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Using the Chemistry of Fireworks To Engage Students in Learning Basic Chemical Principles: A Lesson in Eco-Friendly Pyrotechnics Georg Steinhauser € Atominstitut der Osterreichischen Universitaten, Vienna University of Technology, 1020 Vienna, Austria [email protected] . tke Thomas M. Klapo Department of Chemistry and Biochemistry, Ludwig-Maximilians University of Munich, 81377 Munich, Germany

“So many formulas!” Chemistry may be regarded as a boring subject by many pupils on this planet. A fundamental problem of scientific education seems to be that the curriculum content often is presented in a more formal than descriptive or demonstrative way, which need not always be the case. In this article, we show that many aspects of chemistry are too spectacular and fascinating to be ignored by young audiences. This is especially true of fireworks and pyrotechnics. Why not teach chemical principles, such as redox reactions, combustion, or flame colors using topics from pyrotechnics and pyrotechnic reactions? Pyrotechnics lessons will not only be more fun but also more memorable. We present here some basic pyrotechnic principles as well as examples of typical and less typical pyrotechnics. We also discuss one of the very recent developments in this field: the greening of pyrotechnics. Conventional Pyrotechnics Pyrotechnics comprise a subgroup of the scientific field of energetic materials. Well-known energetic materials include high explosives, such as trinitrotoluene (TNT), nitroglycerine (NG), or pentaerythritol tetranitrate (PETN, nitropenta), that follow the principle of intramolecular oxidation. When ignited by the shock wave of the detonator, the hydrocarbon backbone is rapidly oxidized by the nitro- or organonitrate groups of the molecule, and a large amount of energy and gas is suddenly released within limited volume. In contrast to such explosives, pyrotechnics are traditionally mixtures of several compounds that react exothermically upon ignition. They are designed to react more slowly than explosives and to produce light, color, heat, gas, smoke, sound, and motion. The reactions involved are either electron transfer or oxidation-reduction reactions (1). The fundamental parts of any conventional pyrotechnic device are thus the reductant (fuel) and the oxidizer. The oxidizer provides the oxygen needed for fuel combustion, so that the combustion does not depend on atmospheric oxygen. Pyrotechnics intended to accelerate objects (such as bullets or rockets) are termed propellants. They are designed to produce a large gas volume in a defined speed of reaction. In energetic materials science, we generally differentiate according to the reaction velocity: 150

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• Burning or combustion (in the range of mm s-1 or cm s-1, e.g., flares or pyrotechnic volcanoes) • Deflagration (m s-1, e.g., black powder) • Detonation (km s-1, e.g., high explosives)

Fuels and Oxidizers Several substances can act as pyrotechnic fuels, particularly metals and alloys (magnesium, aluminum, magnesiumaluminum alloy, titanium, iron, zinc, etc.); metalloids (silicon, boron); and nonmetals (charcoal, sulfur, red phosphorus, as well as organic materials and natural products such as stearic acid, flour, or sawdust). Reducing inorganic compounds such as antimony sulfide are also frequently used. In general, metal fuels will produce higher temperatures than nonmetals (up to 3000 K) and consequently emit photons of higher intensity and energy. Nevertheless, a very promising selection of relatively safe and colorful pyrotechnic formulations based on nonmetal fuels has been published and appears to be well suited to pyrotechnic experiments for educational purposes (2). Pyrotechnic oxidizers are compounds that produce oxygen when decomposed. Typical classes of oxidizers include nitrates, perchlorates, and chlorates. The respective cations are usually alkali metal or alkaline earth metal ions or ammonium ions (but not highly unstable ammonium chlorate!).1 Hygroscopic cations such as Mg2þ and Ca2þ are avoided. Also, wherever applicable, Naþ is substituted by the less hygroscopic Kþ. In special applications, chromates (e.g., BaCrO4), permanganates (e.g., KMnO4), metal oxides (e.g., PbO, PbO2, Pb3O4, Fe2O3), and peroxides (e.g., BaO2) are used as well. Most pyrotechnic reactions are solid-solid state reactions, which require an outstanding degree of homogeneity of the mixture to obtain sufficient reactivity for the desired effect. The preparation of a reactive batch of black powder (a mixture of approximately 75% potassium nitrate, 15% charcoal, and 10% sulfur), for example, is not simple at all, as it requires the use of heavy mills that grind the moistened powder intensely. Afterward, the powder must be dried carefully, because water in a pyrotechnic mixture can inhibit a reaction, or, even worse, cause unintended, hazardous reactions. A good and harmless example

