WHITE SMOKE PRODUCTION
The processes used to generate a white smoke by means of a pyrotechnic reaction include:
1. Sublimation of sulfur, using potassium nitrate as the oxidizer: A 1:1 ratio of sulfur to KNO 3 is used in such mixtures. Caution: some toxic sulfur dioxide gas will be formed. Ignition of these mixtures must be done in a well-ventilated area.
2. Combustion of phosphorus: White or red phosphorus burns to produce various oxides of phosphorus, which then attract moisture to form dense white smoke. Research and development work relating to red phosphorus-based smoke mixtures is actively being pursued to find substitutes o the zinc chloride smokes. A typical red phosphorus mixture is given in Table 1. An explosive bursting charge is often used with the very-hazardous white phosphorus. Caution : Phosphorus-based smokes generate acidic compounds which may be irritating to the eyes, skin, and respiratory tract.
TABLE 1. White Smoke Compositions
3. Volatilization of oil: A pyrotechnic reaction produces the heat needed to vaporize high molecular weight hydrocarbons. The subsequent condensation of this oil in air creates a white smoke cloud. The toxicity of this smoke is probably the least of all the materials discussed here.
4. Formation of zinc chloride. A reaction of the type
produces the zinc chloride vapor, which condenses in air and attracts moisture to create an effective white smoke. These mixtures have been widely used for over forty years with an excellent safety record during the manufacturing process. However, ZnC1 2 can cause headaches upon continued exposure and replacements for the HC smokes are actively being sought due to health concerns relating to the various reaction products.
The original HC smoke mixtures (Type A) contained zinc metal and hexachloroethane, but this composition is extremely moisture- sensitive and can ignite spontaneously if moistened. An alternative approach involves adding a small amount of aluminum metal to the composition, and zinc oxide (ZnO) is used in place of the moisture-sensitive metal. Upon ignition, a sequence of reactions ensues of the type.
Alternatively, the original reaction has been proposed to be
In either event, the products are ZnCl 2 , CO, and A120 3. The zinc oxide cools and whitens the smoke by consuming atomic carbon in an endothermic reaction that occurs spontaneously above 1000°C (equation 8.6). The reaction with aluminum is quite exothermic, and this heat evolution controls the burning rate of the smoke mixture. A minimum amount of aluminum metal will yield the best white smoke. Several "HC" smoke compositions are listed in Table 1.
5. "Cold Smoke": White smoke can also be achieved by nonthermal means. A beaker containing concentrated hydrochloric acid placed near a beaker of concentrated ammonia will generate white smoke by the vapor-phase reaction
Similarly, titanium tetrachloride (TiC14) rapidly reacts with moist air to produce a heavy cloud of titanium hydroxide - Ti(OH)4 - and HC1.