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A perchlorate is a chemical compound containing the perchlorate ion, , the conjugate base of perchloric acid ( ionic perchlorate). As counterions, there can be metal cations, quaternary ammonium cations or other ions, for example, Nitronium ion ().
The term perchlorate can also describe perchlorate esters or covalent perchlorates. These are organic compounds that are Alkyl group or Aryl group of perchloric acid. They are characterized by a covalent bond between an oxygen atom of the ClO4 moiety and an organyl group.
In most ionic perchlorates, the cation is non-coordinating. The majority of ionic perchlorates are commercially produced salts commonly used as oxidizers for pyrotechnic devices and for their ability to control static electricity in food packaging. Draft Toxicological Profile for Perchlorates, Agency for Toxic Substances and Disease Registry, U.S. Department of Health and Human Services, September, 2005. Additionally, they have been used in rocket propellants, , and as in the Paper industry and Textile industry.
Perchlorate contamination of food and water endangers human health, primarily affecting the thyroid gland.
Ionic perchlorates are typically colorless solids that exhibit good solubility in water. The perchlorate ion forms when they dissolve in water, dissociating into ions. Many perchlorate salts also exhibit good solubility in non-aqueous . Four perchlorates are of primary commercial interest: ammonium perchlorate , perchloric acid , potassium perchlorate and sodium perchlorate .
Perchlorate salts are typically manufactured through the process of electrolysis, which involves oxidizing aqueous solutions of corresponding . This technique is commonly employed in the production of sodium perchlorate, which finds widespread use as a key ingredient in rocket fuel.Helmut Vogt, Jan Balej, John E. Bennett, Peter Wintzer, Saeed Akbar Sheikh, Patrizio Gallone "Chlorine Oxides and Chlorine Oxygen Acids" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH. Perchlorate salts are also commonly produced by reacting perchloric acid with bases, such as Ammonia solution or sodium hydroxide. Ammonium perchlorate, which is highly valued, can also be produced via an Electrochemistry process.
Perchlorate esters are formed in the presence of a nucleophilic catalyst via a perchlorate salt's nucleophilic substitution onto an alkylating agent. Translated from Uspekhi Khimii volume 57 (1988), pp. 1815-1839.
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These data show that the perchlorate and chlorate are stronger oxidizers in acidic conditions than in basic conditions.
Gas phase measurements of heats of reaction (which allow computation of Δf H°) of various chlorine oxides do follow the expected trend wherein exhibits the largest endothermic value of Δf H° (238.1 kJ/mol) while exhibits the lowest endothermic value of Δf H° (80.3 kJ/mol).Wagman, D. D.; Evans, W. H.; Parker, V. P.; Schumm, R. H.; Halow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. J. Phys. Chem. Ref. Data Vol. 11(2); 1982, American Chemical Society and the American Institute of Physics.
As perchlorate is a weak Lewis base ( i.e., a weak electron pair donor) and a weak Nucleophile anion, it is also a very weakly coordinating anion. This is why it is often used as a supporting electrolyte to study the complexation and the Chemical species of many in aqueous solution or in electroanalytical methods (voltammetry, electrophoresis…). Although the perchlorate reduction is thermodynamics favorable , and that is expected to be a strong oxidant, most often in aqueous solution, it is practically an inert species behaving as an extremely slow oxidant because of severe kinetics limitations. (2025). 9780387311142, Kluwer Academic Publishers. ISBN 9780387311142 The metastable character of perchlorate in the presence of reducing agent cations such as in solution is due to the difficulty to form an activated complex facilitating the electron transfer and the exchange of oxo groups in the opposite direction. These strongly hydrated cations cannot form a sufficiently stable coordination bridge with one of the four oxo groups of the perchlorate anion. Although thermodynamically a mild reductant, ion exhibits a stronger trend to remain coordinated by water molecules to form the corresponding hexa-aquo complex in solution. The high activation energy of the cation binding with perchlorate to form a transient inner sphere complex more favourable to electron transfer considerably hinders the redox reaction. The redox reaction rate is limited by the formation of a favorable activated complex involving an oxo-bridge between the perchlorate anion and the metallic cation. It depends on the molecular orbital rearrangement (HOMO and LUMO orbitals) necessary for a fast oxygen atom transfer (OAT) and the associated electron transfer as studied experimentally by Henry Taube (1983 Nobel Prize in Chemistry) and theoretically by Rudolph A. Marcus (1992 Nobel Prize in Chemistry), both awarded for their respective works on the mechanisms of electron-transfer reactions with metal complexes and in chemical systems.
