Perchlorate

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The structure and dimensions of the perchlorate ion

A space-filling model of the perchlorate ion

Perchlorates are the salts derived from perchloric acid (H Cl O₄). They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders. They are also used as an oxidizer in rocket fuel and explosives and can be found in airbags, fireworks, and Chilean fertilizers. Both potassium perchlorate (K Cl O₄) and ammonium perchlorate (NH₄ Cl O₄) are used extensively within the pyrotechnics industry, whereas ammonium perchlorate is a component of solid rocket fuel. Lithium perchlorate, which decomposes exothermically to give oxygen, is used in oxygen "candles" on spacecraft, submarines and in other esoteric situations where a reliable backup or supplementary oxygen supply is needed. Most perchlorate salts are soluble in water.^[1]

1 Scientific definition
2 Reactivity as an oxidant
3 Environmental presence
4 Health effects
5 Types of perchlorates
6 References
7 External links

[edit] Scientific definition

The perchlorate ion is ClO₄⁻, and it has a molecular mass of 99.45 a.u.

A perchlorate (compound) is a compound containing this group, with chlorine in oxidation state +7.

[edit] Reactivity as an oxidant

The perchlorate ion is the least reactive oxidizer of the generalized chlorates. This is apparently paradoxical, since higher oxidation numbers are expected to be progressively stronger oxidizers, and less stable. Perchlorate does in fact have the highest redox potential and is least stable thermodynamically, but the central chlorine is a closed shell atom and well protected by the four oxygens. Hence, perchlorate reacts sluggishly. Most perchlorate compounds, especially salts of electropositive metals such as sodium perchlorate or potassium perchlorate, are slow to react unless heated. This property is useful in many applications, such as flares, where the device should not explode, or even catch fire spontaneously.

Mixtures of perchlorates with organic compounds are more reactive. Although they do not usually catch fire or explode unless heated, there are a number of exceptions.

[edit] Environmental presence

Low levels of perchlorate have been detected in both drinking water and groundwater in 35 states in the US according to the Environmental Protection Agency. In 2004, the chemical was also found in cow's milk in the area with an average level of 1.3 parts per billion ("ppb" or µg/L), which may have entered the cows through feeding on crops that had exposure to water containing perchlorates.^[2] According to the Impact Area Groundwater Study Program [1], the chemical has been detected as high as 5 µg/L in Massachusetts, well over the state regulation of 2 µg/L^[3].

In some places, perchlorate is detected because of contamination from industrial sites that use or manufacture it. In other places, there is no clear source of perchlorate. In those areas it may be naturally occurring, or could be present because of the use of Chilean fertilizers, which were imported to the U.S. by the hundreds of tons in the early 19th century. One recent area of research has even suggested that perchlorate can be created when lightning strikes a body of water, and perchlorates are created as a byproduct of chlorine generators used in swimming pool chlorination systems.

As of April 2007, the EPA has not yet determined whether perchlorate is present at sufficient levels in the environment to require a nationwide regulation on how much should be allowed in drinking water.^[4] In 2005, U.S. EPA issued a recommended Drinking Water Equivalent Level (DWEL) for perchlorate of 24.5 µg/L. In early 2006, EPA issued a “Cleanup Guidance” for this same amount. Both the DWEL and the Cleanup Guidance were based on a thorough review of the existing research by the National Academy of Science (NAS). This followed numerous other studies, including one which suggested human breast milk had an average of 10.5 µg/L of perchlorate.^[5] Both the Pentagon and some environmental groups have voiced questions about the NAS report, but no credible science has emerged to challenge the NAS findings.

[edit] Health effects

Perchlorate greatly impacts human health by interfering with iodide uptake into the thyroid gland. In adults, the thyroid gland helps regulate the metabolism by releasing hormones, while in children, the thyroid helps in proper development. Perchlorate is becoming a serious threat to human health and water resources.^[6]

The NAS found that perchlorate only affects the thyroid gland. It is not stored in the body, it is not metabolized, and any effects of perchlorate on the thyroid gland are fully reversible once exposure stops^{[citation needed]}. There has been some concern on perchlorates effects on fetuses, newborns and children, but several peer-reviewed studies on children and newborns also provide reason to believe that low levels of perchlorate do not pose a threat to these populations.^{[citation needed]} On October 1, 2004, the American Thyroid Association (ATA) reported that perchlorate may not be as harmful to newborns, pregnant women and other adults as previously thought.^[7]

A study involving healthy adult volunteers determined that at levels above 0.007 milligrams per kilogram per day (mg/kg-d), perchlorate can temporarily inhibit the thyroid gland’s ability to absorb iodine from the bloodstream ("iodide uptake inhibition", thus perchlorate is a known goitrogen).^[8] The EPA converted this dose into a reference dose of 0.0007 mg/kg-d by dividing this level by the standard intraspecies uncertainty factor of 10. The agency then calculated a "drinking water equivalent level" of 24.5 ppb by assuming a person weighs 70 kilograms (154 pounds) and consumes 2 liters (68 ounces) of drinking water per day over a lifetime.^[9] Thus, 25 ppb was set as the recommended drinking water standard (the DWEL). For that reason, most media reports call this the "safe" level of exposure. The NAS report also stated additional research would be helpful, but emphasized that the existing database on perchlorate was sufficient to make its reference dose recommendation and ensure it would be protective for everyone.^{[citation needed]}

Recent research, however, has shown inhibition of iodide uptake in the thyroids of women at much lower levels, levels attainable from normally contaminated water and milk.^[10]