Perchlorates

The perchlorate anion

Perchlorates are the salts of the perchloric acid HClO ₄ . The perchlorate anion ClO ₄ ^- is simply negatively charged and has tetrahedral symmetry. Chlorine has an oxidation number of +7.

Natural occurrence

Perchlorates, which are formed by oxidative processes in the atmosphere, are deposited with the dust. This process is similar to the formation of nitrates and iodate . In areas with regular rainfall, the perchlorates enter the water cycle and are broken down by microorganisms there . In arid desert areas such as the Atacama Desert and in the dry southwestern USA, the perchlorates can accumulate up to concentrations of a few mg / kg. During the mining of Chile's nitrate in the Atacama Desert, perchlorates had to be removed before it was used as fertilizer .

Perchlorates were also detected in the dust of Mars in 2008.

properties

Perchlorates are oxidizing agents and release oxygen when heated :

${\displaystyle \mathrm {ClO_{4}{^{-}}\rightarrow Cl^{-}+2\ O_{2}\!\uparrow } }$

Most perchlorates are easily soluble . Exceptions are the sparingly soluble salts potassium perchlorate , rubidium perchlorate and cesium perchlorate . Potassium, rubidium and cesium salts can, therefore, by the formation of a white precipitate using perchlorate detected are, for example:

${\displaystyle \mathrm {ClO_{4}{^{-}}+K^{+}\rightarrow KClO_{4}\!\downarrow } }$

Perchlorates hardly have an oxidizing effect in aqueous solution, as their reduction takes place very slowly in solutions without a catalyst . They are fire-promoting and are used in pyrotechnics , in explosives and as rocket fuel for solid rocket rockets . Perchlorate cartridges are used in the ISS so that oxygen can be produced quickly in an emergency.

structure

The perchlorate ion has a perfect tetrahedral structure; the chlorine-oxygen bonds are of equal length and value. The bond relationships can be explained completely analogously to the isoelectronic sulfate ion by mesomerism with three delocalized double bonds or by charge separation with a formally triply positively charged chlorine atom.

Manufacturing

Perchlorates are made by the electrolysis of chlorides. The resulting chlorine is not diverted, as is the case with chlor-alkali electrolysis , but absorbed by the lye that forms . This creates hypochlorite , which then disproportionates to form chloride and chlorate . Further electrolysis produces perchlorate.

${\displaystyle \mathrm {2\ Cl^{-}\to Cl_{2}+2\ e^{-}} }$

${\displaystyle \mathrm {3\ Cl_{2}+6\ OH^{-}\to 5\ Cl^{-}+{ClO_{3}}^{-}+3\ H_{2}O} }$

${\displaystyle \mathrm {{ClO_{3}}^{-}+3\ H_{2}O\to {ClO_{4}}^{-}+2\ H_{3}O^{+}+2\ e^{-}} }$

On doped diamond electrodes, perchlorate can be produced with good yield by means of electrolysis from a chloride, hypochlorite or chlorate solution. The mechanisms are not yet fully understood. The formation is probably due to radical reactions (OH, O radicals):

Ammonium perchlorate is by neutralization of ammonia with perchloric acid produced.

Health hazards

Perchlorates inhibit iodine uptake in the body (iodination) and thus the metabolism of the thyroid gland , so they are goitrogens . Perchlorates are therefore used as medicinal substances to regulate the thyroid function.

use

Perchlorates serve u. a. as an oxidizer in rockets and fireworks.

In radiological examinations, hyperthyroid patients are given perchlorate prior to administration of the iodine-containing contrast agent , as this blocks the iodine uptake in the thyroid via competitive inhibition .

proof

Perchlorates can be detected qualitatively by reducing them to chloride . The sample solution is acidified and titanium (IV) sulfate is added. By adding iron or zinc shavings , the Ti ⁴⁺ ion is reduced to Ti ³⁺ . This in turn reduces the ClO ₄ ^- ion to Cl ^- .

${\displaystyle \mathrm {{ClO_{4}}^{-}+8\ Ti^{3+}+8\ H^{+}\to Cl^{-}+8\ Ti^{4+}+4\ H_{2}O} }$

The resulting chloride is usually detected with silver nitrate .

Representative

• Ammonium perchlorate NH ₄ ClO ₄
• Barium perchlorate Ba (ClO ₄ ) ₂
• Potassium perchlorate KClO ₄
• Lithium perchlorate LiClO ₄
• Magnesium perchlorate Mg (ClO ₄ ) ₂
• Silver perchlorate AgClO ₄
• Fluoroperchlorate FClO ₄
• Nickel (II) perchlorate Ni (ClO ₄ ) ₂

Individual evidence

1. ↑ MH Hecht, SP Kounaves, RC Quinn include: Detection of Perchlorate and the Soluble Chemistry of Martian soil at the Phoenix Lander site . In: Science . tape 325 , no. 5936 , 2009, p. 64-67 , doi : 10.1126 / science.1172466 , PMID 19574385 .
2. ^ H. Bergmann, J. Rollin, T. Iourtchouk: The occurrence of perchlorate during drinking water electrolysis using BDD anodes. In: Electrochim. Acta. 2009, 54, pp. 2102-2107.
3. ↑ A. Lasserre, L. Blohm: Short textbook radiology. 3. Edition. Urban and Fischer, Munich 2003, ISBN 3-437-42111-5 , p. 25.
4. ↑ H. Bergmann, T. Iourtchouk, J. Rollin: Perbromate - a new synthesis and detection possibility. In: LABO. June 2010, pp. 8-10.
5. ^ E. Schweda: Jander / Blasius: Inorganic Chemistry I - Introduction & Qualitative Analysis. 17th edition. Hirzel, 2012, ISBN 978-3-7776-2134-0 .