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Chemistry for Everyone Table 1. Typical Compositions of Pyrotechnic Flaresa Flare Types, with Constituent Values Expressed as Weight Percent Constituent

Mk 124: Red Navy Flare

Mk 117: Green Navy Flare

Mk 118: Yellow Navy Flare

Red Highway Flare

White Formulation

Magnesium

24.4

21.0

30.3

-

-

Potassium perchlorate

20.5

32.5

21.0

6.0

-

Strontium nitrate

34.7

-

-

74.0

-

-

22.5

20.0

-

55.0

-

-

-

-

25.0

11.4

12.0

-

-

-

Barium nitrate Potassium nitrate PVC Sodium oxalate

-

-

19.8

-

Copper powder

-

7.0

-

-

-

9.0

-

3.9

-

-

Asphalt Sulfur

-

-

-

10.0

20.0

Binder

-

5.0

5.0

10.0

-

a

Note: Data derived from refs 2, 4, and5. Caution: These formulations are not intended to be used as recipes for educational purposes.

for the reactivity of water in pyrotechnics is the following equation of the combustion reaction of black powder containing a small residue of water (3). 74KNO3 þ 96C þ 30S þ 16H2 O f 35N2 þ 56CO2 þ 14CO þ 3CH4 þ 2H2 S þ 4H2 þ 19K2 CO3 þ 7K2 SO4 þ 8K2 S2 O3 þ 2K2 S þ 2KSCN þ ðNH4 Þ2 CO3 þ C þ S This equation also impressively outlines that pyrotechnic reactions are sometimes much more complicated than one would think at first glance. Colors Colors in pyrotechnics are produced by taking advantage of the flame colors of certain elements: something that can be observed in the undergraduate qualitative analysis lab. Accordingly, a colored firework requires the addition of metal salts as coloring agents to the pyrotechnic mixture: sodium is used for yellow; strontium for red; barium for green; and copper (in presence of chlorine) for blue colors. Compounds such as Ba(NO3)2 or Sr(NO3)2 act as both coloring agents and oxidizers. The white or silvery effect in pyrotechnics is a result of the incandescence caused by extremely hot burning metal fuels such as magnesium or titanium. Reacting particles emit light in the visible range at temperatures above ∼500 °C. In most cases, the reaction of a pyrotechnic mixture is extremely exothermic, burning even hotter. But what exactly emits the colored light? At these temperatures, the ingredients of the pyrotechnic device decompose and occasionally new, unstable, and very short-lived compounds or chemical species are formed, many of which act as “emitting species”. In these species, relaxation of excited electrons causes the emission of a photon with a defined wavelength in the visible spectrum (400-700 nm). For example, for the green flame color of a barium-containing device, two barium(I) compounds have been identified as emitters: BaOH and, most importantly, BaCl. The emitting species for red are primarily SrCl and SrOH, atomic Na for yellow, and CuCl for blue. From this list, it is obvious why chlorinated compounds (e.g., polyvinyl chloride, chlorides, or perchlorates) are prominent constituents in many pyrotechnic

