Perchlorate

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 used extensively within the pyrotechnics industry, and ammonium perchlorate is also 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.

Chemical definition
The chemical notation for the perchlorate ion is. The ion has a molecular mass of 99.45 amu.

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


 * Ammonium perchlorate, NH4ClO4
 * Caesium perchlorate, CsClO4
 * Lithium perchlorate, LiClO4
 * Magnesium perchlorate, Mg(ClO4)2
 * Perchloric acid, HClO4
 * Potassium perchlorate, KClO4
 * Rubidium perchlorate, RbClO4
 * Silver perchlorate, AgClO4
 * Sodium perchlorate, NaClO4

Reactivity as an oxidant
The perchlorate ion is the least reactive oxidizer of the generalized chlorates. This appears to be a paradox, since higher oxidation numbers are expected to be progressively stronger oxidizers, and less stable. A table of reduction potentials of the four chlorates shows that, contrary to expectation, perchlorate is the weakest oxidant among the four in water.

This shows that the chlorates are stronger oxidizers in acidic conditions than basic conditions, while chlorine is unchanged.

Gas phase measurements of heats of reaction (which allow computation of ΔHf°) of various chlorine oxides do follow the expected trend wherein Cl2O7 exhibits the largest endothermic value of ΔHf° (238.1 kJ/mol) while Cl2O exhibits the lowest endothermic value of ΔHf° (80.3 kJ/mol).

The central chlorine in the perchlorate anion is a closed shell atom and is 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.

Other oxyanions
Using Stock nomenclature, if a Roman numeral in brackets follows the word "chlorate", this indicates the oxyanion contains chlorine in the indicated oxidation state, namely

Using this convention, "chlorate" means any chlorine oxyanion. Commonly, "chlorate" refers only to the oxyanion where chlorine is in the +5 oxidation state.

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

Automotive
Perchlorate is used in airbags, seat belt pre-tensioner, TPMS (tire pressure monitor system) valve sensors, batteries for keyless entry system—mentioned in Hyundai TSB 07-00-001 dated 02-07

Medical applications
Perchlorate has been used as a medication to treat hyperthyroidism since the 1950s. 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 1960s, potassium perchlorate was implicated in the development of aplastic anemia—a condition where the bone marrow fails to produce new blood cells in sufficient quantity—in thirteen patients, seven of whom died. Subsequent investigations have indicated the connection between administration of potassium perchlorate and development of aplastic anemia to be "equivocable at best", which means that the benefit of treatment, if it is the only known treatment,  outweighs the risk, and it appeared a contaminant poisoned the 13.

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 contains not only large amounts of nitrates but also trace amounts of perchlorates. The concentration varies but is in the mg/kg range. The dry southwest 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 that is particularly apropos to the discovery of perchlorate at the Mars Phoenix lander site was proposed. 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. In 2010, perchlorate was found at the 1000 ppb levels in a vast section of Antarctica, with implications that it is formed naturally and globally on Earth and probably on Mars. Recent isotopic studies have shown that natural perchlorate is produced on Earth by the oxidation of chlorine species through pathways involving tropospheric ozone or its photochemical products.

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

The electrolysis involves the following reactions:
 * 3 Cl2 + 6 OH&minus; → 5 Cl&minus; + ClO3&minus; + 3 H2O
 * ClO3&minus; + H2O →  ClO4&minus; + 2 H+

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.

Perchlorate-free product development
In response to concerns regarding perchlorate, efforts have been undertaken to produce substitutes for products using perchlorate. For example, efforts to create perchlorate-free flares include both spectrally balanced decoy and colored flare compositions that include nitrate or oxide oxidizers. Because nitrate oxidizers are less reactive than perchlorate oxidizers, high-energy fuels have used to compensate for this energy shortfall. Some of these high-energy fuels were produced using mechanical alloying technology.

Environmental presence
Low levels of perchlorate have been detected in both drinking water and groundwater in 26 states in the U.S., according to the Environmental Protection Agency. In 2004, the chemical was also found in cow's milk in California 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 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. In 2010, perchlorate was found at the 1000ppb levels in a vast section of Antarctica, with implications that it is formed naturally and globally on Earth and, it is presumed, on Mars most likely by photochemical reactions in the atmosphere ].

Fireworks are also a source of perchlorate in lakes.

