T H E POLYMORPHISM O F SODIUM SULFATE: 111. DILATOMETER INVESTIGATIONS B Y F. C. KRACEK AND R. E. GIBSON
Introduction In a recent study Kracek' demonstrated that the numerous breaks in the temperature-time curves obtained when samples of pure anhydrous sodium sulfate are heated and cooled may be accounted for by the hypothesis that this salt has five polymorphic forms between 180' and 26ooC. These modifications were labeled by Roman numerals from SazS041,the variety which crystallizes from the molten salt, to Sa2SO4T'or thenardite, the form which appears from aqueous solutions. The foregoing investigation suggested several problems which might be solved with the aid of a dilatometer and which will now be outlined. The inversions of sodium sulfate fall into two distinct groups-the lower inversions which cluster around zooo and are in general irreversible or pseudomonotropic in the dry salt, and the upper inversions which occur about 240' and appear to be reversible. An important consequence of the pseudomonotropic nature of the lozcer inversions is that sodium sulfate is permanently altered physically by being heated to zooo and hence that two varieties of the salt exist a t room temperature, viz. Ka2S04Vand NaZSO4III. Measurements of optical properties* and densities3 have confirmed these conclusions but at the time these experiments were begun there was still a doubt as to which was the stable form at room temperature. All the transformations which Kracek observed were marked by superheating to a greater or less degree and so it was impossible to estimate the temperatures with any degree of precision. I t was of interest to attempt to reduce the width of the hysteresis loops and to fix more closely the equilibrium temperature of the various transitions. A problem closely allied with this one was an investigation of the promptitude and speed of each inversion. In addition we were interested in the sign and magnitude of the volume changes accompanying each change of phase. At the outset it was discovered that an ordinary glass dilatometer with mercury as index liquid could not be used in a study of the lower inversions, as it was found impossible t o prepare from solution crystals of KazS04which were entirely free from microscopic cavities including solution. A11 attempts t o remove the water from these cavities below 150' were abortive, and only when the crystals started to invert in the dilatometer were the last traces of J. Phys. Chem., 33, 1281 (1929). References are summarized in the first paper of this series. * Kracek and Gibson: J. Phys. Chem., 33, 1304 (1929).
T H E POLYMORPHISM OF SODIUM SCLFATE
189
water evolved. One sample of XarSOa,a part of which lost only 0.1per cent of water on ignition, produced sufficient steam in the dilatometer to distribute the confining liquid violently over the surroundings. Indeed, a sample which contained so little occluded moisture that I O grams lost less than 0.1 mg. on heating to 300°, contained enough t o cause a large anomalous expansion. iVe were therefore led to use an apparatus in which the transformations could be carried on under a pressure of fifty megabaryes-a pressure sufficient t o keep a n y water present in the liquid state below 270'. Although drying the salt above zooo spoils it for investigations of the lower inversions this treatment does not affect the upper inversions, and so it was comparatively simple t o p r e p a r e samples of KaeS04by ignition at about 100' which were suitable for examination of the upper inversions in an ordinary glass dilatometer. It has been suggested' that the speed and promptitude of these polymorphic changes might be enhanced by the presence of water. The pressure dilatometer provided a means of studying the influence of water on all the transformations, and we were able to reverse the lower inversions a t will. I n this article we hope to add to our knowledge of the polymorphism of sodium sulfate: FIG.I Glass dilatometer I. More accurate values of the various inversion temperatures. Conditions under which the lower inversions proceed reversibly. 2. 3. The effect of water on the speed and promptitude of the transformations. 4. The stability relations of the various phases. 5 . The sign and approximate magnitude of the volume changes accompanying the various metamorphoses.
Experimental Apparatus. I t is unnecessary to enlarge upon the glass dilatometer which is sketched in Fig. I . It consisted of a wide glass tube closed a t the upper end and sealed at the lower end to a capillary tube. Mercury was used as the index liquid and the movement of the meniscus in the capillary was carefully Kracek: Op. cit
190
F. C. KRACEK A S D R. E. G I B S O S
FIG.2 Diagrammatic sketch of pressure dilatometer
FIG.3 Photograph of pressure dilatometer and accessories.