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mixtures, as shown in Table 1. The presence of chlorine aids in the formation of an emitting species. Also, the desired emission takes place only when the emitting species is excited in the gas phase. Thus, the increased volatility of inorganic chlorides (compared to other metal salts) is the second important reason for the addition of chlorine donors to a pyrotechnic mixture. This scientific background explains the old pyrotechnic saying “chlorides color”; that is, chlorine acts as a “color potentiator” in pyrotechnics. Indeed, without a chlorine source, even a formulation rich in Ba(NO3)2 produces white luminescence instead of green, as shown in Table 1. It gives the compositions and colors of five pyrotechnic flares. Safety Aspects For teenagers, the manufacture of pyrotechnic mixtures represents both a strong fascination and a considerable health risk. In the course of manufacturing or handling pyrotechnics, numerous grave (and often fatal) injuries and burns occur annually, affecting victims' extremities (6-8), thorax (9), face (10), eyes (11-13), and hearing (14, 15). Thus, the preparation of pyrotechnic devices should be done only under the supervision of experienced personnel, with proper safety measures and on a reasonably small scale. The usual “trial and error” strategy can be extremely hazardous in pyrotechnic experiments because of the incompatibility of several chemicals in a formulation. This is also the reason why only the purest available starting materials should be used. Technical-grade chemicals usually contain impurities in the percent range, which possibly increase the sensitivity of a mixture and make it extremely unpredictable. For example, the “infamous” mixture of sodium chlorate and sugar causes dozens or hundreds of severe injuries among teenagers every year, because sodium chlorate is used in the form of an impure total herbicide. Unless explicitly required for a specific formulation, chlorates should generally be avoided in a formulation because they have the lowest activation energies toward decomposition of any oxidizing material commonly used in pyrotechnics (16). Above all, they exhibit hazardous friction sensitivity in mixtures with numerous reductants, a problem likely to cause accidental ignition. A partial list of incompatible chemicals is shown in Table 2.

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Chemistry for Everyone Table 2. Incompatible, Hazardous Mixtures Potentially Encountered in Pyrotechnicsa Compounds Chlorates Perchlorates Aluminum Magnesium Zinc CIO3-

NAb

-c

Xd

Xe

X

CIO4-

-

NA

?

?

-

Al

X

?

NA

-

-

Mg

X

?

-

NA

-

Zn

X

-

-

-

NA

Acids

X

-

-

X

X

NH4þ

X

-

-

X

X

Water

-

-

?

X

?

Cu2þ

?

-

?f

Xf

Xf

S

X

X

-

X

X

X

X

-

-

-

S

2-

Figure 1. Principle of an electric match (based on ref 42) and photograph of an initiated electric match.

a

Based on ref 16. b NA means not applicable. c - indicates little if any hazard. d X indicates a significantly hazardous combination. e ? indicates that the combination is potentially hazardous, depending on the circumstances. f An exothermic redox reaction of Cu2þ with a less noble metal requires the presence of (traces of) water.

Mixtures that are manufactured on an industrial scale also need to be regarded as potentially hazardous and must be handled with utmost care. Black powder, for example, is more friction and impact sensitive than most other conventional secondary explosives (including TNT, nitropenta, or gelatinous explosives), and above all, black powder is extremely spark sensitive. Therefore, grinding and homogenization of black powder need to be performed in a moistened state. There are several pyrotechnic experiments and formulations published in the literature that can be used in an educational setting. For further reading and pyrotechnic recipes, we recommend the material in ref 2 as well some formulations described in refs 1 and 17 (in German). Environmental Aspects Health aspects in pyrotechnics are not restricted to injuries by accidental ignition of a device. Several recent works identified fireworks and pyrotechnics as environmental polluters. The fundamental problem in pyrotechnics in this respect is the simple idea of “what goes up must come down”. In other words, the chemicals used in pyrotechnics cannot just disappear; they are dispersed in the environment. This includes both reaction products and unburned constituents of a pyrotechnic mixture. Environmental concern in pyrotechnics primarily focuses on two types of emissions: heavy metals (primarily lead and barium) and perchlorates. Perchlorates are emitted in the form of unreacted pyrotechnic oxidizers. In their study, Wilkin and coauthors showed a sudden increase in perchlorate concentration in lake water after a fireworks display (18). Perchlorates are regarded to be persistent in nature and to accumulate in waters and groundwater. The ClO4- ion has an ionic radius similar to that of the I- ion,2 which is essential for the production of thyroidal hormones. Perchlorate is thus mistaken for iodide by the body and taken up into the thyroid gland. The long-term consumption of perchlorate-contaminated water may thus fill the thyroid gland with useless perchlorate and consequently competitively block any iodine uptake by the thyroid gland (21-23). This mechanism does harm to adults, but it gravely effects the development of 152


