Perchlorate
Perchlorates are the salts of perchloric acid HClO4. The perchlorate anion ClO4 - is a single negative charge and has tetrahedral symmetry. Chlorine here 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 iodates. In areas with regular rainfall the perchlorates enter the water cycle, where they are degraded by microorganisms. The perchlorates may accumulate to concentrations of several mg / kg in arid desert areas such as the Atacama Desert and the arid Southwest of the United States. When removing the Chile saltpeter in the Atacama Desert perchlorates had to be removed as fertilizer prior to its use.
Perchlorates were also detected in 2008 in the dust of Mars.
Properties
Perchlorates are oxidants and give off oxygen when heated:
The most perchlorates are readily soluble. An exception is the only moderately soluble potassium perchlorate. Potassium salts can be detected perchlorate therefore by the formation of a white precipitate using:
In aqueous solution, perchlorates hardly seem oxidizing, since their reduction in solutions without catalyst is very slow. They are oxidizing and are used in pyrotechnics, in explosives and as rocket fuel for solid fuel rockets. In the ISS perchlorate cartridges are used to quickly produce oxygen in an emergency.
Structure
Perchlorate ion is constructed perfectly tetrahedral, the chlorine -oxygen bonds are of equal length and equivalent. The bonding can be explained by resonance with three delocalized double bonds or by charge separation with a formal triple- positively charged chlorine atom completely analogous to the isoelectronic sulfate ion.
Production
Perchlorates are produced by the electrolysis of chlorides. The resulting chlorine is not derived as in the chlor-alkali electrolysis, but is absorbed by the forming solution. This produces hypochlorite, which is then disproportionated to chloride and chlorate. By further electrolysis arises perchlorate.
At doped diamond electrodes perchlorate can be prepared in good yield by electrolysis of a chloride solution, hypochlorite or chlorate. The mechanisms are not yet fully understood. Probably the formation but to radical reactions ( OH, O- radicals) is due to:
Ammonium perchlorate is prepared by neutralization of ammonia with perchloric acid.
Health hazards
Perchlorates inhibit iodine uptake in the body ( iodination ) and thus the metabolism of the thyroid, so are goitrogens. Perchlorates are therefore used as a medication to regulate thyroid function.
Use
Perchlorates are used among other things as an oxidizing agent in rockets and fireworks. In radiological examinations hyperthyreotischen patient is administered prior to the administration of iodinated contrast material perchlorate, as this blocks the iodine uptake in the thyroid gland via a competitive inhibition.
Proof
High perchlorates can be detected by the reduction of Chlorations to chloride. Here, the sample solution is acidified and mixed with titanium (IV ) sulfate. By the addition of iron or zinc shavings, the Ti4 ion is reduced to Ti3 . This in turn reduces the ClO4 - ion to Cl -.
The resulting chloride is usually detected with silver nitrate.
Representative
- Ammonium perchlorate NH4ClO4
- Barium perchlorate Ba ( ClO4 ) 2
- Potassium perchlorate KClO4
- Lithium perchlorate LiClO4
- Magnesium perchlorate, Mg ( ClO4 ) 2
- Silver perchlorate AgClO4