T H E POLYMORPHISM O F SODIUM SULFATE
I---
I measured. The capillary was calibrated Steel with a weighed thread of Hg. Sodium Capillary Tubing sulfate wasintroduced in weighed amounts into the apparatus through the tube A which was immediately sealed. The dilatometer was then evacuated and heated to 25ooC for an hour after which the mercury was allowed to flow in a t C. The st,opcock B permitted the arbitrary adjustment of the mercury meniscus. With this apparatus volume changes of the order of 0.2 mm3 could be detected. The complete assembly of the pressure dilatometer is shown diagrammatically in Fig. z and a photograph of the apparatus is given in Fig. 3. The apparatus consisted essentially of a small steel vessel or bomb A connected by a fine steel capillary tube to an Amagat press. The travel of the piston in the press was read on the vernier calipers B the fixed jaw of which was securely clamped to the body of the press while the movable jaw was brought up N against a bar on the piston carriage. As the vernier was graduated to 0.02 mm and the cross section area of the cylinder was j 4 . 5 mm?, the volume changes could be estimated to within 1.1 mm3. The bomb was built of case-hardened machine steel and is illustrated in Fig. 4. To close the bomb and render it absolutely leak proof the conical portion of the steel connection was forced into the copper washer which was supported by a horizontal “flat” and the cylindrical wall of the vessel. J1-e have found this combination very effective -indeed and with slight modification it, FIG.A can be made to hold absolutely tightly Diagram of steel bomb shoving detail a t pressures as high as 15,000 atmos- of packing. pheres. A two-liter Dewar flask filled with highly refined heavy mineral oil served as a heating bath. The flask was fitted with an electrical heater of bare nichrome wire of 2 5 ohms resistance, a stirrer, and a syphon for adjusting the level of the oil. As there was extremely small lag between the bare heater and the oil we were able to change the temperature promptly and maintain it with ease at any particular value. At higher temperatures it was necessary to keep the mouth
>
192
F. C. KRACEK . 4 S D R. E. GIBSOS
of the flask well plugged to eliminate “flashing” of the oil vapors. We found, however, that plugging with absorbent cotton was sufficient t o prevent “flashing” even when the temperature of the oil was 265’. Two copper-constantan thermocouples were placed in a thin glass tube attached to the bomb. One thermocouple was of the differential type with junctions a t the top and at the bottom of the bomb and served t o indicate the magnitude of the temperature gradient throughout’the bath. When the stirring was rapid this difference between the temperatures of the two junctions never exceeded 0.2OC. The junction of the reading thermocouple was placed midway between the junctions of the differential couple. The thermal E.hl.F.’s were read on a potentiometer which has already been described.’ As the constantan wire was cut from the lot used by Adams? in preparing the temperature-E.M.F. tables for copper-constantan elements, it was considered sufficient to check the calibration only at the boiling point of naphthalene ( 2 1 j.95’). By these means we were able to measure the temperature of the bomb and hence of its contents with sufficient accuracy. The material under investigation was weighed into a gold crucible (Fig. 4), and so introduced into the bomb which was then filled with mineral oil. The contents of the crucible were rendered practically air-free by immersing the salt in oil or solution which had previously been placed in the crucible. The upper plug was fitted and screwed into place so that the oil was forced out through the capillary tube. At the same time care was taken to see that the press and its connections were filled to J with the same oil. The bomb was immersed in the oil bath and the capillary connected to the press by the joint J . In this way air was eliminated from the apparatus. The temperature was then gradually raised and held constant at appropriate points. At each stop sufficient time was allowed to permit equalization of the temperature throughout. The volume was then adjusted till the pressure gauge read exactly 50 megabaryes and the position of the piston was read on the vernier calipers. It may be mentioned that the system composed of the dilatometer and the pressure gauge made a very sensitive thermometer and when the gauge needle remained steady we were sure that temperature changes taking The piston displacement was place inside the bomb did not exceed o.I”C. plotted against the temperature to obtain dilatometric curves which will be discussed later.
