Perchlorate

A perchlorate is a chemical compound used mainly in making rocket fuel and fireworks, as well as some other industrial uses. In many countries it is used to treat hyperthyroidism, but, it's an environmental toxin which endangers human health when it contaminates food and water.
Perchlorates contain the perchlorate ion,, the conjugate base of perchloric acid. As counterions, there can be metal cations, quaternary ammonium cations or other ions, for example, nitronium cation.
The term perchlorate can also describe perchlorate esters or covalent perchlorates. These are organic compounds that are alkyl or aryl esters 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. Additionally, they have been used in rocket propellants, fertilizers, and as bleaching agents in the paper and textile industries.
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 solvents. Four perchlorates are of primary commercial interest: ammonium perchlorate, perchloric acid, potassium perchlorate and sodium perchlorate.

Production

Very few chemical oxidants are strong enough to convert chlorate to perchlorate. Persulfate, ozone, or lead dioxide are all known to do so, but the reactions are too delicate and low-yielding for commercial viability.
Perchlorate salts are typically manufactured through the process of electrolysis, which involves oxidizing aqueous solutions of corresponding chlorates. This technique is commonly employed in the production of sodium perchlorate, which finds widespread use as a key ingredient in rocket fuel. Perchlorate salts are also commonly produced by reacting perchloric acid with bases, such as ammonium hydroxide or sodium hydroxide. Ammonium perchlorate, which is highly valued, can also be produced via an electrochemical process.
Perchlorate esters are formed in the presence of a nucleophilic catalyst via a perchlorate salt's nucleophilic substitution onto an alkylating agent.

Uses

The dominant use of perchlorates is as oxidizers in propellants for rockets, fireworks and highway flares. Of particular value is ammonium perchlorate composite propellant as a component of solid rocket fuel. In a related but smaller application, perchlorates are used extensively within the pyrotechnics industry and in certain munitions and for the manufacture of matches. Martian perchlorates might also be used to produce fuel on that planet.
Perchlorate is used to control static electricity in food packaging. Sprayed onto containers it stops statically charged food from clinging to plastic or paper/cardboard surface.
Niche uses include lithium perchlorate, which decomposes exothermically to produce oxygen, useful in oxygen "candles" on spacecraft, submarines, and in other situations where a reliable backup oxygen supply is needed.
Potassium perchlorate has, in the past, been used therapeutically to help manage Graves' disease. It impedes production of the thyroid hormones that contain iodine.
As perchlorate is generally a non-complexing anion and that its sodium salts is particularly soluble, it is commonly used as a background, or supporting, electrolyte in solution chemistry, electrophoresis, and electrochemistry. Although used as a powerful oxidizer in propulsive powders and explosives, quite surprisingly, the perchlorate anion is a weak oxidant in aqueous solution because of kinetics limitations severely hindering the electron transfer.
Chemical properties

The perchlorate ion is the least redox reactive of the generalized chlorates. Perchlorate contains chlorine in its highest oxidation number. A table of reduction potentials of the four chlorates shows that, contrary to expectation, perchlorate in aqueous solution is the weakest oxidant among the four.

Ion	Acidic reaction	E°	Neutral/basic reaction	E°
Hypochlorite		1.63		0.89
Chlorite		1.64		0.78
Chlorate		1.47		0.63
Perchlorate		1.42		0.56

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 of various chlorine oxides do follow the expected trend wherein Dichlorine heptoxide| exhibits the largest endothermic value of Δ_fH° while Dichlorine monoxide| exhibits the lowest endothermic value of Δ_fH°.

Weak base and weak coordinating anion

As perchloric acid is one of the strongest mineral acids, perchlorate is a very weak base in the sense of Brønsted–Lowry acid–base theory.
As it is also generally a weakly coordinating anion, perchlorate is commonly used as a background, or supporting, electrolyte.

Weak oxidant in aqueous solution due to kinetic limitations

Perchlorate compounds oxidize organic compounds, especially when the mixture is heated. The explosive decomposition of ammonium perchlorate is catalyzed by metals and heat.
As perchlorate is a weak Lewis base and a weak nucleophilic 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 speciation of many cations in aqueous solution or in electroanalytical methods. Although the perchlorate reduction is thermodynamically 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. The metastable character of perchlorate in the presence of reducing 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 necessary for a fast oxygen atom transfer and the associated electron transfer as studied experimentally by Henry Taube and theoretically by Rudolph A. Marcus, 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 and Ti 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. When it is the case, metallic cations can readily reduce perchlorate in solution. Ru can reduce to, while V, V, Mo, Cr and Ti can reduce to.
Some metal complexes, especially those of rhenium, and some metalloenzymes can catalyze the reduction of perchlorate under mild conditions. Perchlorate reductase, a molybdoenzyme, also catalyzes the reduction of perchlorate. Both the Re- and Mo-based catalysts operate via metal-oxo intermediates.

Microbiology

Over 40 phylogenetically and metabolically diverse microorganisms capable of growth using perchlorate as an electron acceptor have been isolated since 1996. Most originate from the Pseudomonadota, but others include the Bacillota, Moorella perchloratireducens and Sporomusa sp., and the archaeon Archaeoglobus fulgidus. With the exception of A. fulgidus, microbes that grow via perchlorate reduction utilize the enzymes perchlorate reductase and chlorite dismutase, which collectively take perchlorate to chloride. In the process, free oxygen is generated.

Natural abundance

Terrestrial abundance

Perchlorate is created by lightning discharges in the presence of chloride. Perchlorate has been detected in rain and snow samples from Florida and Lubbock, Texas. It is also present in Martian soil.
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.. 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.

On Mars

Perchlorate was detected in Martian soil at the level of ~0.6% by weight. It was shown that at the Phoenix landing site it was present as a mixture of 60% and 40%. These salts, formed from perchlorates, act as antifreeze and 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 stable in liquid form for a few hours each day during the summer.
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. 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. 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. 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 on board the Mars Reconnaissance Orbiter from four different locations where recurring slope lineae 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.