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fetuses. Therefore, perchlorate is generally regarded as teratogenic. The coincidence of fireworks events and the suddenly increasing number of patients with chronic respiratory diseases was observed decades ago (24). Bach et al. showed that the number of asthma patients doubled during Independence Day celebrations in Hawaii. A study from India showed a similar increase by 12% in the course of the Diwali Festival (the Hindi Festival of Light, usually celebrated with firecrackers) (25). Unfortunately, there have also been reports of fatal asthma attacks in children exposed to fireworks emissions (26). Concerning asthmatic effects, sulfur dioxide has been suspected as the chemical mainly responsible, but in a recent study, we raised the possible interrelationship between respiratory effects and the fireworks' emission of barium-rich aerosols (27). The inhalation of soluble barium compounds is known to have bronchoconstrictor effects, which emerge as an asthmatic symptom (28). Lead aerosols are the result of using lead oxides as pyrotechnic oxidizers, especially in so-called “electric matches”, which will be discussed later. A few years ago, this pyrotechnic emission was estimated to be ∼0.8% at most of the total annual emission and deposition of lead (29). The scientific community has started to investigate the pollution caused by fireworks only very recently (5, 18, 27, 30-35). Some studies focused on the release of gaseous pollutants such as SO2 and NOx as well as particulate matter (36-38). Other studies investigated the problems caused by the disposal of fireworks' wastes (39, 40). Child labor in pyrotechnic factories in developing countries is a serious health issue. Daily exposure of workers to the dust of the pyrotechnic constituents leads to chronic diseases with ongoing symptoms of headaches or dizziness (41). Pyrotechnic Applications and Recent Developments Electric Matches The implementation of new, environmentally friendly pyrotechnics is a slow process because many aspects have to be considered before a new product can be produced on an industrial scale. First and most important is extensive safety testing (impact, friction, heat, long-term stability). One example of a successful innovation is the development of lead-free “electric matches”. These igniters contain a small pyrotechnic pill that is ignited by a high-resistance bridgewire, as shown in Figure 1. Electric matches are used for electrical ignition of devices such as fireworks, airbag igniters, or blasting initiators, remotely and with precise timing. Commercial electric matches


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Chemistry for Everyone Table 3. Examples of Pyrotechnics for Military Use Pyrotechnic Type Heat-generating pyrotechnics

Application

Example

Fuses, fuse cords

Black powder

Pyrotechnic mixtures in detonators (blasting caps, detonators, or primers) Ox: KNO3, BaO2 Fuel: Mg, Ti, Si

Electric matches for igniters (“first fires”) Incendiary devices Percussion primers, stab primers

KClO3 þ TNT þ PbO2; antimony sulfide

Delay compositions (bombs, projectiles, grenades)

Gas generating: black powder Gas free: metal oxides, and chromates with metal powder

Matches

Head: antimony sulfide, or sulfur and KClO3 Rough side: red phosphorus and glass powder (to increase friction)

Smoke-generating pyrotechnics

Camouflage smoke

Red phosphorus and Zr and KNO3

Light-generating pyrotechnics

Signal munition

Red and green flares

Illumination

White illumination

Decoy munition, aerial countermeasure (IR range)

MTVa

a MTV stands for mixtures of magnesium and Teflon or Viton. The perfluorinated polymer acts as the oxidizer in these pyrotechnics, as they react according to the following reaction scheme (see also ref 5): (C2F4)n þ 2n Mg f 2n C þ 2n MgF2.

originally contained several lead compounds, such as lead thiocyanate, lead nitroresorcinate, and lead tetroxide as constituents of the pyrotechnic pill (42). These lead compounds were decomposed upon ignition and set free as inhalable aerosols. The human lung takes up lead compounds efficiently (to approximately 70%), and chronic (occupational) exposure to lead aerosol clouds causes slow and chronic poisoning. Lead-free electric matches have been developed at the Los Alamos National Laboratory and are now at prototype stage (42-44). The innovative key is the use of nanoscale thermite materials based on aluminum and molybdenum trioxide. The proposed mixtures are sensitive to sudden heat stimuli, such as resistive heating, but are safer to use than traditional electric matches because of decreased sensitivity to friction, impact, static, and high ambient temperature. From an environmental standpoint, the absence of toxic lead compounds needs to be highlighted. The novel thermite composites are exceptionally hot burning and can ignite materials that are usually difficult to light. Military Applications In military spheres, pyrotechnics may be used for several purposes. The main applications are heat-, smoke-, and lightgenerating pyrotechnics. An overview with different applications and examples is given in Table 3. Nitrogen-Rich Pyrotechnics Novel developments in pyrotechnics also focus on the application of nitrogen-rich compounds. In contrast to conventional energetic materials, this class of substances does not gain its energy from oxidation of a carbon backbone or a fuel but rather from high heats of formation. In general, the heat of formation of an energetic, nitrogen-rich compound increases as the number of nitrogen atoms increases (45). For example, the heat of formation of imidazole is Δf H °cryst = 14.0 kcal mol-1,


