Abstract
The recent awareness of the impact of strontium on health has stimulated research efforts on lithium-based red pyrotechnic colorants. We have previously shown lithium dihydrobis(azolyl)borates to be promising candidates due to their favorable adjustment to a reductive and low-temperature flame atmosphere. These compounds are assumed to be sufficiently stable only if the pKa values of the heterocycles are between 5 and 20. Apart from their acidities, functionalization of 1H-tetrazole and 1H-pyrazole with nitro or amino groups, respectively, tailors the oxygen balances of the resulting Lewis acid base adducts to enhance the fuel-rich flame environment or to make them oxidizing agents. This work determines whether the lithium salts of dihydrobis(3-nitropyrazol-1-yl)borate and dihydrobis(5-aminotetrazol-1-yl)borate are suitable replacements for strontium-containing color imparters. Furthermore, the influence of potentially green-light-producing boron is evaluated by comparing the emissions of the lithium borates and the corresponding lithium azolates.
Highlights
Introduction disrepute due to their combustion to highly carcinogenic polychlorinated aromatics,[4,5] this is a further advantage of lithium
The presence of chlorine is even considered to have an adverse effect on the light output of a lithium-containing pyrotechnic formulation.[2a]. A fuel-rich flame atmosphere is suggested in order to convert incandescent lithium hydroxide back to gaseous atomic lithium and water vapor in the flame and to avoid paling of the flame color
The radioactivity of radium as well as the high cost of the rare earth elements praseodymium, neodymium, samarium, yttrium, and scandium is prohibitive for their application.[2a]. Red emissions from metastable copper(II) oxide were only observed as a marginal phenomenon in blue pyrotechnics,[2b] whereas the flame color of calcium has a dominant yellowish aspect.[2c]
Summary
In regard of the acidities of the different substituents This means that a pKa value higher than 5 and lower than 20 allows for convenient preparation and sufficient stability of the resulting dihydrobis(azolyl)borate (see Figure 1). Present boron may potentially form metastable green lightemitting boron dioxide in the flame even, if this process should require an oxygen-rich atmosphere to be observable.[13] Its influence on the color performance is evaluated by comparing the emissive properties of the lithium borates with those of the respective azolates (5, 8)
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