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1 JANUARY 1968
THE JOURNAL OF CHEMICAL PHYSICS VOLUME 48, NUMBER 1
Chemistry of Detonations. II. Buffered Equilibria
Mortimer J. Kamlet and J. E. Ablard U.S. Naval Ordnance Laboratory, While Oak, Silver Spring, Maryland (Received 19 June 1967)
At low loading densities, values of N, M, and Q calculated from the HjO-COa “arbitrary” show poor individual agreement with estimates of these quantities from the RUBY computer code. Nevertheless, when substituted into the equation P —15.58 NMV2QmPthey lead to detonation pressures which correspond closely to RUBY predictions. That incorrect input information should yield results which are very nearly “correct” is rationalized on the basis that the equilibria whose shifting engenders the changes in N, M, and Q are “buffered” in the sense that “errors” in N are offset by compensating “errors” in M and Q. As a consequence of the fact that most of the important equilibria in the detonation of C-H-N-O explosives are buffered, calculated (and actual) mechanical properties of detonations appear to he extremely insensitive to exact product compositions. A number of other interesting consequences of these buffered equilibria are discussed.
In Paper I of this series1 it was suggested that detonation pressures of C-H-N-0 high explosives might be estimated by means of the simple empirical equation
P= 15.587VMl/zQl/W (P in kilobars), (1)
where N is the number of moles of gaseous detonation products per gram of explosive, M the average molecular weight of these gases, Q the chemical energy of the detonation reaction (— AHq per gram of explosive), and po the initial density. In a preliminary lest of this relationship, TVruby, ^ruby, and Qruby (the computer estimates of these quantities) were substituted into Eq. (1) for a number of typical organic explosives at representative loading densities. That this led to values of PchIc which differed only nominally from pressures predicted by the ruby and stretch bkw computer codes2 confirmed that, given “proper'’ values of N, M, and Q, the new equation leads to reasonable estimates of P (Table III of Ref. 1).
It was then demonstrated that Ëòèãü, Ìàãú, and Qnrb, as easily calculated from the H2O-CO2 arbitrary assumption of detonation product compositions, corresponded reasonably closely (i.e., to within 5%) to ^Vruby, Ë/ruby, and Qruby at initial densities above 1.40 g/cc, but that differences became., significantly larger at lower loading densities (Table V of Ref. 1). The H3O-CO2 arbitrary represents N2, H2O, and CO2 as being the’ only important gaseous products in the detonation of most C-H-N-0 explosives, with H2O having priority in formation over CO2.
From these findings it was expected that, when used
1M.J. Kamlet and S. J. Jacobs, J. Chem. Phys. 47, 23 (1967), Part I, preceding paper.
* As used herein, the term RUBY includes the results of computations at tbe Los Alamos Scientific Laboratory by the STRETCH BKW computer code and at NOL by the Lawrence Radiation Laboratory’s RUBY code. For the purposes of present discussions, these codes differ only in minor regards and, unless otherwise specified, RUBY computations shall he considered as based on Mader’s most recent covolume factors and the “more appropriate” of his dual K-W parameter sets (Ref. 11 of Part I), with the heat of formation of solid carbon taken as zero. See Part I for other leading references.
in combination with the arbitrary method of estimating N, M, and Q, Eq. (1) might allow rough predictions of detonation pressures of experimental explosives at or near their theoretical maximum densities. Such predictions would require as input information only the elemental composition of a C-H-N-0 explosive, its loading density, and an estimate of its heat of formation, and would require no other calculational aids than are available to the organic-synthesis chemist at his desk. It was felt that the results would be meaningful where arbitrary and ruby values of N, M, and Q did not differ markedly, i.e., only at /çä>1.40 g/cc.
When put to the test, however, the new method showed both higher precision and broader scope than anticipated. Comparisons between hand calculations and machine computations (Table VI of Ref. 1) involved 103 data sets for 27 compounds and compositions at loading densities from 1.00 to 1.96 g/cc. They covered the full gamut of NaTü, Ì^ú, Qnrb, and Ñ^ü (the weight fraction of explosive going over to gaseous detonation products) to be encountered among organic high explosives. The results3 showed good agreement between -Pen 10 [Eq. (1)] and Pruby at the higher densities as was expected. At po> 1.45 g/cc, differences averaged ±1.6% and in only a single instance (of 66 data sets) was the “error” greater than 5%.
Ï. THE SEEMING ANOMALY