PROPERTIES O F AMMONIUM NITRATE
11. The System : Ammonium Nitrate-Ammonium Chloride1 BY N. L. BOWEN
Introduction There has recently appeared a series of papers on the properties of ammonium nitrate, the result of investigations instituted by Professor Lowry but carried on latterly by Professor Perman. One of these has already been referred to in Part I of the present series. Another deals with the reciprocal salt-pair, ammonium nitrate and sodium chloride2. The binary system: NH4hTO3-NH4C1 is included within this salt-pair system and was accordingly investigated by Perman. He concludes that “no isomorphous mixtures or double salts are f ~ r m e d ” . The ~ present investigation confirms his conclusion regarding double salts but shows that with respect to isomorphous mixtures his conclusion is subject to revision. The ordinary method of investigating such a system is to subject mixtures to thermal analysis in the form of heating or cooling curves. When the results from a number of mixtures are assembled it is frequently possible to deduce a complete equilibrium diagram, but not infrequently there may still be some ambiguity. The present investigation arose from the fact that a marked break was obtained a t 1 0 9 T on the heating curve of a mixture of ammonium nitrate with 3 per cent ammonium chloride. Since no break was obtained a t 125’ it was considered probable that the break was due t o the tetragonal (11) --t isometric (I) inversion of the nitrate as changed by the presence of ”&I, a fact which necessitates solid solution. It was felt, however, that multiplication of heating curves would probably still leave ambiguity and that a method of direct observation of the changes should be adopted. Accordingly various mixtures were studied in an oil bath under the microscope, the arrangement of which has already been described in Part I. Thus was permitted, a t measured temperatures, the direct observation of inversions, formation and unmixing of solid solutions, beginning of melting and completion of melting, in fact all the changes necessary for the construction of a complete equilibrium diagram, For the study of those changes that may take place slowly such as the mixing and unmixing of solid solutions, it was possible to keep the temperature of the oil bath constant within about one degree for several hours by using a storage battery as a source of current. The investigation as a whole constitutes an excellent example of one manner in which the petrographic microscope may be used as an aid to the study of chemical equilibrium. On account of the fact that all of the changes Investigations carried on with the co-operation of Peerless Explosives Company, to whom we take this opportunity of expressing our thanks. E. P. Perman: J. Chem. Soc., 121, 2473 (1922). 3 Op. cit., p. 2480.
PROPERTIES O F AMMONIUM NITRATE
727
involved can be viewed actually in progress, the question of alternative interpretations scarcely enters. This is not always true in equilibrium diagrams, especially of systems where solid solution occurs, as evidence of which fact might be cited the several forms of the iron-carbon diagram that are current. The range of variety of the phenomena exhibited and directly observable, the relative ease of attainment of equilibrium, the accessibility of the tem. peratures involved and the consequent simplicity of apparatus required render the present system a very advantageous one to assign to students foi
'"1
l * * , ,, . . .
N4.Tl
In
....
I;wt
%N4C.
FIG.I Equilibrium diagram of the system: ammonium nitrate-ammonium chloride. ABKF -Isometric mix-crystals A E R -Isometric mix-crystals and liquid GKF -Isometric mix-crystals and tetragonal "4x03 DBKH -Isometric mix-crvstals and isometric NHK1 C E D -Liquid and isometric SHlCl GKH -Tetragonal NHaNOa and isometric xH4CI. Below ~~
practice purposes. They would thus be put in the way of acquiring a practical acquaintance with the principles of phase equilibrium and also of gaining some familiarity with the manipulation of the polarizing microscope. Preparation of Mixtures The materials used in the preparation of mixtures were J. T. Baker's analyzed ammonium nitrate and ammonium chloride. These were powdered and dried in an evacuated desiccator over P206after the method recommended by Early and Lomryl. Weighed amounts were then mixed in various proporJ. Chem. SOC.,115, 1389 (1919).
728
N. L. BOWEN
tions and the mixture fused in a short test tube by immersing it in an oil bath to avoid excessive heating. The mixture was well stirred while liquid and then chilled rapidly to prevent segregation. About 2 g, of each mixture to be investigated was thus made; the fused cake obtained was powdered and kept in the vacuum desiccator until needed for study. For the investigation of each mixture a small amount of the powder was placed on a specimen glass, heated gently until melted, covered with a cover slip and placed in the oil bath on the stage of the microscope. For most of the observations a magnification of 80 diameters was used, but for the study of some of the finer structures such as those produced by unmixing of solid solutions resort was had to some 2 0 0 diameters. By working in this manner and noting not only the temperature at which changes occurred but also the nature of the changes the results given in Table I were obtained. It was found that many of the changes were readily overstepped in the cooling direction but were prompt in the opposite direction. I n the case of the solidus and liquidus, especially, it is essential that these be determined in the heating direction.
