Potassium perchlorate is the inorganic salt with the chemical formula KClO4. Like other perchlorates, this salt is a strong oxidizer when the solid is heated at high temperature, although it usually reacts very slowly in solution with reducing agents or organic substances. This colorless crystalline solid is a common oxidizer used in fireworks, ammunition percussion caps, and explosive primers, and is used variously in propellants, flash compositions, stars, and sparklers. It has been used as a solid rocket propellant, although in that application it has mostly been replaced by the more performant ammonium perchlorate. KClO4 has a relatively low solubility in water (1.5 g in 100 mL of water at 25 °C).
Production
Potassium perchlorate is prepared industrially by treating an aqueous solution of sodium perchlorate with potassium chloride. This single precipitation reaction exploits the low solubility of KClO4, which is about 1/100 as much as the solubility of NaClO4 (209.6 g/100 mL at 25 °C). It can also be produced by bubbling chlorine gas through a solution of potassium chlorate and potassium hydroxide, and by the reaction of perchloric acid with potassium hydroxide; however, this is not used widely due to the dangers of perchloric acid. Another preparation involves the electrolysis of a potassium chlorate solution, causing KClO4 to form and precipitate at the anode. This procedure is complicated by the low solubility of both potassium chlorate and potassium perchlorate, the latter of which may precipitate onto the electrodes and impede the current.
Oxidizing properties KClO4 is an oxidizer, greatly increasing the rate of combustion of combustible materials relative to burning in air. Combustion with glucose gives carbon dioxide, water molecules and potassium chloride:
3 KClO4 + C6H12O6 → 6 CO2 + 6 H2O + 3 KCl The conversion of solid glucose into hot gaseous CO2 is the basis of the explosive force of this and other such mixtures. With sugar, KClO4 yields a low explosive, provided a necessary confinement. Otherwise such mixtures simply deflagrate with an intense purple flame characteristic of potassium. Flash compositions used in firecrackers, defined in the US as containing 50 mg of powder or less, usually consist of a mixture of aluminium powder and potassium perchlorate, although this is one of the few instances where potassium chlorate is still allowed as a major component. This mixture, called flash powder, is also used in ground and air fireworks. As an oxidizer, potassium perchlorate can be used safely in the presence of sulfur, whereas potassium chlorate cannot. The greater reactivity of chlorate is typical – perchlorates are kinetically poorer oxidants. Chlorate can produce chloric acid (HClO3) in contact with impure acidic sulfur or certain sulfur compounds, which is highly unstable and can lead to premature ignition of the composition. Otherwise the sensitivity of perchlorate / sulfur mixtures is about the same as chlorate / sulfur mixtures, although it lowers the ignition temperature of chlorate mixtures more. Correspondingly, perchloric acid (HClO4) is quite stable. In commercial use, potassium perchlorate is used in consumer and display pyrotechnics, some types of solid rocket fuels, and specialty black powder substitutes such as Pyrodex. The exact compositions for different types are trade secrets, but the SDS lists the components as:
Depending on the specific mixture, it is classified as either 1.3C or 1.4C for shipping. The 1.4C designation of "no significant blast hazard" allows up to 75 kilograms (165 lb) to be shipped by air.
Debated medical use Potassium perchlorate can be used as an antithyroid agent used to treat hyperthyroidism, usually in combination with one other medication. This application exploits the similar ionic radius and hydrophilicity of perchlorate and iodide. Perchlorate ion, a common low-level water contaminant in the USA due to the aerospace industry, has been shown to reduce iodine uptake and thus is classified as a goitrogen. Perchlorate ion is a competitive inhibitor of the process by which iodide is actively accumulated into the thyroid follicular cells. Studies involving healthy adult volunteers determined that at levels above 7 μg/(kg·d), perchlorate begins to temporarily inhibit the thyroid gland's ability to absorb iodine from the bloodstream. This level is 9000 times greater than has been found in any water supply, however. The reduction of the iodide pool by perchlorate has a dual effect – reduction of excess hormone synthesis and hyperthyroidism, on the one hand, and reduction of thyroid inhibitor synthesis and hypothyroidism on the other. Perchlorate remains very useful as a single dose application in tests measuring the discharge of radioiodide accumulated in the thyroid as a result of many different disruptions in the further metabolism of iodide in the thyroid gland. Treatment of hyperthyroidism (including Graves' disease) with 600–2000 mg potassium perchlorate (430–1400 mg perchlorate) daily for periods of several months, or longer, was once a common practice, particularly in Europe, and perchlorate use at lower doses to treat thyroid problems continues to this day. Although 400 mg of potassium perchlorate divided into four or five daily doses was used initially and found effective, higher doses were introduced when 400 mg/d was discovered not to control thyrotoxicosis in all subjects. Current regimens for treatment of hypothyroidism (including Graves' disease), when a patient is exposed to additional sources of iodine, commonly include 500 mg potassium perchlorate twice per day for 18–40 days. In another related study were subjects drank just 1 litre (34 US fl oz) of perchlorate-containing water per day at a concentration of 10 ppm, i.e. daily 10 mg of perchlorate ions were ingested, an average 38% reduction in the uptake of Iodine was observed. However, when the average perchlorate absorption in perchlorate plant workers subjected to the highest exposure has been estimated as approximately 0.5 mg/(kg·d), as in the above paragraph, a 67% reduction of iodine uptake would be expected. Studies of chronically exposed workers though have thus far failed to detect any abnormalities of thyroid function, including the uptake of iodine. This may well be attributable to sufficient daily exposure, or intake, of stable iodine-127 among these workers and the short 8 hr biological half life of perchlorate in the body.
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