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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 also used as an oxidizer in rocket fuel and can be found in airbags, fireworks, and Chilean fertilizers. Both potassium perchlorate (KClO4) and ammonium perchlorate (NH4ClO4) are used extensively within the pyrotechnics industry, whereas Ammonium perchlorate is 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, giving strongly oxidizing solutions.

Scientific definition

The perchlorate ion is ClO4−.

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

Strength of oxidation

The perchlorate ion is the weakest oxidizer of the generalized chlorates. Lower oxidation numbers are progressively stronger oxidizers, and less stable.

Stability

Most perchlorates, especially salts of electropositive metals such as sodium perchlorate, are slow to react unless heated, the perchlorate ion being largely inert and not oxidizing at lower temperatures. This property is useful in many applications, such as flares, where one obviously does not intend the device to explode, or even catch fire spontaneously.

Similarly, the successful use of ammonium perchlorate in solid rocket engines suggests that its propensity to explode may occasionally be slightly exagerrated.

Mixtures of perchlorates with organic compounds should be treated with more caution; although generally they do not catch fire or explode unless heated, this should not be relied on in any specific case.

Perchlorate in the environment

Low levels of perchlorate have been detected in both drinking water and groundwater in 35 states in the US according to the Environmental Protection Agency. In 2004, the chemical was also found in cow's milk in the area 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 1 µg/L.

In some places it is being detected because of contamination from industrial sites that use or manufacture perchlorate. 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.

As of 2006, the EPA has not yet determined whether perchlorate is present at sufficient levels in the environment to require a nationwide regulation on how much should be allowed in drinking water. In 2005, U.S. 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 which 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.

Perchlorate's health effects

At high levels, above 0.007 milligrams per kilogram per day (mg/kg-d) according to the NAS, perchlorate can temporarily and reversibly inhibit the thyroid gland’s ability to absorb iodine from the bloodstream ("iodide uptake inhibition"). This dose is conventionally converted into a "drinking water equivalent level" of 245 ppb by assuming a person weighs 70 kilograms (154 pounds) and consumes 2 liters (68 ounces) of drinking water per day over a lifetime. This conversion overstates typical exposure because the numerator in the ratio (weight) is below average adult body weight and the denominator in the ratio (daily drinking water consumption) is above the average amount of drinking water adults consume.

Drinking water in the U.S. contains no more than a few ppb perchlorate--typically less than 10 ppb and rarely as much as 25 ppb. At this upper-end concentration, a person could consume 20 liters of drinking water per day before beginning to experience iodide uptake inhibition. Of course, drinking 20 liters of water (5.3 gallons!) during one day almost certainly would be fatal.

While the thyroid uses iodine to produce hormones, NAS says this process of iodide uptake inhibition is not an "adverse," or harmful, affect. There has been some speculation that exposure to extremely high doses of perchlorate, for several months or years could lead to hypothyroidism, but NAS found that iodide uptake inhibition was the only consistently documented health effect of perchlorate in humans.

The NAS also found that perchlorate only affects the thyroid gland. There is no evidence that it causes brain damage, birth defects or cancer in humans. It is also 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 perchlorates 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. (The ATA press release can be viewed at http://www.thyroid.org/professionals/publications/news/04_10_01_perchlorate.html.)

Despite having concluded that iodide uptake inhibition is not harmful and that it does not occur at exposures below 245 ppb, the NAS divided by a safety factor of 10 to derive a recommended “reference dose” of 0.0007 mg/kg-d and declared this would be protective of even the most sensitive subpopulations. Using the 70 kg body weight and 2 liter/day assumptions used above, this dose is conventionally converted to 25 ppb in drinking water. For that reason, most media reports call this the "safe" level of exposure. (Some reports use 24.5 ppb, but this level of precision is meaningless given the 10-fold safety factor and the fact that iodide uptake inhibition itself is not harmful.) 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.

Types of perchlorates

See Category:Perchlorates for a bigger list.

References

External links

Oxoanions | Perchlorates

Perchlorate | Percloratos | Perchlorate

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Perchlorate".

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