of thenardite. This modification has been termed "incerted U w m d l ' l e ' ' by Lliigge, y by IYyrouboff, and corresponds to forin 111 on the scheme of polymorphism advancpd by Kracek, thenardite being form T. A s the conversion of thenardite t o form 111 is undoubtedly accompanied by a volume change, it is reasonable to suppose that the density of a sample of ?;a2S04 should depend upon its previous thermal history, and in particular, on whether or not the sample had been heated above 2ooOC. Earlier determinations of the density of S a ? S 0 4 are summarized in Mellor.4 The results vary from 2 . 6 2 9 according to Filhol to 2 . 7 according to Schroder. The more important values lie between 2.65 and 2 . 7 . The uncertainty in these results is considerably more than the probable error of determination, and seems to indicate an uncertainty of the nature of the samples investigated. The most accurate determination of the density of anhydrous sodium sulfate is that of Richards and Hoover,: who found that sodium sulfate which had been heated to the point of fusion gave, on the Wyrouboff top. cit.) found average, 2.698 i ,002 for the density at 30'. 2.696 for ignited Sa2S04. In this work we have determined the densities of several preparations of pure anhydrous sodium sulfate whose careers have been carefully watched from the time the salt was crystallized. J. Phys. Chem., 33. 1 2 8 1 119291. hliigge: Seues Jahrb. 1Iineral. Ceol.. 1884, ~ ~ 1-14. 1 . 11-yrouboff: Bull. soc. min. F r a m e . 13. 311-6 '18901. ' llellor: "Comprehensive Treatise on Theoretical a n d Inorganic Chemistry," 2 , 662 Richards and Hoover: J. .lm. Chem. Yoc., 37, 108 (191j 1 .
THE POLYMORPHISM O F SODIC31 SCLFATE
I303
Experimental The material used in these experiments was derived from J. T. Raker's C.P. .Inalyzed .Inhydrous Sodium Sulfate and from Kahlbaum's Anhydrou? Sodium Sulfate. I t was essential to secure well-crystallized material which was as free as possible from liquid inclusions. Methods for preparing crystal. of anhydrous sodium sulfate have already been discussed in the preceding pfiper of this series and indeed the preparations used in this work were among those used in the thermal analysis. For the sake of clearness, however, we shall repeat briefly descriptions of the preparations whose densities are given in Table I. Preparation I consisted of small and irregular crystals of ?;a2S04 grown froin a solution a t i o o . The solution was not covered and the crystals separated as a crust on the surface. On ignition the material lost 0.1 per cent water. Preparations z and 3 consisted of large crystals grown slowly a t j o O - 8 0 ~ in a covered vessel. On ignition they lost 0.03 per cent water. Preparation 4 was made in the same way as Preparation 2 except that the solution was alkaline to an extent of z X IO-~S. The crystals lost no water on ignition and did not give an alkaline reaction. Preparation j was similarly obtained from a solution containing I X IO-*?; H2S04. It lost 0 . 0 2 4 per cent of water on ignition, and contained 0.3 per cent HgSOI. I n all cases the solid was separated from the supernatant liquor by suction filtration or by decantation, dried a t about 100' for many days and divided, after careful crushing, into various sizes by appropriate sieves. X sample, usually of the 10-28 mesh size, was then taken and divided into two parts. On one portion density measurements were made immediately. The other portion was heated to 300°-4000 in a platinum vessel and the loss of water during ignition was noted. The sample was cooled and it's density determined. The ignited portions of each preparation are indicated with an A if cooled slowly after heating and an A ' if cooled rapidly. Thus, samples 2 , 2.4 and zd' were taken from the same preparation, sample z was never heated beyond 100') sample 2A was heated to 300°-4000 and cooled slowly and sample z A ' mas heated to the same temperature and cooled rapidly. Part of preparation 2 was fused carefully a t 88 j oand cooled very slowly to room temperature. *I fine clear crystalline product, fused S a 2 S 0 4 ,was obtained. For the density determinations flat-topped pycnometers of the type described by Johnston and Adamsl were used. The balance, weights, thermostat, multiple junction thermocouple and potentiometer used in connection with the thermostat have been referred to in previous papers.' Experiments were made a t 2 j o j, o.01"C. Xylene mas used as the displaced liquid and its density, determined by comparison with pure xater, was found to be 0.8 j z o l 0 . 8 5 2 0 and 0.8519 grams per millilitre. The specific volume was, therefore, 1.1;37 nil. per gram. The experimental results were corrected for buoyancy of the air by the formula proposed by Johnston and Adams. Johnston a n d .*dams: J. .im. Chem. Soc., 34, 563 (1912). Gibson: J. Phys. Chem., 31, 496 (19271.
