Nov., 1958
THEBINARYOXIDANT SYSTEM POTASSIUM PERCHLORATE-BARIUM NITRATE
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A THERMOANALYTICAL STUDY OF THE BINARY OXIDANT SYSTEM POTASSIUM PERCHLORATE-BARIUM NITRATE' BY VIRGINIAD. HOGANAND SAULGORDON Pyroiechnics Chemical Research Laboratory, Picatinny Arsenal, Dover, New Jersey Received May I d t is68
The thermoanalytical techniques of differential thermal analysis and thermogravimetry have been used to elucidate the thermal phenomena occurring as the binary oxidant system potassium perchlorate-barium nitrate is heated to elevated temperatures. The fusion, a t 465O, of a eutectic mixture immediately precedes the barium nitrate catalyzed decomposition of potassium erchlorate. The thermal instability of the eutectic mixture precludes its study by conventional cooling curve techniques. %herefore, the eutectic composition, nominally 2 to 1 potassium perchloratebarium nitrate, was obtained from the integrated areas of the fusion endotherms on the DTA curves for a series of mixtures. It is noted that the presence of potassium ion, in addition to that of barium ion, provides increased thermal stability for the nitrate ion.
Introduction The thermoanalytical techniques of differential thermal analysis (DTA) and thermogravimetry are versatile experimental tools which are finding increasing application in chemical research.2 Differential thermal analysis involves heating the material under study together with a thermally inert reference material t o elevated temperatures a t a constant rate while continuously measuring the temperature difference between them as a function of sample temperature or time. The curves obtained may be used to characterize the system under study in terms of its thermal reactions, both physical and chemical. Integration of areas under endothermal bands has been used to obtain semi-quantitative estimates of one or more of the substances present. Thermogravimetry consists in continuously weighing a sample as it is heated, either a t a constant temperature or to elevated terriperatures a t a constant rate. Curves are obtained as a function of temperature or time. Because thermogravimetric curves are quantitative representations of weight changes, they can be related t o the chemical and physical changes taking place in the sample as it is heated and can often be used to determine the nature of the intermediate and final reaction products. A differential thermal analysis and thermogravimetric study of the system potassium perchloratebarium nitrate indicated that the fusion of a eutectic mixture immediately precedes the barium nitrate catalyzed decomposition of potassium perchlorate.2 In this investigation DTA has been used to determine the composition of the eutectic mixture, and, in conjunction with thermogravimetry, t o elucidate the major thermal phenomena occurring in this system. Although differential thermal analysis has been applied by others to the study of the phase transformations of a variety of systhese investigators have used the technique of DTA to detect the presence of transiently stable phases by the heat effects accompanying (1) Presented before the Division of Physical and Inorganic Chemistry at the 132nd Meeting of the American Chemical Society, New York, N. Y., Sept. 10, 1957. (2) V. Hogan, 8. Gordon and C. Campbell, Anal. Cham., 29, 306 (1957). (3) F. C. Kracek, N. L. Bowen and G. W. Morey, THE JOURNAL, ei, 1183 (1937). (4) E. P. Partridge, V. Hicks and G. W. Smith, J . A m . Cham. Soc., 68, 454 (1941). (5) A. Reisman and F. Holtrberg, ibid., 77, 2115 (1955). (6) F. H. Btroas and 8. T. Abrams, ibid., 73, 2825 (1851).
their formation3 and/or to magnify the breaks in heating or cooling curves so that the transition temperatures may be more easily determined.4-6 However, the thermal instability of the eutectic mixture involved here precludes its study by conpentional cooling curve techniques. Therefore, in this investigation, the eutectic composition was obtained from the integrated areas of the fusion endotherms on the DTA curves for a series of mixtures. Reagents.-Potassium perchlorate, analytical reagent (J. T. Baker Chemical Company); barium nitrate, analytical reagent (Mallinckrodt Chemical Works); potassium nitrate, barium chloride, potassium chloride, analytical reagent (Fisher Scientific Company). The binary mixtures were prepared by weighing out sufficient quantities of the ingredients to make 10 to 50 grams of each mixture and blending them in a ball mill or with a mortar and pestle. The samples for each individual run were taken from these previously prepared mixtures. Instrumentation and Procedures.-The differential thermal analysis and thermogravimetric apparatus employed have been described previously.2 Four-gram samples were taken in the differential thermal analyses and an equal volume of alumina served as reference material. I n the DTA curves the temperature difference between the sample and reference materials is plotted as a function of the sample temDerature. For areas on DTA curves to be quantitatively comparable, the curves must be obtained using the same thermocouple assembly or one of exactly the same dimensions. The curves in Fig. 3 were obtained with a different thermocouple assembly than those in Fig. 1. Areas on these curves were determined by planimetering. Three hundred Wty milligram samples were used for all thermogravimetric analyses with a range of 200 mg. full scale for changes in weight. Thermogravimetric curves are plotted as change in weight as a function of furnace temperature. In both thermoanalytical techniques the furnace is programmed for a linear heating rate of 15' per minute.