TABLE I Temperatures of observed changes in NH4N03-NH4C1mixtures Composition Per cent NH&l 0
1.5 3.0 4.0
5.0 6.0
7.0
8.0 10.0
13.0 14.0
Tetragonal*Isometric inversion "C
Beginning of melting "C
Completion of melting "C
125 * 5 109 (in part) 109 (in part) I09 I09 I09 I09 I09 I09 I09 I09
These results, together with a few others that do not lend themselves to tabulation, are expressed in diagrammatic form in Fig. I .
Correspondence of the Results with those of Perman The diagram obtained by Perman is reproduced in Fig. 2 and comparison shows that the liquids of ammonium nitrate and the composition and temperature of the eutectic are in complete agreement with my results. The upper points on the NH&1 liquidus are somewhat lower than those determined by Perman but these are very difficult of determination because some decomposition sets in and a little gas is evolved so that the error here may be perhaps four or five degrees. It is sufficient to know that the NH&l liquidus rises steeply from the eutectic.
PROPERTIES OF AMMONIUM NITRATE
729
It is especially to be noted that some features of Perman’s results themselves point directly to the existence of solid solution. Thus he obtained a remarkable eutectic arrest in the 11 per cent mixture, the temperature remaining constant for fully seven minutes. On the other hand he obtained no break whatever in the six per cent mixture at that temperature, though in the absence of solid solution the pause should be about half as long’. The absence of the arrest is, of course, due to the fact that the six per cent mixture lies within the limits of solid solution. Moreover, in his later work, in which he separated the crystals from liquid by a method of filter-pressing and then analyzed the crystals, he was unable to get the NH4N03crystals nearly as pure as the NH4C1 crystals obtained on the other side of the eutectic2. The significant difference between the two diagrams is that the new one shows the liquidus of ammonium nitrate to be that of ammonium nitrate mix-crystals and not of the pure salt, The existence of these mix-crystals brings in the FIG.2 The system: ammonium nitratephenomena of melting intervals, inversion intervals and unmixing of ammonium chloride (after Perman). solid solutions which are the really interesting features of the diagram. The existence of such features in mix-crystals or solid solutions is, of course, well known, but the phenomena resulting have seldom been observed actually in progress with the clarity exhibited in the present system, so that some description of the observations would appear to be called for. Melting and Crystallization of the Solid Solutions Both the beginning and completion of melting of the solid solutions can be observed very definitely. Melting begins with the formation of narrow channels of liquid all through the originally homogeneous3 solid. As the temperature is slowly raised these gradually widen at the expense of the intervening areas of solid until they finally occupy the whole field and the solid has completely disappeared. Crystallization is practically the reverse of this. Upon cooling there is ordinarily five degrees or more of undercooling and then crystallization appears, not a t numerous dispersed points, but a t one, or a t most, two or three points from which branching crystal forms spread all through the liquid. As the temperature falls this skeleton grows a t the expense of the intervening areas of liquid until finally all of the liquid is exhausted. Perman: op. cit., p. 2474. Fig. I. Perman and Dawkins: J. Chem. Soc., 125, 1241-2 (1924). a “Homogeneous” is here used to designate the presence of only one phase and not to indicate uniformity of crystallographic orientation. When the latter condition is referred to it is specifically so designated. 2
730
N. L. BOWEN
It can sometimes be definitely observed that, as may be deduced from the diagram, the solid solution first separating is of different composition from that later formed. The index of refraction of ammonium nitrate mix-crystals increases as the content of NH4C1 increases. The branching crystals first formed are relatively poor in NH4C1and therefore of relatively low refractive index, and when crystallization is complete the original skeleton is still distinguishable from the rest of the otherwise homogeneous mass by a lower refractive index, detected by the half-shade method familiar to petrographers. There are, of course, no sharp boundaries, but a general fading of the one into the other. As a result of such zoning of crystals the crystallization interval obtained on cooling is greater than the true equilibrium interval, for such zoning represents a failure of true equilibrium. It should be noted, too, that, for the determination of the melting interval (rising temperature), material which has been crystallized in such a way as to give zoning can not be used because it will give too low a temperature of beginning of melting. Perfectly homogeneous solid solutions for the study of the melting interval were prepared by making the amount in the usual way and dropping it quickly