PROPERTIES OF AMMONIUM KITRATE
I. A Metastable Inversion in Ammonium Kitrate' BY N. L. BOWEN
Introduction Ammonium nitrate has long been known to occur in a number of different crystalline forms and its several inversion points have been the subject of much study. Early and Lowry have investipated in a very careful manner all the changes taking place above room temperature and a t the ordinary ~ 125.2'. Their pressure. They found inversions2 to occur at 32.1', 8 4 . ~ 'and results confirm earlier work, but the actual values given are the most dependable yet found. The stable transformations of ammonium nitrate are expressed in tabular form in Table I where the designations of these forms are given according to the methods of the crystallographer and the physicist.
TABLE I Stability ranges of the modifications of ammonium nitrate at atmospheric pressure. Modification
Liquid Isometric (I) Tetragonal (11) Orthorhombic ? (111) Orthorhombic (IV) Tetragonal (V)
Range of stability "C
169.6 to 125.2 to 169.6 84.2 to 1 2 5 . 2 32.1 to 84.2 -16 to 32.1 - to-16
Experimental Method The writer has recently studied these transformations of ammonium nitrate by a method which permits direct observation of the changes and, a t the same time, control and measurement of the temperature. A small quantity of the nitrate, previously dried, is melted on an ordinary microscopic slide and, while liquid, is covered with a cover-slip which is pressed down lightly so that upon cooling one obtains a thin film of the nitrate analogous to the thin section of the mineralogist. This is substantially the method used by Wallerant of preparing slides of nitrate for a similar purpose3. In the present study the slide was then placed on a wire support in a small oil bath which was mounted on the stage of a petrographic microscope and which was electrically heated by means of a coil of nichrome wire immersed in the oil. ~~
Investigations carried on with the co-operation of Peerless Explosives Company, to whom we take this opportunity of expressing our thanks. The properties of ammonium nitrate. Part I. The freezing point and transitiontemperatures. J. Chem. Soc., 115, 1387 (1919). 3 Sur le polymorphisme et l'isomorphisme des azotes alcalins. Bull. Soc. Min. Fr., 28, 311 (1905).
N. L. BOWEN
722
The container for the oil was a glass crystallizing dish with flat bottom and the slide could thus be observed by transmitted light as it is ordinarily in the absence of the bath. The temperature was controlled by varying a resistance in series with the nichrome heating coil and was measured by means of a potentiometer and thermoelement of copper-constantan which dipped into the oil and whose junction rested on the mount, being separated from the charge only by the thin cover-slip. Experimental Results
b
General observations. With this arrangement it was possible to observe all the stable transformations of ammonium nitrate above room temperature in a very satisfactory manner. The temperatures of the changes correspond with those given by Early and Lowry within the limits of error, especially when the change was in the direction induced by rising temperature, With falling temperature there was often much lag and it was necessary to make the cooling rate very small in order to obtain a value corresponding approximately with that obtained in the reverse direction. The various forms of ammonium nitrate exhibit sufficient difference of optical properties that they may be distinguished from each other readily as thus viewed under the microscope. The orthorhombic form, stable a t ordinary temperature, has refractive indices y = 1.64-, p = 1.61, a! = 1.41+, and 2V is about 3 5'. The birefringence is thus extremely high, considerably higher than that of calcite1. The orthorhombic (?) or monoclinic form stable between 32' and 84' has a distinctly lower birefringence, though it would still be described as very high. It is further distinguished by an optic axial angle (zV) of nearly goo. The tetragonal form stable between 84' and 125' is uniaxial and positive. The birefringence is again distinctly lower, but would still be described as high. The isometric form is, of course, isotropic. On account of these differences in optical properties the transformations are readily observed between crossed nicols under the microscope and the various forms identified even when they persist in a temperature region where they are not stable. T h e metastable inversion. In addition to, and, in some measure, in place of the stable transformations that have already received so much study, another transformation was observed which was usually more dependable and prompt than the stable transformations. This was the metastable inversion, orthorhombic (IV) tetragonal (11) which occurs a t approximately 50°C. When the molten film of nitrate is cooled it crystallizes, usually with considerable undercooling, to the isometric form which is transformed to the tetragonal form upon further cooling. The actual inversion point may be overstepped as much as 18'. The tetragonal form may then be cooled through the temperature of 84' without showing any change and in practically all
=
The index a as given by Thomas and Hallimond at 1.45 (Trans. Faraday Soc., 20, 56 (1924) ) is much too high, indeed the match with amyl valerate of index 1.411 is sensibly perfect.