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for 1,2,4-triazole Δf H °cryst = 26.1 kcal mol-1, and for tetrazole Δf H °cryst = 56.7 kcal mol-1 (see ref 46). For pyrotechnics, these high-energy-density materials serve as potential propellants or coloring agents and fuels, eventually in combination with less toxic metal ions, such as Cuþ or Cu2þ, instead of Ba2þ. Nitrogen-rich materials combine several advantages (5), including products that: • • • • •

Are only or mostly gaseous products (smokeless combustion) Have high heats of formation Have high propulsive power Have high specific impulse Have high flame temperatures

“Green” pyrotechnics, regardless of whether for military or civil purposes, should primarily avoid perchlorates and heavy metals. Compounds applicable in fireworks should be inexpensive, easy to synthesize, and nonhygroscopic. High nitrogen content is desirable for reduction of smoke and particulate matter, as dinitrogen will be the main reaction product. A smokeless pyrotechnic device based on nitrogen-rich compounds offers the advantage that smoke does not cloud the pyrotechnic effect with solid reaction products. Conventional pyrotechnics usually require the application of much more of the energetic mixture in order to compensate for this effect. The additional material, of course, obscures the sky more and more, in a vicious cycle. In other words, smokeless pyrotechnics are characterized by higher efficiency. One can regard this as a special form of “atom economy”, which is a fundamental principle of green chemistry (47). On an economic level, nitrogen-rich pyrotechnics cannot compete with the large-scale industrial production of “traditional” fireworks yet. However, when taking the costs for environmental remediation into account, high-nitrogen compositions might become competitive soon (31). The literature suggests different compounds with high nitrogen content for application in pyrotechnics (48). Two classes of nitrogen-rich heterocycles have proven to be most


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Figure 3. Combustion of two novel, environmentally friendly compositions developed at LMU Munich in collaboration with the Armament Research, Development, and Engineering Center.

Figure 2. Chemical structures of 5,50 -bistetrazole (BT); 3,6-dihydrazino1,2,3,5-tetrazine (DHT); N,N-bis(1H-tetrazol-5-yl)amine (BTA); triazoloaminotriazinyl-1,2,3,5-tetrazine (TATTz); 1H-tetrazole (Tz); 5-aminotetrazole (5-At); and 1-methyl-5-nitriminotetrazole (1-MeAtHNO2).

promising: tetrazoles (five-membered rings with four nitrogen atoms) and tetrazines (six-membered rings with four nitrogen atoms) (5, 49). Structures of some typical tetrazoles and tetrazines are provided in Figure 2. Chavez et al. (48) synthesized several salts of 5,50 -bistetrazole (BT) and investigated their application as additives in pyrotechnics with regard to smokeless combustion. Furthermore, these researchers demonstrated that 3,6-dihydrazino-1,2,3,5tetrazine (DHT) could be used as a colorful pyrotechnic fuel if small amounts of coloring agents were added. Koppes et al. (50, 51) suggested the use of salts of triazoloaminotriazinyl-1,2,3,5-tetrazine (TATTz) in pyrotechnics. The hydrogen atom of the tetrazine moiety can be abstracted, resulting in a TATTz- anion. In combination with nitrogenrich cations, this anion showed excellent properties for use in propellants. In the Ludwig-Maximilians University (LMU) research group, salts and copper complexes of N,N-bis(1H-tetrazol-5yl)-amine (BTA) and their use in pyrotechnics were investigated. These compounds have several favorable properties, such as bright emission of colored light and low sensitivity to external stimuli (46, 52). Other Cu-based pyrotechnical compositions have also recently been studied in this research group (53, 54). These compounds have also been suggested for use in gas generators, automotive airbags, and as rocket-propellant additives. A technical application of BTA complexes is the synthesis of nanostructured metal foams by combustion under controlled conditions (55-59). Pyrotechnic reactions may thus be used as a pathway toward the production of novel functional materials, as well as with other applications such as the storage of (hydrogen) gas. Who says a firework is only an entertaining extravagance without any sustained value? Another class of suitable additives in pyrotechnics may be the salts of 1H-tetrazole (Tz) (60). In particular, strontium bistetrazolate pentahydrate shows promising properties; it shows a brilliant red color, is easy to prepare, and is not sensitive to either impact or friction. The salt was investigated as a coloring agent in new pyrotechnic compositions based on 154