Results and Discussion The DTA curves for a series of potassium perchlorate-barium nitrate mixtures illustrated in Fig. 1 by the 50/50 composition, show an endothermic peak a t 465" which cannot be assigned to any of the thermal reactions exhibited by either of the ingredients also shown in this figure. In addition, they exhibit the 300" transition endotherm for potassium perchlorate and decomposition patterns for perchlorate and nitrate above 500". Both DTA and thermogravimetric curves show that potassium perchlorate undergoes no thermal reactions other than crystalline transition at 300", and melting and decomposition at approximately 600". Barium nitrate undergoes no thermal reaction prior to fusion a t 600") followed by decomposition in the region of 700". Thermogravimetric
VIRGINIA D. HOGANAND SAULGORDON
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Vol. 62
Temp., "C. 100 200 300 400 500 600 700 800 900 I
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1 2 3 4 5 6 7 8 Mole ratio, KC104/Ba(NO& Fig. 2.-The determination of the eutectic com osition from the area of fusion endotherms on a series of digrentid thermal analysis curves; eutectic fusion, 465".
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200 400 600 8000 200 400 600 800 Temp., "C. Temp., "C. Fig. 3.-Differential thermal anal sis curves for the system potassium perchlorate-t mium nitrate. 0
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The small interval between eutectic fusion and perchlorate decomposition shown by DTA sug150 gested that it would be difficult, if not impossible, to obtain a stable eutectic melt. This was con0 100 200 300 400 500 600 700 800 900 firmed by simultaneously recording differential Temp., "C. Fig. 1.-Differential thermal analysis and thermogravi- temperature and amount of gas evolved as a funcmetric curves for the system potassium perchlorate-barium tion of time on a two-pen recorder' and by trials nitrate. in a muffle furnace. This technique of simultaneous DTA and thermovolumetric analysis shows that analyses of the mixtures show that the potassium approximately 7% of the oxygen content of the perchlorate present does not lose its oxygen until perchlorate has been evolved by the end of the temperatures above 500". Therefore the 465" eutectic fusion endotherm. While heating in a endotherm cannot be attributed to the barium muffle furnace a t 465' for 15 minutes does not nitrate catalyzed decomposition of potassium completely melt the 50-50 potassium perchlorateperchlorate. Since partial melting is observed barium nitrate mixture, heating a t 470" for i 5 while this endotherm is occurring, the heat ab- minutes results in vigorous bubbling and a marked, sorption must be due to the fusion of an eutectic up to lo%, loss in weight. mixture. (7) 8. Gordon and C. Campbell, Anal. Chem., 89, 1708 (1957).