Materials. Purified anhydrous sodium sulfate was recrystallized in the way already described;3 in fact, Preparation 2 (large crystals of dry neutral Na2S04) and Preparation I O are the same as those bearing these numbers in the paper to which reference has been made. Preparation 11 was crystallized by slow evaporation from a o.ojN acid solution, and cont,ained 0.4 per cent H2S04. The pellets described as PreparaIT. P. White: Z. Instrumentenkunde, 34, 7 1 ( 1 9 1 4 ) . “International Critical Tables,” 1, j8. a Kracek: Op. cit.
* L. H. Adams:
THE POLYMORPHISM O F SODIUM SCLFATE
I93
tion 1 2 were made by compressing the very fine powder obtained by the slow dehydration of Na2SO4.7H2O. The differential pressure employed in making these pellets exceeded jooo kg/cm2.
Results The experiments fell into three distinct groups according to the nature of the material enclosed in the dilatometer. We shall, therefore, consider first the behavior of dry neutral sodium sulfate, second, the behavior of the salt in contact with its saturated solution, and third, that of sodium sulfate crystallized from the faintly acid solution. Temperature-volume curues for d r y Na2S04. (a) Measurements in the glass dilatometer. Owing to the experimental difficulties mentioned in the introduction, ignited salt was the only type of preparation which could be in-
4 0
3
5
> 2
I
200'
210'
220'
233'
240'
250'
260 C
FIG.5 Temperature-volume curve for dry SatSO, in the neighborhood of the upper inversions. (Glass dilatometer.)
vestigated in an ordinary dilatometer and then only the upper inversions could be studied. As these experiments carried on under atmospheric pressure are of the more conventional type we shall discuss them first. In all of the diagrams that follow, the change in volume from an arbitrary zero is plotted as ordinate with the temperature as abscissa. Readings taken on heating are indicated by open circles which are plain when the readings were absolutely steady and provided nTith an arrow when the volume change still continued slowly even though the temperature had been held constant for a t least fifteen minutes. Points taken on cooling are similarly distinguished but are represented by black dots. Those wishing to know the exact preparation used in each experiment will find the information in Tables I t o IV. The diagram in Fig. j represents heating and cooling volum curves for K a 2 S 0 4in the neighborhood of the upper inversions as observed in the glass dilatometer filled with Preparation 2 . C p to 241' the expansion proceeded regularly but beyond this temperature the inversion started and went very
I94
F. C. KRACEK A S D R. E. GIBSOh
slowly. I n the vicinity of 245' the volume increased rapidly but not until 255'-260' was the reaction complete. The time required to complete the change was of the order of one hour. When the apparatus was cooled regular contraction took place until the temperature fell to 23 j", and then the volume decreased rapidly. This break a t 235' was one of the most definite and reproducible points we encountered in all the investigation. The volume, however, did not return to its original value at 235', but when slightly over half the expected volume change had occurred the readings ceased to drift at constant temperature, and then not until 22oo-23o0 did any further abnormal diminution in volume indicate that a change of phase was taking place. At about this temperature the volume again decreased rapidly at first, but then quite slowly, and the cooling curve finally coincided with the heating curve. Although certain quantitative details of Fig. 5 were not exactly reproducible in all our experiments, the salient features cited above were always present and we would again emphasize the single large change beyond 241' on heating and the two changes-the first proceeding sharply a t 23s0, the second between 220' and 23oo-0n cooling. The volume changes accompanying these upper inversions are all positive; that is to say, the high-temperature forms have the larger specific volumes. The magnitude of the volume changes on heating and on cooling are collected in Tables I1 and IT. Reference to our previous papers' shows that ignited Na2S04remains in the form Na2S04111below the temperatures of the upper inversions; accordingly, we are dealing here with the phase changes Na2S04111-+NazS041,Na2SO4I+Ka2SO4II and ~ a 2 S 0 ~ I I - + N a ~ S 0 ~ I I I . Reversibility experiments (one of which is represented in Fig. 6) indicate that the equilibrium temperature of the I e I I inversion was 236'. I n the reversibility experiments the temperature was successively raised and lowered about a certain point and the direction of the volume change was noted a t each constant temperature. On the diagram the arrows connect consecutive readings and it will be seen that a t 235' the I+II reaction was proceeding whereas a t 237' the reverse change was taking place and phase I1 was passing t o phase I. An interesting reversibility experiment which demonstrated that rather than pass to Xa2S0411, Na2S04111will remain inert up to 241' and then pass to Na2S041is depicted in Fig. 7. The experiment was carried on in the usual way until the break on the cooling curve indicated that the reaction Na2S041+Xa~SOJI had begun. At the point A where this reaction was nearly complete the temperature was raised. The path of the curve may be followed by the arrows and shows that the change had reversed a t ~ 3 7 'and ~ a t 239' the reading was back on the original cooling curve. Again the temperature was lowered and a t the point B where it was certain the I+II inversion was complete but the 114111 change had not begun, the temperature was again raised. The point at 238' shows that again the reaction 1141 'Kracek: J. Phys. Chem., 33, 1281 (1929); Kracek and Gibson: J. Phys. Chem., 33,
1304 (1929).