PROPERTIES O F AMMONIUM NITRATE
723
cases may be cooled to a temperature somewhat below 50' before the appearance of a new form which is not the orthorhombic ? (111)form stable a t this temperature, but the orthorhombic (IV) form stable below 32'. Even with very rapid cooling I have not observed the temperature to fall below 44' without the appearance of the new form, and when the rate of cooling is about one degree per minute the transformation always takes place promptly If the orthorhombic (IV) form thus obtained is now reheated, a t 47'-49'. transformation to the tetragonal (11) form takes place a t a temperature a little above 50'. Even with a rapid rate of heating I have not observed the orthorhombic (IV) form to persist above 54O, and with a rate of one degree This metastable per minute the change takes place promptly a t 51'-52.5'. transformation is thus readily reversible and rather less readily overstepped than the stable transformations when both directions are considered. One may sometimes heat the nitrate back and forth through the inversion point a score or more of times with repeated inversions in both directions and thus retain the one or the other of the forms in its metastable region for a total of several hours without the appearance of the stable, orthorhombic ? (111)form. If the temperature is carried below 32' no change occurs a t that point, since the nitrate is already in the orthorhombic form stable below that temperature, and likewise if it is carried above 84' no change occurs a t that point, since it is already in the form stable above that temperature. Moreover, when the temperature is thus carried upward or downward into the regions of stable existence of the respective forms no change is apparent in the tendency of these forms to persist in the metastable region and to exhibit the metastable inversion at 50' when the temperature is brought back to that value. Eventually, however, any given charge of nitrate appears to change its mind and decide to assume the stable modification, I have observed this to take place in only one way and that immediately after the metastable transformation in the cooling direction. The tetragonal form is observed to change to the orthorhombic in the usual way and then immediately the stable orthorhombic ? (111)form puts in an appearance. It starts at one or two centers from which it spreads with a circular front and ordinarily begins to form before the metastable transformation is complete in all parts of the charge, so that the two changes can frequently be seen passing across the field of the microscope at the same time, the metastable inversion somewhat in advance of the change to the stable form. It thus affords an excellent example of Ostwald's law of successive reactions. If the charge is now cooled below 3 2 O , transformation to the orthorhombic (IV) form occurs at that temperature, and if it is heated above ..4O, inversion to the tetragonal (11) form occurs, but the metastable transformation a t 50' is now no longer obtained. By carrying the charge above 125' and thus changing it into the isometric (I) form the metastable inversion can frequently be induced to reappear when the charge is again cooled and by remelting the charge its reappearance is practically assured.
724
N . L. BOWEN
Earlier Work indicating the Existence of the Metastable Inversion Certain aspects of the observations here recorded have been noted by others. Thus Early and Lowry note that undercooling readily occurs a t the 84' inversion point and state that "in many cases the salt was cooled t o 32' without any indication that this change of crystalline form had taken place".l While there is nothing inherently improbable in such a degree of undercooling, it was never observed in the present work, the metastable transformation always occurring a t a temperature well above 32'. Wallerant noted that ammonium nitrate could be cooled through the 84' inversion point without inversion and that the tetragonal (11) form might change directly to the orthorhombic (IV) form2. He did not, however, note
30
:
:
FIG.
PRESSURE I
%& /
Curve of variation of the orthorhombic (IV) tetragonal (11)inversion of ammonium nitrate (after Bridgman) showing its metastable extension (broken curve) and indicating an inversion point of 50.5" a t atmospheric pressure.
that the change took place a t any particular temperature. Behn, studying the inversions dilatometrically, noted that the tetragonal modification could easily be cooled to a temperature of 45', and definitely suggested that this temperature was determined by the 11-IV inversion, though the final result was always the form 111. He calls attention to the fact that Tammann had studied the 11-IV inversion a t high pressures, where it becomes a stable inversion, and that extrapolation of his curve to atmospheric pressure would indicate an inversion point of 45°.s Op. cit., p. 1392. Op. cit., p. 319. aProc. Roy. Soc., 80, 449 (1908). 1
PROPERTIES O F AMMONIUM NITRATE
725
There have thus been a number of indications of the existence of this transformation with falling temperature, but it has not been observed that it was definitely reversible and could be obtained with rising temperature as well. Concerning the actual temperature of the inversion it is to be noted that the temperature of 45’ where inversion was noted by Behn is definitely too low. While with rapid cooling it may fall to that temperature, with slower cooling it takes place a t a temperature of from 47’-49’ and with slow heating Having regard for the fact that all the inversions of it occurs a t 51’-52.5’. ammonium nitrate are more easily overstepped in the cooling direction than in the opposite direction, me may place the inversion point a t 50’ with the suggestion that it is a little higher rather than lower, but the accuracy of the determinations does not warrant any more definite figure. If we turn to Bridgman’s determinations of the transformations of ammonium nitrate under pressure we find that he has studied the 11-IV transformation throughout its stable range of some 8000 atmospheres’. A relatively short extrapolation of his values into the metastable region at low pressures indicates a value of 50.5’ for the transformation at atmospheric pressure. This value is in good agreement with that observed in the present study. Bridgman’s results on ammonium nitrate are here reproduced in Fig. I , with the metastable extension of the 11-IV curve added. Perhaps the temperature 50. so, as determined by extrapolation from Brjdgman’s results, would be a more satisfactory value for recording as a constant of ammonium nitrate than any that could be determined directly in the metastable region. d
Summary Besides the several well-known transformations of crystalline ammonium nitrate it is found that a reversible transformation takes place at approximately 5oOC. The method of studying the inversion is that of direct observation under the petrographic microscope by which means the various ,forms can be identified in virtue of differences of optical properties. The inversion a t 50’C is found to correspond with the change, orthorhombic (IV) tetragonal (11). At this temperature both forms are metastable, nevertheless the transformationis, in many respects, more prompt and dependable than most of the stable inversions. Extrapolation of Bridgman’s values for the I1 e I V inversion a t high pressures, where it is a stable transformation, indicates a value of 50.5’ for the temperature of this change a t atmospheric pressure. This temperature is in good agreement with that here directly determined. P. W. Bridgman: “Polymorphic Changes under Pressure of the Univalent Nitrates”. Proc. Am. Acad. Arts Sci., 51, 605 (1916).
