Potassium Chlorate (KCIO 3)
One of the very best, and certainly the most controversial, of the common oxidizers is potassium chlorate, KC1O 3. It is a white, crystalline material of low hygroscopicity, with 39.2% oxygen by weight. It is prepared by electrolysis from the chloride salt.
Potassium chlorate was used in the first successful coloredflame compositions in the mid-1800's and it remains in wide use today in colored smoke, firecrackers, toy pistol caps, matches, and color-producing fireworks.
However, potassium chlorate has been involved in a large percentage of the serious accidents at fireworks manufacturing plants, and it must be treated with great care if it is used at all. Other oxidizers are strongly recommended over this material, one can be found that will produce the desired pyrotechnic effect.
Potassium chlorate compositions are quite prone to accidental ignition, especially if sulfur is also present. Chlorate / phosphorus mixtures are so reactive that they can only be worked with when quite wet. The high hazard of KC1O 3 mixtures was gradually recognized in the late 19th century, and England banned all chlorate /sulfur compositions in 1894. United States factories have greatly reduced their use of potassium chlorate as well, replacing it with the less-sensitive potassium perchlorate in many formulas. The Chinese, however, continue to use potas- sium chlorate in firecracker and color compositions. Details on their safety record are not available, although several accidents are known to have occurred at their plants in recent years.
Several factors contribute to the instability of potassium chlorate-containing compositions. The first is the low (356°C) melting point and low decomposition temperature of the oxidizer. Soon after melting, KC1O 3 decomposes according to equation.
This reaction is quite vigorous, and becomes violent at temperatures above 500°C. The actual decomposition mechanism may be more complex than equation suggests. Intermediate formation of potassium perchlorate has been reported at temperatures just above the melting point, with the perchlorate then decomposing to yield potassium chloride and oxygen.
The decomposition reaction of potassium chlorate is rare among the common oxidizers because it is exothermic, with a heat of reaction value of approximately -10.6 kcal/mole. While most other oxidizers require a net heat input for their decomposition, potassium chlorate dissociates into KC1 and 0 2 with the liberation of heat. This heat output can lead to rate acceleration, and allows the ignition of potassium chlorate-containing compositions with a minimum of external energy input (ignition stimulus).
Potassium chlorate is particularly sensitive when mixed with sulfur, a low-melting (119°C) fuel. It is also sensitive when combined with low-melting organic compounds, and low ignition temperatures are observed for most such compositions. Higher ignition temperatures are found for KC1O 3 /metal mixtures, attributable to the higher melting points and rigid crystalline lattices of these metallic fuels. However, these mixtures can be quite sensitive to ignition because of their substantial heat out- put, and should be regarded as quite hazardous. Ignition tem- peratures for some KC1O 3 mixtures are given in Table. Note: Ignition temperatures are quite dependent upon the experimental conditions; a range of +/-500 may be observed, depending on sample size, heating rate, degree of confinement, etc.
Ignition Temperatures of Potassium Chlorate/Fuel Mixtures
Mixtures containing potassium chlorate can be quite suscepible to the presence of a variety of chemical species. Acids have a dramatic effect - the addition of a drop of concentrated sulfuric acid (H 2SO 4) to most KCIO 3 /fuel mixtures results in immediate inflammation of the composition. This dramatic reactivity has been attributed to the formation of chlorine dioxide (C10 2) gas, a powerful oxidizer. The presence of basic "neutralizers" such as magnesium carbonate and sodium bicarbonate in KC1O 3 mixtures can greatly lower the sensitivity of these compositions to trace amounts of acidic impurities.
The ability of a variety of metal oxides -- most notably manganese dioxide, Mn0 2 - to catalyze the thermal decomposition of potassium chlorate into potassium chloride and oxygen has been known for years. Little use is made of this behavior in pyrotechnics, however, because KC10 3 is almost too reactive in its normal state and ways are not needed to enhance its reactivity. Materials and methods to retard its decomposition are desired instead. However, knowledge of the ability of many materials to accelerate the decomposition of KC10 3 suggests that impurities could be quite an important factor in determining the reactivity and ignition temperature of chlorate-containing mixtures. It is vitally important that the KCIO 3 used in pyrotechnic manufacturing operations be of the highest possible purity, and that all possible precautions be taken in storage and handling to prevent contamination of the material.
McLain has reported that potassium chlorate containing 2.8 mole% copper chlorate as an intentionally-added impurity (or "dopant") reacted explosively with sulfur at room temperature. A pressed mixture of potassium chlorate with realgar (arsenic sulfide, As2S 2) temperature.
Ammonium chlorate, NH,,C103 , is an extremely unstable compound that decomposes violently at temperatures well below 100° C. If a mixture containing both potassium chlorate and an ammonium salt is prepared, there is a good possibility that an exchange reaction will occur -- especially in the presence of moisture - to form some of the ammonium chlorate
If this reaction occurs, the chance of spontaneous ignition of the mixture is likely. Therefore, any composition containing both a chlorate salt and an ammonium salt must be considered extremely hazardous. The shipping regulations of the United States Department of Transportation classify any such mixtures as "forbidden explosives" because of their instability. However, compositions consisting of potassium chlorate, ammonium chlor- ide, and organic fuels have been used, reportedly safely, for white smoke production.
Colored smoke compositions are a major user of potassium chlorate, and the safety record of these mixtures is excellent. A neutralizer (e.g., MgCO 3 or NaHCO 3 ) is typically added for storage stability, as well as to lower the reaction temperature through an endothermic decomposition, in the flame, of the type
Colored smoke mixtures also contain either sulfur or a carbohydrate as the fuel, and a volatile organic dye that sublimes from the reaction mixture to produce the colored smoke. These compositions contain a large excess of potential fuel, and their explosive properties are greatly diminished as a result. Smoke mixtures must react with low flame temperatures (500°C or less) or the complex dye molecules will decompose, producing black soot instead of a brilliantly colored smoke. Potassium chlorate is far and away the best oxidizer for use in these compositions. Potassium chlorate is truly a unique material. Shimizu has stated that no other oxidizer can surpass it for burning speed, ease of ignition, or noise production using a minimum quantity of composition. It is also among the very best oxidizers for producing colored flames, with ammonium perchlorate as its closest rival. Chlorate-containing compositions can be prepared that will ignite and propagate at low flame temperatures - a property invaluable in colored smoke mixtures. By altering the fuel and the fuel/oxidizer ratio, much higher flame temperatures can be achieved for use in colored flame formulations. KC10 3 is a versatile material, but the inherent danger associated with it requires that alternate oxidizers be employed wherever possible. It is just too unstable and unpredictable to be safely used by the pyrotechnician in anything but colored smoke compositions, and even here coolants and considerable care are required!