THE POLYMORPHISM O F SODIUM SULFATE
I95
was going rapidly. Next time cooling was continued as far as C where undoubtedly part of the 11-111 change had taken place. When the temperature was again raised a certain increase in volume took place but after 239" the curve ran parallel to the original V-t curve until at 245' another sharp volume increase brought the reading up to the original cooling curve. The reaction at 237-238" is the inversion of Na&0411 left unconverted at C t o Na&O4I. The reaction at 245-246" is the XazSO4III+Pia~SOJ transformation which we always find on the heating curves for Na2S04111. Fig. 7 is in complete agreement with Kracek's reversal experiments1 and enables
FIG.6 Temperature-volume curve for dry Na2SOd showing the reversibility of the I d 1 inversion. (Glass dilatometer.)
us to conclude (a) that the change N a 2 S O J ~ i X a z S O d Iproceeds promptly and rapidly in both directions and that the change Xa~SO~III+NazS0411 will not occur in the dry state, but that when Na2SOJII is heated it remains inert until the temperature is high enough for the inversion Na2S04111-+ h'azSo4I to take place. I n Fig. 8 we have collected the results of other experiments which show the resemblances and differences which were encountered. (b) Measurements i n the pressure dilatometer. I n this series of experiments we were able to study all the inversions from 160' to 260'. The first material used was very dry-it was Preparation 2 which had been in an air oven a t 110' for two to three months. Microscopic examination revealed that it was thenardite. Fig. 9 gives the complete behavior of this material a t a pressure of 50 megabaryes the first time it was heated from 160' t o 260". The small break A is identified with the change S ~ Z S O ~ T ' - - + X ~ ~ SAfter O~IV. this break the curve runs steadily up to 220' when a change of slope is seen. The increase in volume begins beyond 241' and ends a t 255-260". The distribution of the points shows that the reaction was comparatively slow. 1
Op.cit., p.
1294.
196
F. C. XRACEX A S D R. E. GIBSON
Here we may contrast the heating curve in Fig. 9 with the heating curve for moist NazS04 in Fig. I I . We notice at once that the large volume decrease beyond 186' is absent in Fig. 9 and that the 241' inversion involves less volume change in Fig. 9 than in Fig, I I ; in other words the change XazS041V+ Na2S04111 has been delayed in the dry sample until at least 220-230'. On cooling, the 236' inversion takes place promptly and the usual flattening of the curve is followed by the second volume decrease. The final part of the cooling curve lies below the initial heating curve, a circumstance which is explained by the observation that the phases are not the same on
FIQ.7 Temperature-volume curve for dry Na2S04 illustrating the reversibility
of the Id11 invereion and the irreversibility of the II#III inversion. (Glass dilatometer.)