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Nov., 1958
THEBINARY OXIDANTSYSTEM POTASSIUM PERCHLORATE-BARIUM NITRATE
Since the irreversibility of the decomposition of the eutectic system precluded obtaining a stable melt for conventional thermal analysis, the feasibility of determining the eutectic composition from the DTA curves was considered. The area of an endotherm on a DTA curve is a measure of the gross heat eflect of a physical or chemical reaction and it has been shown to be proportional to the heat of reaction and the mass of material reacting.8 In the case of the fusion of a eutectic mixture, the amount of material reacting at a given temperature should vary with the composition of the sample. As the sample composition approaches the eutectic composition, the amount of material reacting increases, and the sample will absorb increasingly larger amounts of heat and exhibit progressively larger endotherms on the DTA curves. Consequently, a plot of the areas of the eutectic fusion endotherms versus sample composition should show a maximum a t the eutectic composition, assuming that the endothermic heat of fusion of the eutectic composition exceeds the heat of solution or mixing of the components. In Fig. 2 we have this graph for the system. The maximum area occurs a t a composition of approximately 52% potassium perchlorate by weight, corresponding to a 2:l mole ratio of potassium perchlorate to barium nitrate. The DTA curves of the system, Fig. 3, show that the 50-50 mixture, which approaches the eutectic composition of nominally fifty-two per cent. potassium perchlorate, exhibits strongly endothermic perchlorate decomposition in contrast to the marked exothermicity observed for this reaction in pure potassium perchlorate and in mixtures other than the eutectic composition. Thermodynamic data have been published9 to show that the exothermicity of potassium perchlorate decomposition is due to the deposition of solid potassium chloride from the melt of potassium perchlorate and its decomposition products, potassium chloride and chlorate. In this process the heat of fusion of potassium chloride, 6.1 kcal./mole, is evolved. This suggests, then, that the solubility of potassium chloride in the melt minimizes or precludes the “freezing out” phenomenon, leaving the endothermic decomposition of potassium perchlorate as the only thermal effect a t this point. Therefore, in mixtures containing less potassium perchlorate than the eutectic composition, the decomposition is exothermic because of the limited solubility of potassium chloride in the excess barium nitrate; and in mixtures containing more than this quantity of potassium perchlorate the decomposition is exothermic due to the limited solubility of potassium chloride in the resulting melt. However, it must be noted that the DTA curves for the 50-50 mixtures indicate a greater (8) 8.Speil, L. H. Berkelhamer, J. Pask and B. Davies, “Differential Thermal Analysis.” U. 8. Bur. of Mines, Technioal Paper 664, 5-8, 1945. (9) M. M. Markowitz, THIEJOURNAL, 61, 506 (1957).
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absorption of heat than can be accounted for solely by the 0.8-1.7 kcal./mole that has been estimated for the heat of decomposition of potassium perchlorate. The angle of the endothermal side of the decomposition band for the 50-50 composition in Fig. 3 and the decomposition loop seen in Fig. 1 shows that the sample cools during the decomposition despite the constantly increasing furnace temperature. By comparison, the melting of barium nitrate, which in Fig. 1 produced a narrow endotherm with no indication of cooling, absorbs 5.9 kcal./mole. Since the relatively large heat absorption occurs rapidly and only when the sample is substantially liquid, it is apparent that the phenomenon responsible is not a solid state reaction. This endothermicity may, be accounted for by the integral effect of the endothermic decomposition of the potassium perchlorate and the subsequent dissolution of the resulting potassium chloride in the melt. The mechanism of a slightly endothermic decomposition followed by a very exothermic “freezing out” phenomenon for the mixtures illustrated in Fig. 3, is corroborated by the arrests or small endotherms observed on the DTA curves between the eutectic fusion endotherm and the endotherm or exotherm accompanying the visible evolution of oxygen. The decomposition process is also complicated by vigorous foaming which accompanies the oxygen evolution. This may further alter the thermal properties of the reacting sample, and consequently the nature of the corresponding DTA band. Above 700°, these mixtures give DTA and thermogravimetric evidence of nitrate decomposition. The thermogravimetric curves show that the decomposition temperature of the nitrate ion in these salt solutions is inversely proportional to the amount of barium nitrate present in the mixture. It varies from 25” below the decomposition temperature of potassium nitrate for 80-20 potassium perchlorate-barium nitrate to 10” above the decomposition temperature of barium nitrate for 20-80 potassium perchlorate-barium nitrate. Figure 4 shows thermogravimetric curves for potassium nitrate, barium nitrate and binary mixtures containing two moles of potassium nitrate to one mole of barium chloride and two moles of potassium chloride to one mole of barium nitrate, the stoichiometric metathetically related compositions. The curves of the mixtures are essentially identical. Decomposition temperatures for the binary mixtures are not quite as high as that of potassium nitrate, but they are definitely higher than that for barium nitrate. Since the samples are completely liquid at the decomposition temperature of the nitrate ion, we cannot say that it is either potassium nitrate or barium nitrate which decomposes. However, it seems clear that the presence of the alkali metal cation in the melt provides greater thermal stability for the nitrate anion than does the alkaline earth metal cation.