Regulatory Activity
On February 11, 2011, the U.S. Environmental Protection Agency (EPA) issued a "regulatory determination" that perchlorate meets the Safe Drinking Water Act criteria for regulation as a contaminant. The Agency found that perchlorate may have an adverse effect on the health of persons and is known to occur in public water systems with a frequency and at levels that it presents a public health concern. As a result of EPA's regulatory determination, it begins a process to determine what level of contamination is the appropriate level for regulation. EPA prepared, as part of its regulatory determination, extensive responses to submitted public comments. The "docket ID" for EPA's regulatory action is EPA-HQ-OW-2009-0297 and can be found on regulations.gov.

Prior to issuance of its regulatory determination, 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 that 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. In March 2009, a Centers for Disease Control study found 15 brands of infant formula contaminated with perchlorate. Combined with existing perchlorate drinking water contamination, infants could be at risk for exposure to perchlorate above the levels considered safe by E.P.A.

The US Environmental Protection Agency has issued substantial guidance and analysis concerning the impacts of perchlorate on the environment as well as drinking water. California has also issued guidance regarding perchlorate use. 

Several states have enacted drinking water standard for perchlorate including Massachusetts in 2006. California's legislature enacted AB 826, the Perchlorate Contamination Prevention Act of 2003, requiring California's Department of Toxic Substance Control (DTSC) to adopt regulations specifying best management practices for perchlorate and perchlorate-containing substances. The Perchlorate Best Management Practices were adopted on December 31, 2005 and became operative on July 1, 2006. California issued drinking water standards in 2007. Several other states, including Arizona, Maryland, Nevada, New Mexico, New York, and Texas have established non-enforceable, advisory levels for perchlorate.

Courts have been called upon to take action with regard to perchlorate. For example, in 2003, a federal district court in California found that Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) applied because perchlorate is ignitable and therefore a “characteristic” hazardous waste. (see Castaic Lake Water Agency v. Whittaker, 272 F. Supp. 2d 1053, 1059-61 (C.D. Cal. 2003)).

One example of perchlorate related problems was found at the Olin Flare Facility, Morgan Hill, California - Perchlorate contamination beneath a former flare manufacturing plant in California was first discovered in 2000, several years after the plant had closed. The plant had used potassium perchlorate as one of the ingredients during its 40 years of operation. By late 2003, the state of California and the Santa Clara Valley Water District had confirmed a groundwater plume currently extending over nine miles through residential and agricultural communities.

The Regional Water Quality Control Board and the Santa Clara Valley Water District have engaged in a major outreach effort that has received extensive press and community response. A well testing program is underway for approximately 1,200 residential, municipal, and agricultural wells in the area. Large ion exchange treatment units are operating in three public water supply systems that include seven municipal wells where perchlorate has been detected. The potentially responsible parties, Olin Corporation and Standard Fuse Incorporated, are supplying bottled water to nearly 800 households with private wells. The Regional Water Quality Control Board is overseeing potentially responsible party (PRP) cleanup efforts.

The two production sites of PEPCON and Kerr-McGee in Henderson, Nevada, which were the biggest producers until 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.

Biological functions
Over 40 phylogenetically and metabolically diverse microorganisms capable of perchlorate reduction have been isolated since 1996, including members of the Proteobacteria as well as two recently identified Firmicutes, Moorella perchloratireducens and Sporomusa sp.

Health effects
Perchlorate adversely affects human health by interfering with iodine 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 affects only 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 perchlorate's 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.

Discovery of perchlorate on Mars
In June 2008, the Wet Chemistry Laboratory (WCL) on board the 2007 Phoenix Mars Lander performed the first wet chemical analysis of martian soil. The analyses on three samples, two from the surface and one from 5 cm depth, revealed a slightly alkaline soil and low levels of salts typically found on Earth. Most unexpected though was the presence of ~ 0.6 wt % perchlorate (ClO4-), most likely as a Mg(ClO4)2 phase.

The extreme temperatures found on Mars typically lead to either crystallization or evaporation of water, making it difficult to imagine that water could be found in liquid form. The salts formed from perchlorates discovered at the Phoenix landing site act as “anti-freeze” and will substantially lower the freezing point of water. Based on the temperature and pressure conditions on present-day Mars at the Phoenix lander site, conditions would allow a perchlorate salt solution to be present in liquid form for a few hours each day during the summer.

The possibility that the perchlorate was a contaminant brought from Earth has been eliminated by several lines of evidence. The Phoenix retro-rockets used ultra pure hydrazine and launch propellants consisted of ammonium perchlorate. Sensors on board Phoenix found no traces of ammonium, and thus the perchlorate in the quantities present in all three soil samples is indigenous to the martian soil.