38
Chemistry of Pyrotechnics
Basic Chemical Principles
39
(KOH), and calcium hydroxide, Ca(OH)
ACIDS AND BASES
2 .
Ammonia (NH 3 ) is a
weak base, capable of reacting with H+ to form the ammonium ion,
+
An acid is commonly defined as a molecule or ion that can serve as N H,
Acids catalyze a variety of chemical reactions, even when pres-a hydrogen ion (H + ) donor. The hydrogen ion is identical to the ent in small quantity. The presence of trace amounts of acidic ma-proton - it contains one proton in the nucleus, and has no elec-terials in many high-energy compounds and mixtures can lead to trons surrounding the nucleus. H + is a light, mobile, reactive instability. The chlorate ion, C10 - , is notoriously unstable in species. A base is a species that functions as a hydrogen ion 3
the presence of strong acids. Chlorate-containing mixtures will acceptor. The transfer of a hydrogen ion (proton) from a good usually ignite if a drop of concentrated sulfuric acid is added.
donor to a good acceptor is called an acid/base reaction. Materi-Many metals are also vulnerable to acids, undergoing an oxi-als that are neither acidic nor basic in nature are said to be neu-dation /reduction reaction that produces the metal ion and hydro-tral
gen gas. The balanced equation for the reaction between HCl Hydrogen chloride (HC1) is a gas that readily dissolves in wa-and magnesium is
ter. In water, HCl is called hydrochloric acid and the HC1 molecule serves as a good proton donor, readily undergoing the re-Mg + 2 HC1 } Mg +2 + H 2 + 2 CI- +heat
action
Consequently, most metal-containing compositions must be free HC1 , H+ + Cl -
of acidic impurities or extensive decomposition (and possibly ignition) may occur.
to produce a hydrogen ion and a chloride ion in solution. The As protection against acidic impurities, high-energy mixtures concentration of hydrogen ions in water can be measured by a will frequently contain a small percentage of a neutralizer. So-variety of methods and provides a measure of the acidity of an dium bicarbonate (NaHCO
aqueous system. The most common measure of acidity is pH, a 3 ) and magnesium carbonate (MgCO
-23 )
are two frequently-used materials. The carbonate ion, C0
,
number representing the negative common logarithm of the hy-3
re-
acts with H +
drogen ion concentration
-2
2H+ +CO
i H
pH = -log [H+]
3
2O+CO 2
to form two neutral species - water and carbon dioxide.
If a solution also contains hydroxide ion (OH - ), a good proton ac-Boric acid (H
ceptor, the reaction
3 BO 3 ) - a solid material that is a weak H + donor -
is sometimes used as a neutralizer for base-sensitive compositions.
H+ + OH + H
Mixtures containing aluminum metal and a nitrate salt are notably 2 O
sensitive to excess hydroxide ion, and a small percentage of boric occurs, forming water - a neutral species. The overall reaction acid can be quite effective in stabilizing such compositions.
is represented by an equation such as
HCl + NaOH -> H 2O + NaCl
I NSTRUMENTAL ANALYSIS
Acids usually contain a bond between hydrogen and an electronegative element such as F, 0, or Cl. The electronegative ele-Modern instrumental methods of analysis have provided scientists ment pulls electron density away from the hydrogen atom, giving with a wealth of information regarding the nature of the solid state it partial positive character and making it willing to leave as H + .
and the reactivity of solids. Knowledge of the structure of solids The presence of additional F, 0, and Cl atoms in the molecule and an ability to study thermal behavior are essential to an under-further enhances the acidity of the species. Examples of strong standing of the behavior of high-energy materials.
acids include sulfuric acid
hydrochloric acid (HC1),
(H2SO4),
X-ray crystallography has provided the crystal type and lat-perchloric acid (HC10,,), and nitric acid (HNO 3).
Most of the common bases are ionic compounds consisting of a tice dimensions for numerous solids. In this technique, high-energy x-rays strike the crystal and are diffracted in a pattern positive metal ion and the negatively-charged hydroxide ion, OH - .
characteristic of the particular lattice type.
Examples include sodium hydroxide (NaOH), potassium hydroxide Complex mathematical
40
Chemistry of Pyrotechnics
analysis can convert the diffraction pattern into the actual crystal structure. Advances in computer technology have revolutionized this field in the past few years. Complex structures, formerly requiring months or years to determine, can now be analyzed in short order. Even huge protein and nucleic acid chains can be worked out by the crystallographer [9].
Differential thermal analysis (DTA) has provided a wealth of information regarding the thermal behavior of pure solids as well as solid mixtures [10] . Melting points, boiling points, transitions from one crystalline form to another, and decomposition temperatures can be obtained for pure materials. Reaction temperatures can be determined for mixtures, such as ignition temperatures for pyrotechnic and explosive compositions.
Differential thermal analysis detects the absorption or release of heat by a sample as it is heated at a constant rate from room temperature to an upper limit, commonly 500°C. Any heat-absorbing changes occurring in the sample (e.g. , melting or boiling) will be detected, as will processes that evolve heat (e.g. , exothermic reactions). These changes are detected by continually comparing the temperature of the sample with that of a thermally-inert reference material (frequently aluminum oxide) FIG. 2.4
The thermogram for pure 2,4,6-trinitrotoluene (TNT).
that undergoes no phase changes or reactions over the tempera-The major features are an endotherm corresponding to melting at ture range being studied. Both sample and reference are placed 81°C and an exothermic decomposition peak beginning near 280°.
in glass capillary tubes, a thermocouple is inserted in each, and The x axis represents the temperature of the heating block in de-the tubes are placed in a metal heating block. Current is applied grees centigrade. The y axis indicates the difference in tempera-to the electric heater to produce a linear temperature increase ture, AT, between the sample and an identically-heated reference (typically 20-50 degrees/minute) [7].
solid, typically glass beads or aluminum oxide.