In contrast to the cations which remain unoxidized in deaerated perchlorate aqueous solutions free of dissolved oxygen, other cations such as Ru(II) and Ti(III) can form a more stable bridge between the metal centre and one of the oxo groups of . In the inner sphere electron transfer mechanism to observe the perchlorate reduction, the anion must quickly transfer an oxygen atom to the reducing cation. (1982). 9780841207349, American Chemical Society. ISBN 9780841207349 When it is the case, metallic cations can readily reduce perchlorate in solution. Ru(II) can reduce to , while V(II), V(III), Mo(III), Cr(II) and Ti(III) can reduce to .Urbansky, Edward T. (1998). Perchlorate Chemistry: Implications for Analysis and Remediation
Some metal complexes, especially those of rhenium, and some metalloenzymes can Catalyst the reduction of perchlorate under mild conditions. Perchlorate reductase (see below), a molybdoenzyme, also catalyzes the reduction of perchlorate. Both the Re- and Mo-based operate via metal-oxo intermediates.
Naturally occurring perchlorate at its most abundant can be found commingled with deposits of sodium nitrate in the Atacama Desert of northern Chile. These deposits have been heavily mined as sources for nitrate-based fertilizers. Chilean nitrate is in fact estimated to be the source of around of perchlorate imported to the U.S. (1909–1997). Results from surveys of ground water, ice, and relatively unperturbed deserts have been used to estimate a "global inventory" of natural perchlorate presently on Earth.
(2025). 9783319124148, Springer. ISBN 9783319124148
The possibility that the perchlorate was a contaminant brought from Earth was eliminated by several lines of evidence. The Phoenix retro-rockets used ultra pure hydrazine and launch propellants consisting of ammonium perchlorate or ammonium nitrate. Sensors on board Phoenix found no traces of ammonium nitrate, and thus the nitrate in the quantities present in all three soil samples is indigenous to the Martian soil. Perchlorate is widespread in Martian soils at concentrations between 0.5 and 1%. At such concentrations, perchlorate could be an important source of oxygen, but it could also become a critical chemical hazard to astronauts.
In 2006, a mechanism was proposed for the formation of perchlorates that is particularly relevant to the discovery of perchlorate at the Phoenix lander site. It was shown that soils with high concentrations of chloride converted to perchlorate in the presence of titanium dioxide and sunlight/ultraviolet light. The conversion was reproduced in the lab using chloride-rich soils from Death Valley.Miller, Glen. " Photooxidation of chloride to perchlorate in the presence of desert soils and titanium dioxide ". American Chemical Society. March 29, 2006 Other experiments have demonstrated that the formation of perchlorate is associated with wide band gap semiconducting oxides. In 2014, it was shown that perchlorate and chlorate can be produced from chloride minerals under Martian conditions via UV using only NaCl and silicate.
Further findings of perchlorate and chlorate in the Martian meteorite EETA79001 and by the Mars Curiosity rover in 2012-2013 support the notion that perchlorates are globally distributed throughout the Martian surface.Adam Mann. " Look What We Found on Mars – Curiosity Rover Serves Up Awesome Science". Slate (magazine). 26 September 2013. With concentrations approaching 0.5% and exceeding toxic levels on Martian soil, Martian perchlorates would present a serious challenge to human settlement, as well as microorganisms. Mars covered in toxic chemicals that can wipe out living organisms, tests reveal. Ian Sample, The Guardian. 6 July 2017. On the other hand, the perchlorate would provide a convenient source of oxygen for the settlements.
