NOTES
358
thereby making the mixture lcss acidic. However, another significant point worth considering is that with mixtures having a molar ratio 1: I , the variations in pH with time are negligible (Tablc I). Acknowledgment.-Thanks are due to Dr. A. R. Kidwai for providing facilities and C.S.I.R. (India) for the award of fellowship to F.A.S.
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RETARDATION OF THE THERMAL DECOMPOSITION OF LITHIUM PERCHLORATE BY MEYCRM. MAREOWITZ AND D A ~ EA. LBORYT4 Poole .%ftneral Company, Research and Engineetrng C e d e r , Chemical
Dcarnon, P. 0 . Boz 615, West Chester, Pennsylvania Recetved h'ovember 20. 1961
The perchlorates frequently are regarded as prime examples of materials susceptible to catalytic thermal decomposition. Consistent with this viewpoint is the care usually taken to maintain high levels of purity when perchlorates are used or studied. Accordingly, it is of interest to report in this note some of the results from a continuing program aimed a t resolving the decomposition processes of perchlorate salts. Specifically, a significant degree of retardation of the thermal breakdown of lithium perchlorate has been achieved by the use of certain silver salts as inhibitory additives. An earlier paper' had presented evidence for an autocatalytic mechanism during the decomposition of lithium perchlorate; lithium chloride, the primary residual prodiwt, had been determined to be the autocatalytic agent. The top half of Fig. 1 shows the usual decomposition-time curves obtained for pure lithium perchlorate and for B 5 mole yo lithium chloride-95 mole % ' lithium perchlorate mixture, each maintained at 41 7.8O. As these curves illustrate, decomposition is essentially complete after 2 4 hours. The luueer half of Fig. 1 depicts the decomposition-time patterns for various mixtures of lithium perchlorate and silver nitrate, also kept a t 41'7.8'. Clearly, there is a pronounced stabilization of the lithium perchlorate due to the presence of the silver nitrate and decomposition is almost complete only after about 3945 hours. This stabilization is manifested by the prolongation of the induction period prior to the rapid, autocatalytic decomposition of the lithium perchlorate. Similar results have been obtained with silver perchlorate as the stabilizing additive. The decomposition-time ciirves were obtained by means of an automatic recording thermobalance.ln2 Though lithium and silvcr nitrates individually decompose slowly a t 417.8', it appears that i i i the reaction mixtures studicd here the nitrate contents are conserved to a good degrce. Decomposition residues from several lithium perchlorate-silver nitrate mixtures showed but very faint traces of nitrite and yielded more than 90% (1) M hl Markowit1 and D A Baryta, J. F'hy8. Chem , 65, 141'5 (1961). (2) M M. Marko\rits and D A. Baryta, A n d . Chem., 32, 1588
(1860).
4'7'80c'
,t 9
i
30
35 49 45 Time, hours. Fig. 1.-Decomposition-time curves for lithium perchlorate samples decomposed at 417.8'.
of the original nitrogen, recovered as ammonia after reduction with Devarda's alloy and distillation with sodium hydroxide solution. Accordingly, thc percentage of lithium perchlorate decomposed was computed directly from the continuously recorded weight losses. The mechanism of the stabilization cffect appears to be related to the insolubility of silver chloride in mixtures rich in lithium perchlorate. As a consequence, the chloride-catalyzed decomposition is suppressed by immediate rcmoval of the chloride ion because of formation of the insoluble silver chloride. Decomposition-time ciirvcs for mixtures of lithium perchlorate and silver chloride a t 417.8' follow rather closely that characteristic of pure lithium perchlorate at the same temperature. Differential thermal analyses of mixturcs of lithium perchlorate and silver chloride show the melting point of the lithium perchlorate (247') to be virtually unaffected by the presence of the silver chloride ; however, a t the clearly defined melting point of silver chloride (455 ") the decomposition of the lithium perchlorate becomes rapid, indicating that fusion of the silver chloride had permitted the introduction of chloride ions into the mass of molten lithium perchlorate. It should be observed that as the proportion of silver nitrate to lithium perchlorate incrcases in the samples studied, the percentage of pcrchlorate decomposition increases for any given time prior to rapid decomposition. Apparently, though the silver ion does inhibit the autocatalytic reaction promoted by chloride iori, the d y e r ion also uppears to catalyze to some small extent an alternate route of decomposition of lithium perchlorate.