heating and cooling. The heating curve refers to Sa2S041V;the cooling curve to the denser Na2S04111. Microscopic examination at the conclusion of the experiment identified the salt as Na2S04111. When the sample was heated a second time the phase in the dilatometer was h'a2S0,111 and Fig. I O shows that the volume-temperature curve was absolutely straight from 195' to above 241" when the large volume change occurred. The transformations on cooling were again divided into two parts but the cooling curve fell below the heating curve. This effect was certainly not all due to leakage, but we have been unable to find a satisfactory explanation for it. The volume changes are summarized in Tables 111 and I\-. Volume-temperature curties for Na2S04 in contact with its aqueous solution. The gold crucible was filled within 5 mm of the top with known amounts of coarsely crystalline thenardite and its saturated solution. 3Iineral oil was added to fill the crucible to the brim. The crucible with its contents was introduced into the bomb of the pressure dilatometer which was also filled with the same oil and closed in the usual way. Fig. 11 shows a heating curve
T H E POLYMORPHISM OF SODIVM S'L'LFATE
I97
obtained from this type of experiment. Soteworthy features of this curve are the volume increase a t 186", the large volume decrease at 19j", and the rapidity of the 241' inversion which was all completed a t only a little above 241' and required less than fifteen minutes. There are also irregularities between 220' and 230' which are not due to the apparatus, as a comparison with Fig. I O will show, but which are connected with the metastable inversions. h detailed study of the lower inversions led to the conclusion that the V-IV transition proceeds below 180" and that the equilibrium temperature
FIQ.8 Temperature-volume cumes for dry KatSO,. (Glaas dilatometer.)
of the important IV-+III change is 185 i IO. This change is accompanied by a decrease in volume. An interesting property of this inversion is that its speed is apparently different in the two directions and that the rate of reaction method of determining the equilibrium temperature is not applicable. I n the left-hand curve of Fig. 1 2 details of a reversal experiment are given. As before, the arrows are drawn between consecutive observations and the directions of the tails on the circles indicate the direction of the volume drift. At 187.0' NaZSOJV was changing to Na2S04111;a t I S ~ O ,the volume was constant, and a t 183.5', with both phases present, a slight increase in volume showed that Na2S04111was transforming to Na2S041V. Even in presence of water, however, the III-+IT' transition showed great reluctance to proceed if there were no NazS041Vin contact with the Sa2S04111. Turning to the upper inversion we see, by comparing curve B of Fig. 1 2 which is a cooling curve, with the heating curve in Fig. I I, that the hysteresis loop is very small; on heating the change is finished at 246' and on cooling it is already proceeding rapidly at 240'. Indeed it was easily possible to observe
198
F. C. KRACEK AXD R. E. GIBSON
this change taking place in one direction a t 243' and in the opposite direction at 240'. We estimated the equilibrium temperature as 2 4 1 IO. The cooling curve also lacks the double break a t 236' and 220-230' which is characteristic of the behavior of the dry sodium sulfate and, except for minor irregularities, no change appears to take place until below 180' when Na2S041V and then NatS04V are slowly re-formed. On the day following the experiment which gave curve B in Fig. 12, the temperature having fallen overnight to 100') observations were made on
*
FIQ.g Temperature-volume curve (heating and cooling) for dry Na2S04in pressure dilatometer. Original phase Na2S04V.
reheating the same material and these are recorded in curve C of the same figure. The appearance of both breaks characteristic of the lower inversions shows that during the night the sodium sulfate had all transformed to thenardite, which evidently is the stable form at lower temperatures. The volume changes measured in these experiments with the moist salt are to be found under the appropriate headings in Tables I and 111. Temperature-volume curves for slightly acid NU2804. The behavior of Preparation I I , which it must be emphasized was free from superficial moisture, in the pressure dilatometer is recorded in Fig. 13. The heating and
THE POLYMORPHISM O F SODIEM SULFATE
I99
cooling curves over the whole range are similar to those for Sa,SO, in contact with solution. The lower inversions, the minor effects around z 2 0 ° , the rapidity and small hysteresis loop of the upper inversion are all noteworthy features of resemblance, while the absence of the breaks on the cooling curve a t 2 3 j oand 2 2 5 ' carry the similarity still further. Curve A in Fig. 13 shows the results of a cycle of observations. After the lower inversions had been brought about heating was continued up to 206' and then the apparatus was cooled. Instead of following the broken
6
5
A
"?u .?
zl
2
I
11
FIG.I O Temperature-volume curve for dry S a 2 S 0 4in pressure dilatometer. Repetition experiment after Fig. 9 . Original phase S a ~ 8 0 r I I I .
line the points on the smooth cooling curve fell somewhat higher. At 176' the point lay considerably above the extension of the upper branch of the heating curve. After se;.eral hours the volume changed to a value almost equal to that required by the broken line and shortly thereafter the III-+Is' inversion started. When this change was complete the observations again fell on the initial heating curve for thenardite, as may be seen from the two points marked with the double circles. This experiment and others convinced us of the reversibility of the 18s"change and that leaks in the apparatus were quite inappreciable.