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potassium permanganate as the oxidizer instead of perchlorates (61). Alkali salts of 5-aminotetrazole (62), easily prepared by deprotonation of 5-aminotetrazole (5-At), show suitable color performance and have a high nitrogen content, which makes them promising coloring agents in pyrotechnics. Even more promising may be the alkali salts of 1-methyl-5-nitriminotetrazole (63) because their additional nitro group leads to a better oxygen balance. We recently published a new pyrotechnic composition containing strontium bis(1-methyl-5-nitriminotetrazolate) monohydrate (61). Generally, 5-nitriminotetrazoles (64-66) form a unique class of energetic tetrazole derivatives because (i) they combine both the oxidizer (the nitro group) and the fuel (the tetrazole backbone) in one molecule and (ii) they can be deprotonated to form thermally stable anions, which can be used in nitrogen-rich salts or as ligands in several metal complexes (67). Figure 3 provides an example of a successful laboratorylevel-development of colored pyrotechnics based on nitrogenrich compounds. This figure shows the bright red and green emissions of novel, environmentally friendly red and green compositions for use in hand-held signal flares. The red composition is perchlorate-free, while the green formulation does not contain the heavy metal barium. Conclusions This article has described the basic principles of pyrochemistry and the environmental impact of traditional pyrotechnics and fireworks. The scientific community in this field is currently looking for possible substitutes for heavy metals (e.g., lead and barium) and perchlorate. Several institutions worldwide (including in Vienna and Munich) are currently searching for novel, eco-friendly pyrotechnics, with very different approaches. Lead-free electric matches have been developed at Los Alamos National Laboratory on the basis of mixtures of molybdenum trioxide oxidizer and aluminum fuel. A completely different approach would be implementation of nitrogen-based pyrotechnics, as they are, in theory, applied in the form of the pure compound and not as mixtures of several substances, which has long been a typical trait of pyrotechnics. Nitrogen-rich tetrazole and tetrazine derivatives gain their energy from high heats of formation. When they are ignited, these substances primarily decompose into the elements and thus mainly form dinitrogen, which is the most eco-friendly reaction product one can imagine. So, after all, can we regard pyrotechnics as a suitable topic for the chemical and environmental education of young people? Yes, we can.


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Acknowledgment We thank Johannes H. Sterba for the photograph of the igniting electric match. Financial support of this work by the Ludwig-Maximilian University of Munich (LMU), the Fonds der Chemischen Industrie (FCI), the European Research Office (ERO) of the U.S. Army Research Laboratory (ARL), and ARDEC (Armament Research, Development, and Engineering Center) under contract nos. N 62558-05-C-0027, R&D 1284CH-01, R&D 1285-CH-01, 9939-AN-01, W911NF-07-10569, W911NF-08-1-0372, W911NF-08-1-0380 and R&D 1324-CH-01 and the Bundeswehr Research Institute for Materials, Explosives, Fuels, and Lubricants (WIWEB) under contract nos. E/E210/4D004/X5143 and E/E210/7D002/4F088 is gratefully acknowledged. The authors acknowledge collaborations with Mila Krupka (OZM Research, Czech Republic) in the development of new testing and evaluation methods for energetic materials and with Muhamed Sucesca (Brodarski Institute, Croatia) in the development of new computational codes to predict the detonation parameters of high-nitrogen explosives. We are indebted to and thank Betsy M. Rice (ARL, Aberdeen Proving Ground, MD) and Gary Chen (ARDEC, Picatinny Arsenal, NJ) for many helpful and inspiring discussions and support for our work. Notes 1. Ammonium ions should never come into contact with chlorate ions because such mixtures are highly explosive and unpredictable. 2. Ionic radii of I-, 2.20 Å (CN = 6) (see ref 19), and ClO4-, 2.40 Å (thermochemical radius), cited in ref 20.

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