On September 28, 2015, NASA announced that analyses of spectral data from the Compact Reconnaissance Imaging Spectrometer for Mars instrument (CRISM) on board the Mars Reconnaissance Orbiter from four different locations where recurring slope lineae (RSL) are present found evidence for hydrated salts. The hydrated salts most consistent with the spectral absorption features are magnesium perchlorate, magnesium chlorate and sodium perchlorate. The findings strongly support the hypothesis that RSL form as a result of contemporary water activity on Mars.
The source of perchlorate in California was mainly attributed to two manufacturers in the southeast portion of the Las Vegas Valley in Nevada, where perchlorate has been produced for industrial use. This led to perchlorate release into Lake Mead in Nevada and the Colorado River which affected regions of Nevada, California and Arizona, where water from this reservoir is used for consumption, irrigation and recreation for approximately half the population of these states. Lake Mead has been attributed as the source of 90% of the perchlorate in Southern Nevada's drinking water. Based on sampling, perchlorate has been affecting 20 million people, with highest detection in Texas, southern California, New Jersey, and Massachusetts, but intensive sampling of the Great Plains and other middle state regions may lead to revised estimates with additional affected regions. An action level of 18 μg/L has been adopted by several affected states.
In 2001, the chemical was detected at levels as high as 5 μg/L at Joint Base Cape Cod (formerly Massachusetts Military Reservation), over the Massachusetts then state regulation of 2 μg/L.
As of 2009, low levels of perchlorate had been detected in both drinking water and groundwater in 26 states in the U.S., according to the Environmental Protection Agency (EPA).
Despite its importance to environmental contamination, the specific source and processes involved in natural perchlorate production remain poorly understood. Laboratory experiments in conjunction with isotopic studies have implied that perchlorate may be produced on earth by oxidation of chlorine species through pathways involving ozone or its photochemical products. Other studies have suggested that perchlorate can also be formed by lightning activated oxidation of chloride aerosols (e.g., chloride in sea salt sprays), and ultraviolet or thermal oxidation of chlorine (e.g., bleach solutions used in swimming pools) in water.
One of the main sources of perchlorate contamination from natural nitrate fertilizer use was found to come from the fertilizer derived from Chilean caliche (calcium carbonate), because Chile has rich source of naturally occurring perchlorate anion. Perchlorate concentration was the highest in Chilean nitrate, ranging from 3.3 to 3.98%. Perchlorate in the solid fertilizer ranged from 0.7 to 2.0 mg g−1, variation of less than a factor of 3 and it is estimated that sodium nitrate fertilizers derived from Chilean caliche contain approximately 0.5–2 mg g−1 of perchlorate anion. The direct ecological effect of perchlorate is not well known; its impact can be influenced by factors including rainfall and irrigation, dilution, natural attenuation, soil adsorption, and bioavailability. Quantification of perchlorate concentrations in nitrate fertilizer components via ion chromatography revealed that in horticultural fertilizer components contained perchlorate ranging between 0.1 and 0.46%.
Ex situ treatments include ion exchange using perchlorate-selective or nitrite-specific resins, bioremediation using packed-bed or fluidized-bed , and membrane technologies via electrodialysis and reverse osmosis. In ex situ treatment via ion exchange, contaminants are attracted and adhere to the ion exchange resin because such resins and ions of contaminants have opposite charge. As the ion of the contaminant adheres to the resin, another charged ion is expelled into the water being treated, in which then ion is exchanged for the contaminant. Ion exchange technology has advantages of being well-suitable for perchlorate treatment and high volume throughput but has a downside that it does not treat organochloride. In addition, ex situ technology of liquid phase carbon adsorption is employed, where granular activated carbon (GAC) is used to eliminate low levels of perchlorate and pretreatment may be required in arranging GAC for perchlorate elimination.