into the oil bath, which was already at a temperature of 130'. The preparation is thus chilled instantly to 130' and crystallizes to a homogeneous solid solution. Melting and Crystallization of Eutectiferous Mixtures Eutectiferous mixtures such as that containing I O per cent JSHJ.21 are best crystallized in a similar manner. They then consist of areas of homogeneous mix-crystal with interstitial patches of a fine-grained intergrowth of NHdC1 and nitrate mix-crystals. When this is heated the fine-grained intergrowth is seen to melt as a whole a t 141' and then with rising temperature the excess crystals dissolve in the liquid. One thus obtains direct visual evidence that the change occurring at 141' is the melting of the eutectic intergrowth. If crystallized relatively slowly by cooling in the oil bath, so that zoning of mix-crystals occurs, even mixtures that lie within the limits of solid solution may show some residual eutectic intergrowth. Thus the seven per cent mixture, though it can readily be shown, when properly crystallized, to be within the solid solution area, may at times show some eutectic. This is due to the fact that the first crystals formed have only about 4 per cent NHdC1 and unless these can, with falling temperature, be completely made over into crystals richer in KH&1 by reaction with the liquid, the final liquid must have considerably more NH4C1than 7 per cent, that is, must lie within the eutectiferous region. Thus the 7 per cent mixture may or may not develop some eutectic mixture, depending on the conditions of crystallization. In other words free N H E 1 as a separate crystalline phase may or may not develop, depending on the opportunity for reaction (interchange of material) between crystals and liquid during crystallization. This factor is so important in silicate systems where solid solution is so prevalent that I have sought to emphasize it by
PROPERTIES O F AMMONIUM NITRATE
731
giving it a name for which I have suggested the “reaction principle”.l In mineral systems, as in the present system, the working of the reaction principle permits the development or the non-development of certain compounds as separate crystalline phases depending on the conditions of crystallization. Desch has offered objection to the principle and refers to it as “abandonment of the equilibrium diagram” though he fails to point out just where the deviation comes in2. It is, as a matter of fact, a direct deduction from equilibrium diagrams and is by no means confined to silicates. Here we have direct visual evidence of its working in a salt system and one could cite many systems involving either solid solution or incongruent melting and coming from Desch’s own field of metallurgy where the working of the reaction principle is of paramount importance in determining the phases present in an alloy and therefore in determining the properties of the alloy. Limit of Solid Solution
By crystallizing rapidly a t 130’ in the manner described it can readily be observed that most of the mixtures in the solid solution area form a single homogeneous crystalline phase. In the 8 per cent mixture such a phase forms first a t 130’ but, immediately, separation of NH4C1 occurs as skeletons of cubic pattern throughout the mass. Though both the nitrate mix-crystals and the NH4C1 are isotropic, the presence of two substances and the identity of each is readily established on account of the much higher refractive index of the “4C1. If, now, the temperature of the mass is raised to 138’, a temperature only a little below the eutectic, the NH4C1is observed to decrease in amount slowly. After 13 hours a t that temperature there is only a bare trace left and it persists with further heating at this temperature. If the temperature could be held exactly a t the eutectic without passing a little above it, and thus inducing melting, it is probable that this last trace of SH&1 would disappear. At any rate the amount left a t 138’ is so small that the extent of solid solution may be said to be nearly if not quite 8 per cent. Metastable Solid Solutions Even mixtures well beyond the limit of solid solution, indeed on the NH4C1 side of the eutectic, may crystallize in such a way as to give transitory, metastable, solid solutions. Thus the mixture with 13 per cent n’H4C1will cool in the oil bath t o about 136’ (well below the eutectic) before crystals form and it then crystallizes to a single, homogeneous phase. This phase is scarcely formed, however, when it begins to separate into two phases. This unmixing starts a t one or two centers from which it spreads after the manner of an inversion. It is plainly the unmixing of a homogenous phase and not the gradual coarsening of a sub-microscopic intergrowth of two phases, for this latter would take place very gradually and all parts of the mass would, a t any time, be a t the same stage of coarsening. Contrary to this, the actual change N. L. Bowen: “The Reaction Principle in Petrogenesis”. J. Geoi., 30, 177-198 (1922). (1925); and also Presidential Address Section B, Chemistry, British A. A. S., Southampton, 1925, p. 20.