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F. C. KR.4CEIi ATD R. E. GIBSOS
TABLE I Density of S a z S 0 4at Pre,paratloii
C; H ? 0 occluded
n - t . of Sa:SOa
13.110.; 16.440; 14.j629 13.8350 13.6384
I
0.103
2
0.028
3 4
0.040
1-01. of xylene displaced
1.9300 6.1712 j.j437 5.1860 j.1239
o.ooo 5 0.024 Densit,y of Sa?SO4 dried at about -1v. for 2 , 3 and 4
IA
i
o.I'C
Density T'ncorrected corrected density for buoyancy
Density corrected for occluded Hz0
2,6593 2,6640 2.6630 2.6678 2.6617
2.661 2.663 2,663 2.665 2.661
2.6368 2.661j 2.6605 2.66j3 2.6592 av. = 2.6625 = 2.664*0.001
2.692; 2,6899 2.6948 2.6922 2.4 ' 2.69j1 2.6925 3A 2.6921 2.689; 4 .A 2.6988 2.6962 5 -4 2.6896 2.6870 58' 15,1407 z ,6909 2.6883 Fused 18.7643 2.6971 2.6997 Density of ignited S a z S 0 4 ,gross av. = 2.692 = 2.69j+0.001 Av. for qA and Fused 2A
'
11.3313 16.1143 1j.6678 1j.91j6 14.0163 17.3482
IOO',
25.00
4.2096 5,9799 6.5jj3 5.9118 5.193; 6.4j02 5.6267 6.9j04
2.690 z ,692 2.692 2.690 2 ,6962 2.687 2.688 z .69jl
Discussion of Results The accuracy of the foregoing results is strictly limited by our inability to prepare from solutions crystals of anhydrous sodium sulfate which were entirely free from inclusions. I n the case of the unignited material an approximate correction for the volume of the water included was applied, bringing the resultsfor preparations 2 , 3 and 4 to an average value of 2.664 =t0.001. The results for preparations I and 5 are lower; the amount of included water in preparation I makes the correction uncertain, while j contains about 0.3 per cent HzSO, (determined by titration of a sample with standard alkali), and hence, is not strictly pure SazS04. Ignited preparation 5 also yields lower density than the others. Our best result is that for preparation 4 which contained so little occluded water that the loss of weight on ignition could not be measured, but even this mat,erial contained some microscopic inclusions. In the case of the ignited preparations the results are vitiated by the uncertainty of filling of the cavities resulting from the inclusions, with xylene. It is significant that the values for preparation 4 and for the fused l;apS04 agree closely while all the others give lower densities. In view of these considerations we estimate the density of the unignited crystals (thenardite, KasS04 V) as 2.664 i 0.001, and that of the ignited, cooled crystals (Sa?SOr 111) as 2.697 i 0.001,at 2 i . 0 0 i o.01"C. The latter value checks closely xith the best published determinations.
THE POLYMORPHISM OF SODICTY SULFATE
1307
The purity and homogeneity of the preparat'ions used were checked by careful microscopic examination for which we are indebted t.o Dr. J. W. Greig. T'ery little information could be obtained on the nature of the inclusions. These are generally so minute in size that even with the highest power of the microscope it was not possible to tell whether they were filled with liquid or air after ignition. The nature of the phases present could be told with certainty for the modifications V and 111. The samples examined before ignition consisted of thenardite I S a 2 S 0 4T-) with only traces of other material. hft,er ignition, and in the case of the fused sample, the bulk of the material was S a 2 S 0 4111. Xnother phase was usually present, in amount from a trace to perhaps as much as I per cent in preparation j. This phase resembles thenardite but has higher birefringence ; its low index is considerably lower than the low index of thenardite. The differentiation between the two is difficult n-hen they are present in small quantities mixed with a large amount of modification 111. This highly birefringent phase has been classified as modification IT in the first paper of this series.
Conclusion The results of this investigation show clearly that Ta2S04can exist at ordinary temperatures in a t least, two modifications. One of these is t,he ordinary low-temperatnre form. thenardite (iYa2SOi 1-1,with a density of 2.664 ==I 0.001. When this is heated above zoo' it becomes altered, passing through a series of polSmorphic changes, as indicated in the first paper. On subsequent cooling the salt does not revert to thenardite, if kept dry, but remains in the form of another modification, iYa2S04111, which has the density 2.69; i 0.001. In this connection result P A ' is of interest as the density determination vas made one week after the preparaticn was ignited. In the preceding papei of this series it was also shown that S a 2 S O 4kept after a cycle of inversiox at various temperatures below zoo' failed to invert to the low temperature form. The volume change accompanying the mutual conversion of these two phases is 0 . 0 0 4 j cm3,g or 1.2 per cent of the volume of S a 2 S 0 4Vat 2 jo.If the two modifications have sensibly equal coefficients of expansion up to the inversion point we should expect, a contraction of approximately 1.2 per cent when the reaction takes place. These density results have been combined with microscopic examination, and furnish a confirmation of the results of thermal study of this salt presented in the preceding paper. They are in accord with reliable data, both optical and on densities, published by other investigators. In view of the known stability of thenardite (5a2S04 T-) they firmly establish the pseudomonotropic nature of the mutual conversion in the dry state of the two modifications discussed. Nuch vague discussion has appeared in print on the metastability of matter. The phenomena involved are neither vague nor mysterious, and, admittedly, they abound in nature to a great,er extent than has for long been
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F. C . KRACEK A S D R . E. G I B S O S
suspected. Accurate work is required in their investigat'ion, but more than that, a varied attack is necessary, for phenomena which escape interpretation in one mode of att'ack can often be elucidated by a combination of methods. To take as example the present work on densities; alone it would have little meaning. Combined with .optical examination, thermal analysis, and the other methods employed, the results fall in line with a consistent scheme. The next paper of this series will deal with dilatometric measurements.
Summary At least two modifications of anhydrous sodium sulfate can e w t indefinitely at ordinary temperature and pressure. These forms, thenardite 0 1 Na2S04 Y, and ?u'anS04I11 have at z j . 0 0 i O.OI"Cdensities of z 664 0.001 and 2.697 =t0.001respectively.
*
Geophystcal Laboratory. Carnegze Iiist~tutioiiof Il'ashangton, May, 1929.
