72
Chemistry of Pyrotechnics
Components of High-Energy Mixtures
73
-2
mixtures, winding up as the sulfide ion (S ) in species such as has traditionally been used in toy pistol caps and trick noise-potassium sulfide (K 2S), a detectable component of black powder makers ("party poppers").
combustion residue.
Phosphorus is available in two forms, white (or yellow) and When present in large excess, sulfur may volatilize out of the red. White phosphorus appears to be molecular, with a formula burning mixture as yellowish-white smoke. A 1:1 ratio of po-of P,,. It is a waxy solid with a melting point of 44 0C, and ig-tassium nitrate and sulfur makes a respectable smoke composi-nites spontaneously on exposure to air. It must be kept cool and tion employing this behavior.
is usually stored under water. It is highly toxic in both the solid and vapor form and causes burns on contact with the skin. Its Boron
use in pyrotechnics is limited to incendiary and white smoke compositions. The white smoke consists of the combustion product, Boron is a stable element, and can be oxidized to yield good heat primarily phosphoric acid (H 3PO,,).
output. The low atomic weight of boron (10.8) makes it an ex-Red phosphorus is somewhat more stable, and is a reddish-cellent fuel on a calories/gram basis. Boron has a high melting brown powder with a melting point of approximately 590°C (in point (2300°C), and it can prove hard to ignite when combined the absence of air). In the presence of air, red phosphorus ig-with a high-melting oxidizer. With low-melting oxidizers, such nites near 260°C [2]. Red phosphorus is insoluble in water. It as potassium nitrate, boron ignites more readily yielding good is easily ignited by spark or friction, and is quite hazardous any heat production. The low melting point of the oxide product time it is mixed with oxidizers or flammable materials. Its fumes (B 20 3 ) can interfere with the attainment of high reaction tem-are highly toxic [3].
peratures, however, [1].
Red phosphorus is mixed as a water slurry with potassium Boron is a relatively expensive fuel, but it frequently proves chlorate for use in toy caps and noisemakers. These mixtures acceptable for use on a cost basis because only a small percentage are quite sensitive to friction, impact, and heat, and a large is required (remember, it has a low atomic weight). For example, amount of such mixtures must never be allowed to dry out in the reaction
bulk form. Red phosphorus is also used in white smoke mix-BaCrO,, + B - products (B
tures, and several examples can be found in Chapter 8.
2 0 3 , BaO, Cr 20 3 )
burns well with only 5% by weight boron in the composition [5, 6]. Boron is virtually unknown in the fireworks industry, but Sulfide Compounds
is a widely-used fuel in igniter and delay compositions for mili-Several metallic sulfide compounds have been used as fuels in tary and aerospace applications.
pyrotechnic compositions. Antimony trisulfide, Sb 2S 3 , is a reasonably low-melting material (m.p. 548°C) with a heat of combus-Silicon
tion of approximately 1 kcal/gram. It is easily ignited and can be used to aid in the ignition of more difficult fuels, serving as In many ways similar to boron, silicon is a safe, relatively inex-a "tinder" in the same way that elemental sulfur does. It has pensive fuel used in igniter and delay compositions. It has a high been used in the fireworks industry for white fire compositions melting point (1410°C), and combinations of this material with a and has been used in place of sulfur in "flash and sound" mix-high-melting oxidizer may be difficult to ignite. The oxidation tures with potassium perchlorate and aluminum.
product, silicon dioxide (Si0 2 ) , is high melting and, importantly, Realgar (arsenic disulfide, As 2S2 ) is an orange powder with is environmentally acceptable.
a melting point of 308°C and a boiling point of 565°C [2]. Due to its low boiling point, it has been used in yellow smoke composi-Phosphorus
tions (in spite of its toxicity!) , and has also been used to aid in the ignition of difficult mixtures.
Phosphorus is an example of a material that is too reactive to be The use of all arsenic compounds -- including realgar - is proof any general use as a pyrotechnic fuel, although it is increas-hibited in "common fireworks" (the type purchased by individuals) ingly being employed in military white smoke compositions, and it by regulations of the U. S. Consumer Product Safety Commission [ 121.
74
Chemistry of Pyrotechnics
a ,
Components of High-Energy Mixtures
75
Organic Fuels
A variety of organic (carbon-containing) fuels are commonly employed in high-energy compositions. In addition to providing heat, these materials also generate significant gas pressure through the production of carbon dioxide (C0 2) and water vapor in the reaction zone.
The carbon atoms in these molecules are oxidized to carbon dioxide if sufficient oxygen is present. Carbon monoxide (CO) or elemental carbon are produced in an oxygen-deficient atmosphere, and a "sooty" flame is observed if a substantial amount of carbon is generated. The hydrogen present in organic compounds winds up as water molecules. For a fuel of formula C x HyOz , x moles of C0 2 and y/2 moles of water will be produced per mole of fuel that is burned. To completely combust this fuel, x + y/2 moles of oxygen gas (2x + y moles of oxygen atoms) will be required. The amount of oxygen that must be provided by the oxidizer in a high-energy mixture is reduced by the presence of oxygen atoms in the fuel molecule. The balanced equation for the combustion of glucose is shown below C6H 1206 + 6 0 2 - 6 CO2 + 6 H 2O
Only six oxygen molecules are required to oxidize one glucose molecule, due to the presence of six "internal" oxygen atoms in glucose. There are 18 oxygen atoms on both sides of the balanced equation.
A fuel that contains only carbon and hydrogen - termed a hydrocarbon - will require more moles of oxygen for complete combustion than will an equal weight of glucose or other oxygen-containing compound. A greater weight of oxidizer is therefore required per gram of fuel when a hydrocarbon-type material is used.
The grams of oxygen needed to completely combust one gram of a given fuel can be calculated from the balanced chemical equa -
tion. Table 3.6 lists the oxygen requirement for a variety of organic fuels. A sample calculation is shown in Figure 3.1.
To determine the proper ratio of oxidizer to fuel for a stoichio -
metric composition, the grams of oxygen required by a given fuel (Tables 3.4-3.6) must be matched with the grams of oxygen delivered by the desired oxidizer (given in Table 3. 2). For the reaction between potassium chlorate (KC10 3 ) and glucose (C6H1206) , 2.55 grams of KC1O 3 donates 1.00 grams of oxygen, and 0.938