* C. H. Desch: Trans. Faraday Soriety, 20, 472-3
732
N. L. BOWEN
a t any point is an instantaneous one from a perfectly clear, homogeneous mass to a turbid intergrowth of two phases which does not thereafter change any further. The change is propagated from point to point as a circular wave which may take two, three or even five minutes to traverse one centimeter. At the end of this 5 minutes the portion not yet reached by the wave of unmixing is still the clear, homogeneous solid solution that it was originally. While there may thus elapse several minutes between the time of formation of the metastable solution and the completion of unmixing in all parts of the mass, I have never observed any appreciable interval between the formation of the homogeneous crystalline phase and the beginning of unmixing a t some point in the mass. It is not suggested that this is a necessary property of metastable solid solutions. It is merely pointed out as a fact in the present instance. Mixing and Unmixing of Solid Solutions Reference has already been made to the unmixing of solid solutions especially in connection with the metastable solid solutions and also to both mixing and unmixing in describing the determination of the limits of solid solution on the 8 per cent mixture. The phenomena observed on crossing the unmixing curve BK in any of the mixtures between 3 I/Z per cent ( K ) and 8 per cent ( B ) are in most respects the same as those already described for the 8 per cent mixture: Thus the 5 per cent mixture, when crystallized rapidly a t 1 4 2 O , gives a single homogeneous mix-crystal phase and it remains indefinitely as such if the temperature is maintained at that value. If, however, the temperature is lowered to I I ~ ' ,unmixing occurs in the form of the separation of a skeleton of I\U"4C1 of cubic pattern. This separation is somewhat different from that occurring in the metastable solid solutions. It takes place gradually and uniformly in all parts of the mount, appearing first as a slight turbidity which increases until finally definite, though minute, stripes of higher refractive index (NH4C1) are to be made out. If the homogeneous mass is now slowly reheated (in the actual example the heating rate was IO' per hour) the NH4C1 can be seen to decrease in amount. At 130' it has finally disappeared and the preparation is again a single homogeneous phase. It was also ascertained that a t 125Osome NH4C1persists after two hours and without apparent diminution in amount after the first few minutes. It was taken, therefore, that disappearance a t 130' represents equilibrium at. the point on the curve BK at 5 per cent NH4C1was accordingly placed a t that temperature. Inversion and the Inversion Intervals of the Solid Solutions When a mount of pure ",NOs is slowly heated the inversion from the birefracting tetragonal form to the isotropic form is observed to take place at 1 2j.5'. I n all mixtures containing (none with less than 1.5 per cent was examined) the same change takes place at 109' (GKH). Below that temperature all mixtures are definitely inhomogeneous, both tetragonal NHdN03 and cubic NH&1 being recognizable. This is true even of the mixture containing only 1.5 per cent NH&l so that from direct observation
PROPERTIES O F AMMONIUM NITRATE
733
we know that there is little if any solid solution of NH4C1 in the tetragonal modification of NH4N03. Immediately above 109" the condition of the mixture depends upon whether it is of a composition lying to the right or to the left of K . Those compositions to the right of K (richer in NH4C1) ccnsist of isotropic mix-crystals and excess iCHdC1 as such. Those to the left of K consist of isotropic mix-crystals and excess birefracting (tetragonal) ",NO3. Indeed it was on this basis that the composition of the point K was established. The 4 per cent mixture was found to belong to the one class and the 3 per cent to the other. The point K was believed to be thus located with sufficient accuracy and was placed midway or a t 3.5 per cent. It may be of interest to note that if we calculate the concentration of the solid solution which should have an inversion point of 109"by means of the
RT2 N
approximate formula A T = - - we obtain a value of approximately 3.4 L No weight per cent NH4Cl which is in agreement with the experimental finding that it is between three and four per cent. I n the formula A T is the difference between the temperature of any phase-change (melting, boiling or inversion) of a pure solvent and that of the same change in a solution containing N mols of solute to No mols of solvent. T is the temperature of the phase-change in the pure solvent, L the latent heat of the phase-change per mol and R, the gas constant. In the present case the temperature of the inversion of the pure nitrate is 12 j"C whence T = 398' and A T = 16". The latent heat of inversion per gram = 13 o calories1, whence L = 1040 calories. The further behavior, with temperature rising above 109O, of mixtures to the right of K (those with excess NH4C1) has already been described. Those to the left of K remain to be discussed. They contain in addition to isotropic mix-crystals an excess of tetragonal NH4N03. The latter is very conspicuous under crossed nicols, for it occurs as bright birefracting filaments ramifying through the dark, isotropic ground mass. When the temperature is slowly raised these filaments gradually narrow as a result of the absorption of their substance into the surrounding isotropic phase. Thus, if the 1.5 per cent mixture is held at I 18' for 3 5 minutes there is at first marked absorption of the filaments, but the change finally comes to rest with minute filaments still persisting. On the other hand, a t 120' disappearance of the filaments is complete in 2 0 minutes. On the basis of these determinations the point on the curve FK corresponding to the I. j per cent mixture was placed a t I 19'. Under conditions of relatively rapid heating, these filaments of tetragonal NH4N03 behave in a different manner. There is then insufficient opportunity for diffusion into the adjacent phase and they act as they would in its absence. The temperature may accordingly be raised to 1 2 j.5' where they promptly invert t o the isotropic form. We have thus additional proof that these filaments are pure, tetragonal NH4N03 and that this form of N H 4 N 0 3takes no NH4C1 into solid solution. Even after inversion of the filaments to the iso-
-
This value was calculated by Bridgman from the results of his study of the effect of pressure on the transition point. Proc. Am. Acad. Arts Sei., 51, 614 (1916).