200
F. C. KRACEK A S D R. E. GIBSOS
Discussion of Results In$uence of Eater on the potymwph2c tratasfomiations. A comparison of the results for the dry salt on the one hand and for the moist and slightly acid salts on the other shows conclusively that the presence of water not only accelerates the stable transformations but d s o materially reduces the hysteresis. Figs. 9 and 1 1 show how true this is for the IV+III change which IS almost completely lost in the dry salt, and Figs. 9 and 1 3 show that the hysteresis in the upper inwrsion is reduced by water and the reaction
FIG.I I Temperature-volume curve for SazSOd in contact with saturated aqueous solution. (Pressure Ilatometer.)
velocity increased. Indeed, the time required for the 11141 change was reduced from more than an hour to about ten minutes by the presence of water. Such an effect may be due to rapid solution and recrystallization but we doubt if this is the whole story. It seems reasonable to conclude that recrystallization may provide a plentiful supply of nuclei of the new form and that from these nuclei the inversion sweeps through each solid crystal. Even in the case of the dry salt we have shown (Fig. 7) how much
THE POLYMORPHISM OF SODIUM SULFATE
201
more easily reversible the transformations may be when two phases are present. Stability of the phases. Enough has been said to show that thenardite is the stable phase at room temperature, that thenardite passes reversibly to Na2S041S'which in turn undergoes an enantiotropic change to XazSO,III a t ,185 .'1 The pressure dilatometer experiments make it impossible to avoid the conclusion that SazSOrIII is in equilibrium with P\'aaSOqIat 241' and that NazS0411therefore has no region of stability at low pressures.
*
tc
FIG.1 2 Temperature-volume curves (heating and cooling) for NaL301 in contact with saturated aqueous solution. Illustrative of the reversibility of the I V s I I I inversion.
The experiments with dry T\'a2SOIin the glass dilatometer showed that at 236' the transition Na2S041f.iSa2S0411proceeded sharply and reversibly, that it was impossible to bring about the transformation Na2S0dII-+ ~ a 2 S 0 4 1even 1 when both phases were in contact, and that no definite temperature could be assigned to the slow reaction ~azS0411-+NazS04111. We have the following explanation to offer. Na2S041shows great reluctance to pass to ~ a 2 S 0 4 1 1in1 the dry state and so when it is cooled the temperature
202
F. C. KRACEK A S D R. E. GIBSON
falls below the equilibrium temperature to that of the reaction IiazS041% Ka:SOJI, a change which we have shown to take place promptly and rapidly at 236', and so instead of the stable phase I11 we have the temporary appearance of the metastable phase T\'a2S0411. When the temperature falls far enough I1 passes slowly and irreversibly to 111. We have seen that water cuts down the hysteresis accompanying all the changes. Hence in the moist ?rTa2S04the reversible Is111 change occurs promptly a t 241' and XazS0411 has no chance to appear. I n view of these phenomena we must modify
FIG.13 Temperature-volume curves for Sa?SOa containing 0.4 per cent H?SO1. (Pressure dilatometer.)
Kracek's equilibrium diagram by displacing the hypothetical thermo dynamic potential-temperature curve for Sa2SOlII parallel to itself so that below 240' it always lies above the curve for Xa2S04111. The modified diagram drawn so as to place the intersections representing the phase changes I Y S I I I , I I I e I and I I e I at the appropriate temperatures is given in Fig. 14. Volume changes. The inversion of Na2SOaVto ?Ja2SOaIT' was not very rapid and the change in volume involved was very small. We were unable to obtain any definite information about this reaction except that the volume
THE POLYMORPHISM O F SODIUM SULFATE
203
change is positive, NaZSO4IVhas a higher specific volume than Na2S04V, and that its magnitude is certainly less than 0.0005 cm3 per gram. The transition KazS041V+Xa~S04111 was much more amenable to investigation and after we had established the equilibrium temperature close to 185', the change in volume accompanying the transition was determined by measuring the distance between the two portions of the heating curve at 185'. As already emphasized, it was not possible to study the inversion at 18join the dry material; all these results refer either to the moist salt or t o
-.,A .