In situ treatments, such as bioremediation via perchlorate-selective microbes and permeable reactive barrier, are also being used to treat perchlorate. In situ bioremediation has advantages of minimal above-ground infrastructure and its ability to treat chlorinated solvents, perchlorate, nitrate, and RDX simultaneously. However, it has a downside that it may negatively affect secondary water quality. In situ technology of phytoremediation could also be utilized, even though perchlorate phytoremediation mechanism is not fully founded yet.
Bioremediation using perchlorate-reducing bacteria, which reduce perchlorate ions to harmless chloride, has also been proposed.
In large amounts perchlorate interferes 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. The NAS, in its 2005 report, Health Implications of Perchlorate Ingestion, emphasized that this effect, also known as Iodide Uptake Inhibition (IUI) is not an adverse health effect. However, in January 2008, California's Department of Toxic Substances Control stated that perchlorate is becoming a serious threat to human health and water resources. In 2010, the EPA's Office of the Inspector General determined that the agency's own perchlorate reference dose (RfD) of 24.5 parts per billion protects against all human biological effects from exposure, as the federal government is responsible for all US military base groundwater contamination. This finding was due to a significant shift in policy at the EPA in basing its risk assessment on non-adverse effects such as IUI instead of adverse effects. The Office of the Inspector General also found that because the EPA's perchlorate reference dose is conservative and protective of human health further reducing perchlorate exposure below the reference dose does not effectively lower risk.
Because of ammonium perchlorate's adverse effects upon children, Massachusetts set its maximum allowed limit of ammonium perchlorate in drinking water at 2 parts per billion (2 ppb = 2 micrograms per liter).
Perchlorate affects only thyroid hormone. Because it is neither stored nor metabolism, effects of perchlorate on the thyroid gland are reversible, though effects on brain development from lack of thyroid hormone in , , and children are not.
Toxic effects of perchlorate have been studied in a survey of industrial plant workers who had been exposed to perchlorate, compared to a control group of other industrial plant workers who had no known exposure to perchlorate. After undergoing multiple tests, workers exposed to perchlorate were found to have a significant systolic blood pressure rise compared to the workers who were not exposed to perchlorate, as well as a significant decreased thyroid function compared to the control workers.
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 ("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 and consumes of drinking water per day over a lifetime.
In 2006, a study reported a statistical association between environmental levels of perchlorate and changes in thyroid hormones of women with low iodine. The study authors were careful to point out that hormone levels in all the study subjects remained within normal ranges. The authors also indicated that they did not originally normalize their findings for creatinine, which would have essentially accounted for fluctuations in the concentrations of one-time urine samples like those used in this study. When the Blount research was re-analyzed with the creatinine adjustment made, the study population limited to women of reproductive age, and results not shown in the original analysis, any remaining association between the results and perchlorate intake disappeared. Soon after the revised Blount Study was released, Robert Utiger, a doctor with the Harvard Institute of Medicine, testified before the US Congress and stated: "I continue to believe that that reference dose, 0.007 milligrams per kilo (24.5 ppb), which includes a factor of 10 to protect those who might be more vulnerable, is quite adequate."
In 2014, a study was published, showing that environmental exposure to perchlorate in pregnant women with hypothyroidism is associated with a significant risk of low IQ in their children.
In 2002, the EPA completed its draft toxicological review of perchlorate and proposed an reference dose of 0.00003 milligrams per kilogram per day (mg/kg/day) based primarily on studies that identified neurodevelopmental deficits in rat pups. These deficits were linked to maternal exposure to perchlorate.
In 2003, a federal district court in California found that the CERCLA applied, because perchlorate is ignitable, and therefore was a "characteristic" hazardous waste. Castaic Lake Water Agency v. Whittaker, 272 F. Supp. 2d 1053, 1059–61 (C.D. Cal. 2003).
