perchlorate

perchlorate

[per-klawr-eyt, -klohr-]
perchlorate: see chlorate.

Perchlorates are the salts derived from perchloric acid (HClO4). 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 and fireworks. Both potassium perchlorate (KClO4) and ammonium perchlorate (NH4ClO4) 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.

Scientific definition

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

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

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. Large amounts of improperly stored ammonium perchlorate led to the PEPCON disaster, in which an explosion destroyed one of the two large scale production plants for ammonium perchlorate in the US.

Use

Oxidizer

The high oxygen content and the high stability of perchlorates make them ideal oxidizers for fireworks, airbags and as key compound in solid rocket fuel. The solid rocket boosters of the space shuttle contain 350 metric tons of ammonium perchlorate each.

Medical applications

Perchlorate has been used as a medication to treat hyperthyroidism for many decades. At very high doses, 70,000 – 300,000 ppb, the administration of potassium perchlorate was considered the standard of care in the United States, and remains the approved pharmacologic intervention for many countries. In the early ’60s, potassium perchlorate was implicated in the development of aplastic anemia among several patients. Aplastic anemia is a condition where the bone marrow fails to produce cells that lead to red blood cells. Subsequent investigations indicated the absence of a connection between the administration of potassium perchlorate and the development of the disease.

Production

Natural formation of perchlorates

There are several well-documented mechanisms for natural formation of perchlorate. Involving ozone or hydroxyl radicals as oxidizer for sodium chloride from the sea and are somewhat similar to the formation processes of iodates also present in the atmosphere.

As most perchlorates are readily soluble in water, an accumulation of perchlorates in the environment only occurs in arid areas with little or no rainfall. It is known since the beginning of the 20th century that the Atacama Desert not only contains large amounts of nitrates but also trace amounts of perchlorates. The concentration varies but is in the mg/kg range. The dry south west of the United States also shows accumulation of perchlorates. With the use of nitrates from the Atacama Desert, so called Chile saltpeter as fertilizer the chlorates were also distributed into the environment. As the Chile saltpeter was mostly substituted by nitrates produced by the Haber Bosch process, which contains no perchlorates this source of perchlorates nearly vanished.

In 2006 a mechanism for the formation of perchlorates was proposed which is particularly apropos to the discovery of perchlorate at the Mars Phoenix lander site. It was shown that soils with high concentrations of natural salts could have some of their chloride converted to perchlorate in the presence of sunlight and/or ultraviolet light. The mechanism was reproduced in the lab using chloride rich soils from Death Valley.

Industrial production

Perchlorates are either produced by electrolysis of chloride salts or by the neutralisation of perchloric acid, which is produced by electrolysis of chlorine, with ammonia or other base.

The electrolysis involves the following reactions:

3Cl2 + 6OH- → 5Cl- + ClO3- + 3H2O
ClO3- + H2O → ClO4- + 2H+

For example the industrial scale synthesis for sodium perchlorates starts from sodium chloride. If the electrolysis is not done with the method described at chlorine, but a mixing of the chlorine evolved and the sodium hydroxide is allowed, the reaction mentioned above takes place. The hypochlorite and the chlorate are intermediates in this process.

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. According to the Impact Area Groundwater Study Program, the chemical has been detected as high as 5 µg/L in Massachusetts, well over the state regulation of 2 µg/L.

In a statement issued after the discovery of perchlorate on Mars (see below), NASA declared, among other things, that perchlorates are found naturally on Earth at such places as Chile's hyper-arid Atacama Desert.

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.

Fireworks are also a source of perchlorate in lakes.

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. 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. 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. In February 2008, U.S. Food and Drug Administration said that U.S. toddlers on average are being exposed to more than half of the U.S. EPA's safe dose from food alone.

The two production sites of PEPCON and Kerr McGee in Henderson, Nevada, which were the biggest producers till the explosion of PEPCON in 1988 and the closure of the Kerr McGee plant in 1998, leaked significant amounts of perchlorates into the Las Vegas Wash and from there into Lake Mead and the Colorado River.

The disposal of unused rocket motors and ammunition has led to contamination by perchlorates of several military installations.

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.

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. 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. 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.

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). 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. 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.

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.

Biological functions

Several microorganisms have been identified, which are capable to use perchlorates as electron acceptor. The phylogenetically diverse microorganisms are Proteobacteria.

Discovery of perchlorate on Mars

NASA reports that: "Within the last month [July 2008], two samples have been analyzed by the Wet Chemistry Lab (WCL) of the lander's Microscopy, Electrochemistry, and Conductivity Analyzer, or MECA, suggesting one of the soil constituents may be perchlorate...."

The source of the perchlorate has not yet been evaluated fully, and may represent possible extra-Martian (Earth-sourced) contamination via the Phoenix lander itself. Nevertheless, the ultrapure hydrazine used in the Phoenix retro rockets make this type of contamination unlikely. In addition, perchlorate found below the surface is more highly concentrated than would be expected from contamination during Earth launch operations.

Types of perchlorates

References

External links

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