I
I60
200'
220'
260c
240'
FIG.14
Modified equilibrium diagram illustrating the polymorphism of KatSOd.
che acid salt. The results are to be found in Table I and are, on the whole, consistent. We may summarize them by saying that the volume change accompanying the reaction Na?S041V+NazSOJII is negative and amounts to 0.0034 cc per gram. TABLE I Changes in volume accompanying the transition XazS041V+NazS0~III in the pressure dilatometer Preparation
I1
-
I1
13 B
I1
2 2 2
2 2 2
Figure in text
+ Hz0 + Hz0 + Hz0
+ dil.HzSOa + dil.HzS04 + dil.HzSOc
13 11 I2
A
c
Volume change in mm3 a t 185"
Volume change in cm3 per gram a t 185'
18.30 18.30 18.30
-66.5 -64.4 -61 .o
- 0.00363 - .00351
21.7**
-75.7
21.7**
Weight in grams
I2
21.7**
-77.4 -76.3
-
19,oj** 19.05**
-55.6 -67 . o
19 os**
-51.2
-
,00333
- 0.00349 .003j6
- .00352*** - 0.00289
-
00352
- .00269***
Average -0.003 43 ** Refers to amount of salt in contact \\-ith saturated solution at 185". * * * Reversibility experiment.
204
F. C. KRACEK A S D R. E . GIBSOS
I n the second paper of this series' we showed that at 2 j othe specific volume of NazSOIIII is 0.0045 cc less than that of thenardite. Making allowances for the differences in thermal expansion we conclude that the agreement is close enough to warrant confidence in the estimate made with the pressure dilatometer. I t was possible to obtain estimates of the change in volume during the I I I G I transition both on heating and on cooling, in the glass dilatometer and in the pressure dilatometer. The results obtained with the former were quite consistent, as Table I1 shows, but those obtained with the pressure dilatometer (Table 111) were not so precise and their agreement with the results in Table 11 leaves something to be desired.
TABLEI1 Changes in volume accompanying the transition NazSOIIII--tSa&OJ in the glass dilatometer Preparation
Figure in text
2
6
2
7 5
2
I2
-
I2
8D
IO
8B 8 *A 8C 8C
IO 10
IO
Weight in grams
Volume change in mm3 a t 241'
22.97 2 2 97 2 2 97
161 o 160 o 176.5
17.42 17.42
1 2 5 . j*
.00720
120.0
,00689
148.9 142.5 141.5 143.5
,00725
20
55
20.55 20.55
20.55
Average
Volume change in cm8 per gram a t 2419
o
00701
,0069 7 ,00768
,00693 ,00689 ,00698 0.00709
* hleasure of I-tIII. Bearing in mind that the object of the pressure apparatus was to investigate questions of stability rather than to make accurate volumetric measurements, we shall give more weight to Table I1 and conclude that during the reaction ~ a z S 0 1 1 1 1 ~ N a z S the 0 4 1change in volume is positive and equal to 0.0070 cc per gram. We estimated the change in volume when Na2S041passes to NazSOJI by measuring the distance between the two parts of the cooling curve a t 236'. Owing to uncertainty of the end product which is metastable we can not claim that these figures are more than rough estimates. They are given in Table IV, and indicate that at 236' the specific volume of KazSOII is approximately 0.004 cc greater than that of Ka2SO4II. Kracek and Gibson: J. Phys. Chem., 33, 1304(1929).
T H E POLYMORPHISX OF SODIUN SlTLF.4TE
2 0j
TABLE111 Changes in volume accompanying the transition NaZSO4III%iNazSO4Iin the ~ressuredilatometer Preparation
Figure in text
2
-
2
9
2
IO 10
2
-
2 2
IO
II
+
2 dil.HzSO4 * * See Table I.
