Jan., 1959
THEBIKARY OXIDANTSYSTEM BARIUM PERCHLORATE-POTASSIUM NITRATE
After some manipulations one obtains = Mi P
- lOOOHi
-
(I
Hi = ( b / a C i ) T , P , C , f i
(A-7)
HiCi j-1
where
93 (-4-8)
The derivatives H i may be determined readily when the density is known as a function of the solute molarities. Equation A-3 may be employed directly to obtain Vi for solute i if the density is known as a function of mi.
A THERMOANALYTICAL STUDY OF THE BINARY OXIDANT SYSTEM BARIUM PERCHLORATE-POTASSIUM NITRATE BY VIRGINIA D. HOGAN AND SAULGORDON Pyrotechnics Chemical Research Laboratory, Picatinny Arsenal, Dover, New Jersey Received August 4, 19.58
A differential thermal analysis and thermogravimetric study of the thermally unstable barium perchlorate-potassium nitrate system revealed that its physico-chemical behavior at elevated temperatures varies widely with composition. Differential thermal analysis first revealed the occurrence. of an exothermal, solid state, metathetical reaction which takes place a t 3 2 5 O , predominantly close to the stoichiometric composition. The reaction, 'which is thermodynamically spontaneous and involves cations of equal crystal radii, was found by X-ray diffraction analysis to go to completion; therefore, the potassium perchlorate-barium nitrate system which is produced must be the stable salt pair. The diversity in the thermal behavior of this system at elevated temperatures can be elucidated satisfactorily on a basis of (a) the physicochemical properties and reactions characteristic of the four reciprocal salts which are present as a result of the solid state metathesis, and (b) the fusion of an exceedingly potassium nitrate-rich mixture of the original ingredients.
Introduction The thermoanalytical techniques of differential thermal analysis and thermogravimetry are versatile experimental tools which are finding increasing application in chemical research. Differential thermal analyses furnish data relative to the evolution or absorption of heat as a result of the high temperature physico-chemical phenomena characteristic of the sample under study, while thermogravimetric curves yield data concerning any physical or chemical reactions accompanied by weight changes that occur a t elevated temperatures. A thermoanalytical study of the system potassium perchlorate-barium nitrate utilizing these techniques' revealed varied and interesting thermal effects which suggested an investigation of the high temperature phenomena characteristic of the reciprocal system barium perchlorate-potassium nitrate. I n common with the potassium perchlorate-barium nitrate system, the system considered here exhibits thermal instability which precludes its study by conventional cooling curve techniques. Therefore, differential thermal analysis and thermogravimetry were employed. The thermal behavior of the barium perchlorate-potassium nitrate system varies widely with the composition of the particular mixture under study. However, this variation in thermogravimetric and differential thermal analysis behavior with composition can be accounted for satisfactorily by assuming the fusion of a eutectic mixture of the ingredients and the occurrence of a metathetical reaction between them, in addition to the physicochemical reactions of the pure materials. Reagents.-Potassium nitrate, analytical reagent grade Fisher Scientific Co.); barium perchlorate, Desicchlora made by G . Frederick Smith Chemical C o . for J. T. Baker Chemical C o . )were used. It was found necessary to prepare the individual samples from reagents freshly dried at 200'. I n the presence of moisture, binary mixtures of these salts
I
(1)
V. Hogan and 9. Gordon, THISJOURNAL,
62, 1433 (1958).
apparently undergo a metathetical reaction which destroys the potassium nitrate at the temperature necessary to completely dry barium perchlorate. Apparatus and Procedures .-The differentia 1 thermal analysis and thermogravimetric apparatus employed have been described previously.* Four gram samples were taken for differential thermal analyses and an equal volume of alumina served as the reference material. In the DTA curves the temperature difference between the sample and reference materials is plotted as a function of the sample temperature. Conventionally, exothermal reactions appear as upward deflections while endothermal reactions appear as downward deflections. Three hundred and fifty milligram samples were used in the thermogravimetric analyses with a range of 200 mg. full-scale for changes in weight. Thermogravimetric curves are plotted as loss in weight as a funct,ion of furnace temperature. This furnace temperature was found to be five to thirty degrees higher than the actual samDle temperature at the heating rate used. In both thermoanalytical techniques the furnace is set for a linear heating rate of 15' per minute.
Results and Discussion Differential thermal analysis and thermogravimetric curves for the ingredients are illustrated in Fig. 1. Differential thermal analysis of pure barium perchlorate reveals one endothermic crystalline transition a t 290" and possibly a second a t about 365" prior to melting a t 490' and vigorously exothermic decomposition a t 500". The accompanying thermogravimetric curve displays a loss in weight equivalent to decomposition to barium chloride a t 535". The DTA curve of pure potassium nitrate exhibits first an endothermic crystalline transition a t about 130". A small exothermic spike at 330" culminates in endothermic fusion a t 340". A slight bubbling reaction starts a t about 675O, although no definite thermal reaction is occurring. The bubbling most probably is due to slight impurities which are oxidized and escape as carbon dioxide or oxides of nitrogen. The sample does not begin t o decompose visibly until temperatures above 750". The thermogravimetric curve (2) V. Hogan, 6. Gordon and C . Campbell, Anal. Chem., 29, 306 (1957).
VIRGINIAD. HOGAN AND SAUL GORDON
94 0
Vol. 63
200 400 600 800
which cannot be attributed to any of the physical or chemical reactions of the pure ingredients, while that of the mixture containing 70% potassium r: nitrate shows a fusion endotherm a t 315" instead 100 of the exotherm a t 325". However, all the DTA 150 % and TGA behavior of the mixtures can be satiss factorily elucidated, if one assumes, in addition to the physico-chemical reactions of the pure ingredients, the fusion of a potassium nitrate-rich i 100 eutectic mixture of the ingredients and the occur_ _ _ ~ " 140 rence of a solid state, metathetical reaction such 0 200 400 600 800 0 200 400 600 800 1000 as .-_
U IO 3S
I
,
+
+
Sample temp., "C. Furnace temp., "C. Ba(C104)2 2I