73 4
N . L. BOWEN
tropic form, whereby they have become the same phase as their surroundings, they are, nevertheless, a different concentration of that phase and are still distinguishable by virtue of a lower refractive index, This difference gradually fades out, however, and in a few minutes no observable difference persists. The freedom of diffusion in these solids, which is evidenced by this and other phenomena already described, is very remarkable when it is considered that the temperatures concerned are but little above the boiling point of water. The inversion of the solid solutions (G-K) with falling temperature should take place when the curve FK is crossed but as a matter of fact undercooling ordinarily occurs. Filaments of birefracting, tetragonal NH4N03 can be developed, however, in a mass that was originally a homogeneous isotropic solid solution by cooling it into the region FKG and holding it there. It should be pointed out that some mixtures can be crystallized in such a way as to give both the effect characteristic of mixtures to the left of K and that characteristic of mixtures to the right of K . This is accomplished by the relatively slow crystallization induced by cooling the oil bath with the mount in place and was observed to occur in the 4 per cent mixture. As has already been stated, the crystallization begins with the separation of a framework with less than 4 per cent NH4C1which is later filled in by material with more than 4 per cent NH4C1 and the difference is apparent in the lower refractive index of the original framework. Further cooling of a mass so constituted and holding it a t a temperature of 113' induce separation of birefracting N H 4 N 0 3in the parts corresponding with the original framework and of a cubic skeleton of NH4C1 in the other parts. Lack of Solid Solution in the other Modifications of Ammonium Nitrate We have already noted the evidence that there is no solid solution of NH&l in the tetragonal form of NH4N03. A study of the other inversions of the nitrate, that a t 84', that a t 3 2 ' , and the metastable inversion at 50°, shows that there is no appreciable change in any of them in the presence of NH4C1. From this fact it may be deduced that, like the tetragonal form, none of the other low-temperature forms of ammonium nitrate takes ammonium chloride into solid solution. Therefore for the complete diagram of stable equilibrium above room temperature Fig. I requires to have added to it only two horizontal lines, one a t 84' and one at 32'. Summary A re-examination of the system ammonium nitrate-ammonium chloride gives results that agree with those of Perman as to the position of the liquidus curves and the eutectic but reveals a series of solid solutions not found by him. The method of investigation is that of direct observation under the microscope whereby the various crystalline phases can be identified by means of their optical properties and phase-changes can be observed actually in progress. It is thus found that all mixtures from pure ",NO3 as far as eight per cent NH4C1 crystallize to a homogeneous. mix-crvstal phase of isometric
PROPERTIES O F AMMONIUM NITRATE
73 5
symmetry, I n consequence of the existence of this mix-crystal or solid solution series the phenomenon of the eutectic does not enter until after the limits of solid solution are passed, each member of the series beginning to melt at a successively higher temperature as pure r\THdK03 is approached. I n addition to these melting intervals of the solid solutions their inversion intervals can likewise be observed directly. The isometric (I) +tetragonal (11) inversion changes from 125.5' for pure xH4x03 to 109' for the solid solution containing 3.5 per cent KH4Cl. At 109' the curve of unmixing of solid solutions is encountered and along this curve the phenomena of mixing and unmixing of solid solutions take place and can be directly observed, Thus even changes involving diffusion in the solid state take place with comparative readiness in these materials. All of the facts are embodied in the equilibrium diagram, Fig. I .
