Nov., 1960
DIFFERENTIAL THERMAL ANALYSIS OF PERCHLORATES
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Acknowledgment.-We should like to express teresting and useful discussions and to Dr. A. R. our appreciation to Dr. F. A. Matsen and Dr. Katritzky of the University Chemical Laboratory, E. E. Ferguson of the University of Texas for in- Cambridge, for helpful correspondence.
THE DIFFEREKTIAL THERMAL ANALYSIS OF PERCHLORATES. IV. A SPURIOUS HEAT EFFECT BY MEYERM. MARKOWITZ AND DANIELA. BORYTA Foote Mineral Company, Research and Development Laboratories, Chemicals Division, Berwyn, Penna. Received May IS, 1960
A serious source of error has been found in DTA studies of highly turbulent condensed phase-gas reactions (via, the thermal decompositions of the alkali metal perchlorates) when carried out in an open furnace exposed to the ambient atmosphere. Recession of the sample from the thermocouple is shown to occur, giving rise to endothermal breaks which have been incorrectly attributed to part of the decomposition mechanism of the material under study. This spurious thermal behavior appears to be eliminated when a closed furnace is used. The thermal decompositions of lithium and potassium perchlorates are discussed in some detail.
Introduction Experimental Procedures DTA runs on the purified anhydrous perchlorates of During the course of a continuing program of lithium, potassium and rubidium were performed both in study concerned with the thermal behavior of per- set-up A and in set-up B. Five-gram samples were heated chlorate salt,s,1-3 an equipment-dependent effect at 10" per minute using a motorized variable transformer was found in the different'ial thermal analysis to program the current to the furnace. The recording (DTA) of lithium, pot'assium and rubidium per- equipment consisted of a 5.5 mv. span strip-chart potentiorecorder (Fisher Scientific Co., 717 Forbes Street, chlorates. In order t'o dist'inguish between DTA metric Pittsburgh 19, Penna., "Recordall"), the inputs to which breaks due t,o the samples and those introduced by were alternated through an automatic stepping switch the equipment, t'he origin of the anomalous effects (Fisher Scientific Co., "Auto-Step Switch"). The sample temperature signal was attenuated with a 10 was sought. turn, 1000 ohm linear precision potentiometer held a t a In a frequently applied DTA apparat'us (set- setting of 0.15, The range for the differential input was up A),4 the sample and inert reference material -2.75-0-2.75 mv. on the recorder, achieved by addition (alumina), each contained in a separate test-tube, of a bias voltage. Thus, T and AT were recorded on the are set symmetrically into a steel block which fillsthe same time basis. The T value corresponding to any point cylindrical cavity of a crucible furnace. Since the on the A T curve is readily found by drawing a horizontal between the two curves. entire arrangement is open to the atmosphere, a lineThe curves given represent differential temperaturesharp temperature gradient exists between the sur- time plots with the areas indicative of thermal effects desigface of the steel block and its environment. Many nated by Roman numerals. The temperature values for DTA studies on alkali metal perchlorates using these areas are given where appropriate. this equipment gave results in good agreement with Experimental Results those previously r e p ~ r t e d . ~ , ~ -Subsequently, * a The DTA curves for anhydrous lithium perchlorate, obnew furnace arrangement employing an enclosed tained with both set-ups, are depicted in Fig. 1. Curves cavity within a muffle furnace (K. H. Huppert Co., iz and B differ markedly in the decomposition regions (III6830 Cottage Grove Ave., Chicago, Ill., Model 434 IV-V in A and I11 in B). The small endothermal breaks IA4and I B correspond to the dissociation reaction: LiC104. Deluxe Furnace without pyrometer and controller) H20+LiC104 + H?Oat about 150°2*8;water may be absorbed was adopted. The only contact between the heated by anhydrous lithum perchlorate during handling and exchamber and the atmosphere is by way of a narrow posure to the air.10 Endotherms IIA and I I B correspond to slit in the top of the heavily insulated furnace the fusion of anhydrous lithium perchlorate a t about 247°.2 The sequence IIIA (exotherm>-IVA (endotherm)-VA through which the differential thermocouples enter. (exotherm) from approximately 480 to 5 5 5 O , Surprisingly, the DTA curves obtained with this whereas the extends sole decomposition break IIIB (exotherm) equipment (set-up B) were significantly different extends from about 480 to about 560". Endotherms VIA from those using the open furnace. The reasons and IVB coincide with the melting point of the reaction lithium chloride, a t about 615'. for these differences and their importance in the product, Figure 2 shows the corresponding DTA curveP for potasproper applicat'ion of DTA comprise the subject sium perchlorate. Endothermal breaks IA and I B are atmatter of this paper. tributable to a reversible crystallographic transition in the (1) M. M. Markowitz, T H IJOURNAL, ~ 61,505 (1957). (2) M. M. Markowitz, ibid., 62, 827 (1958). (3) hl. M. hlarkowits and R. F. Harris, {bid., 63,1519 (1959). (4) S. Gordon and C. Campbell, Anal. Chem., 27, 1102 11955). (5) V. D. Hogan and S. Gordon, Tms JOURNAL, 62, 1433 (1958). ( 6 , S. Gordon and C. Campbell, Bull. A m . Ceram. Soe., 34, 372 (19%). (7) V. D. Hogan, S. Gordon and C. Campbell, Anal. Chem., 29, 306 (19.57). ( 8 ) 8. Gordon and C. Campbell, "Proceedings of the Fifth Symposium on Combustion," Reinhold Publ. Corp., New York, N. Y., 1965, pp. 277-284.
solid potassium perrhlorate at 300".4J1 The decomposition sequence I I A (fusion endotherm)-111.4 (exotherm)IVA (endothermtIV'A (exotherm) extends from about 595 to about 685O, and the sequence IIB (fusion endotherm)IITB (exotherm) covers approximately the snme tempma-
(9) J . P. Simmons and C. D. 1,. Ropp, J . A m . C h e m . S O C . . S O , 1 6 3 (1928). (10) R. F. Muraca and L. L. Taylor, Progress Report No. 20-347, J e t Propulsion Laboratory, California Institute of Technology, Pasadena, Jan. 17, 1958. (11) D.Vorlaender and E. Kaascht, Ber., 66, 1157 (1923).
MEYERM. MARKOWITZ AND DANIEL A. BORYTA
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