Weight in grams
Volume change in cm3 per gram at 241'
2 0 .I
19.93 19.93 19.93 19.93 19.93
0.00651 ,00667 .oo61o .00788 .00580
,00767
Remarks
Cooling Cooling Heating, repeat Cooling, repeat Heating, repeat Cooling, repeat
16.8 16.8
0.00691 ,00723
Heating Cooling
18.30
0.00598
I3
18.30
,00586
Mean, heating and cooling ,Mean, heating and cooling
I1
2 1 .7** 21.7
0.00587
I2
-
18.9**
o ,0065
-
18.9 TABLE
.OOjj'I
,0064
Heating Cooling Mean, heating and cooling Cooling
Iv
Changes in volume accompanying the transition Ka2S041%sNazS0411. Preparation
Figure in text
Weight in grams
2
5
2
7
IO
8 8
22.97 22.97 17.42 I7 . 4 2 20.55
IO
8
2 0 . 5 5
I2 I2
2
-
10
2
-
IO
-
'9.93 19.93 16.8
Volume change in cm3 per gram at 236'
0,0037 ,0038 0.0042
,0042 0.0035
,003j 0.0048
.0047
0.0039
Remarks
Glass dilatometer Jl
*
11 1)
7) 19
Pressure dilatometer
,,
Jl
Concluding Remarks The results of this investigation are in good accord with the facts recorded in the first two papers of this series. The previous conclusion that I\'azS04is pentamorphic is confirmed in detail with the exception that we must now conclude that KaZSO4IIhas no region of stable existence at low pressures.
F. C. KRACEK AND R. E. G I B S O S
206
The metastable appearance of Sa2SOlII in the very dry salt is not surprising when we consider that the temperature of the reversible metastable transition Ka&O4II%iSa~SO4I,viz. 236', lies within the hysteresis loop of the stable Na2SO4III%Ka&3OaI inversion. The thermal analysis and the experiments in the glass dilatometer fix the extreme limits of this hysteresis loop as approximately 220' and 2 j o " . Establishing I8j' as the equilibrium temperature of the IV+III change helps to clear up a previously noted but unexplained difficulty. In the thermal analysis no breaks were detected below 19jo, yet, as observed in the first paper of this series,' Na&04111could be detected by microscopic examination in preparations which had been heated for some time a t constant temperatures as low as 190'. It is now evident that SazSOIIII may appear over the region from 18jO to z ~ I ' , in amounts which vary with the character and previous thermal treatment of the preparation. It was also pointed out in the first paper that as a consequence of the hysteresis the actual equilibrium temperatures could not be deduced from the thermal analysis curves. A comparison of the curves for the pressure dilameter experiments on the dry and the moist salt (see Figs. g and I I ) lends support to this statement, and Fig. 9 especially explains the occurrence of the lower inversions a t temperatures considerably above the equilibrium points.
Summary With the help of an ordinary dilatometer and a dilatometer specially developed for studying volume changes under a pressure of fifty megabaryes we have been able to confirm and amplify the results of the first two papers on the polymorphic transforinations of XazS04between 180" and 260'. These changes take place slowly and are accompanied by marked hysteresis when the salt is dry, but in the presence of water or a trace of H2SOp the salt inverts promptly and rapidly. In consequence of the more accutate estimate. of the transition temperature which we were thus able to make, we can assign to each modification the ranges of stability indicated by the following scheme where the figures below the arrows indicate the temperatures of each change and the figures above are the volume changes in cm3 per gram. 0.000j
+
-0.0034
a
0.0070
e
Sa2SOlT Na2SOlIJ7 iYa2S04111 Sa2S041 185' 2410 160-180' Sa2S0411has no region of stability at low pressures, but if the reaction NapS041+SazS04111is inhibited, the transformation of Fa2S041 to Sa2S0411 takes place reversibly a t 236", with a volume decrease of 0.004 cm3/'g. Geophysical Laboratory, Carnegie Institution of It*ashingto?i, J u n e , 1929. 1
Op. cit.