Subsequently, the U.S. National Research Council of the National Academy of Sciences (NAS) reviewed the health implications of perchlorate, and in 2005 proposed a much higher reference dose of 0.0007 mg/kg/day based primarily on a 2002 study by Greer et al. During that study, 37 adult human subjects were split into four exposure groups exposed to 0.007 (7 subjects), 0.02 (10 subjects), 0.1 (10 subjects), and 0.5 (10 subjects) mg/kg/day. Significant decreases in iodide uptake were found in the three highest exposure groups. Iodide uptake was not significantly reduced in the lowest exposed group, but four of the seven subjects in this group experienced inhibited iodide uptake. In 2005, the Reference dose proposed by NAS was accepted by EPA and added to its integrated risk information system (IRIS).
Although there has generally been consensus with the Greer et al. study, there has been no consensus with regard to developing a perchlorate RfD. One of the key differences results from how the point of departure is viewed (i.e., NOEL or "lowest-observed-adverse-effect level", LOAEL), or whether a benchmark dose should be used to derive the RfD. Defining the point of departure as a NOEL or LOAEL has implications when it comes to applying appropriate safety factors to the point of departure to derive the RfD.
In early 2006, EPA issued a "Cleanup Guidance" and recommended a Drinking Water Equivalent Level (DWEL) for perchlorate of 24.5 μg/L. Both DWEL and Cleanup Guidance were based on a 2005 review of the existing research by the National Academy of Sciences (NAS).Committee to Assess the Health Implications of Perchlorate Ingestion, National Research Council (2025). 9780309095686, The National Academies Press. . ISBN 9780309095686
Lacking a federal drinking water standard, several states subsequently published their own standards for perchlorate including Massachusetts in 2006 and California in 2007. Other states, including Arizona, Maryland, Nevada, New Mexico, New York, and Texas have established non-enforceable, advisory levels for perchlorate.
In 2008, EPA issued an interim drinking water health advisory for perchlorate and with it a guidance and analysis concerning the impacts on the environment and drinking water. California also issued guidance regarding perchlorate use. Both the Department of Defense and some environmental groups voiced questions about the NAS report, but no credible science has emerged to challenge the NAS findings.
In February 2008, the U.S. Food and Drug Administration (FDA) reported that U.S. toddlers on average were being exposed to more than half of 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 and that combined with existing perchlorate drinking water contamination, infants could be at risk for perchlorate exposure above the levels considered safe by EPA.
In 2010, the Massachusetts Department of Environmental Protection set a 10 fold lower RfD (0.07 μg/kg/day) than the NAS RfD using a much higher uncertainty factor of 100. They also calculated an Infant drinking water value, which neither US EPA nor CalEPA had done.
On February 11, 2011, EPA determined that perchlorate meets the Safe Drinking Water Act criteria for regulation as a contaminant.EPA (2011-02-11). "Drinking Water: Regulatory Determination on Perchlorate." 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. Since then EPA has continued to determine what level of contamination is appropriate. EPA prepared extensive responses to submitted public comments. EPA-HQ-OW-2009-0297 "Docket ID" for EPA
In 2016, the Natural Resources Defense Council (NRDC) filed a lawsuit to accelerate EPA's regulation of perchlorate.
In 2019, EPA proposed a Maximum Contaminant Level of 0.056 mg/L for public water systems.EPA (2019-06-26). "National Primary Drinking Water Regulations: Perchlorate." Proposed Rule. Federal Register. .
On June 18, 2020, EPA announced that it was withdrawing its 2011 regulatory determination and its 2019 proposal, stating that it had taken "proactive steps" with state and local governments to address perchlorate contamination. In September 2020 NRDC filed suit against EPA for its failure to regulate perchlorate, and stated that 26 million people may be affected by perchlorate in their drinking water. On March 31, 2022, the EPA announced that a review confirmed its 2020 decision. Following the NRDC lawsuit, in 2023 the US Court of Appeals for the DC Circuit ordered EPA to develop a perchlorate standard for public water systems. EPA stated that it will publish a proposed standard for perchlorate in 2025, and issue a final rule in 2027.
Several perchlorate esters are known. For example, methyl perchlorate is a high energy material that is a strong alkylating agent. Chlorine perchlorate is a covalent inorganic analog.
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