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Perchloric acid is a strong acid that is explosive when concentrated. It is a common reagent in chemical laboratories, where it requires a special fume hood and training to use. The uses of perchloric acid range from etching liquid crystal displays and oxide layers to digesting organic matter. Its primary use is as an oxidizing agent to catalyze chemical reactions, or to cause explosions.
The perchloric acid chemical formula is HClO4, making it an oxoacid of chlorine — an acid that contains oxygen. Commercially, perchloric acid is a liquid sold at 70-72% strength. At this strength, it is highly caustic. At room temperature, it can cause severe burns to the skin, eyes, and mucous membranes.
A derivative of perchloric acid is perchlorate, which contains ClO4-. Perchlorate can be found naturally or be manufactured, and has been found on Mars. Several perchlorate salts are used industrially. These include sodium perchlorate, known chemically as NaClO4, and ammonium perchlorate, also known as NH4ClO4, among others. The salts are solid at room temperature and are generally soluble in water. They are hygroscopic, meaning they have a tendency to absorb water from the atmosphere.
The most prominent use of the salts is that of ammonium perchlorate as an oxidizer for rocket propellants. This involves the transfer of oxygen atoms from the ammonium perchlorate to the fuel. Such oxidation can generate combustion that continues without external ignition. Its uses in rockets have ranged from the U.S. Space Shuttle and military rockets, to fireworks displays.
Originally, sodium perchlorate was used as the main oxidizer for rocket propellants. Ammonium perchlorate was found to be superior, because it is less hygroscopic. The salts of perchlorate can also be explosive if mixed with organic compounds, but they are more stable than the compounds that used to found in fireworks. There was, however, a large and lethal explosion in the United States, in which an ammonium perchlorate plant was destroyed in 1988. This is known as the PEPCON disaster.
Perchlorate is used in many countries to treat hyperthyroidism, the over-production of thyroid hormone. It can be used in this manner because it inhibits the uptake of iodide into the thyroid gland. This is an issue of concern, since perchlorate has been found in the drinking water of over four percent of the public water systems in the United States. It has also been found in cow’s milk and in several brands of infant formula in the United States.
The build-up of perchlorate in drinking water and groundwater can occur naturally, from industrial contamination, or from fireworks displays over lakes. The United States government is considering regulating perchlorate levels. Several states have their own regulations. One can remove perchlorate from drinking water with a home reverse-osmosis water treatment system.
Not all perchlorate salts are hygroscopic. Potassium perchlorate is not hygroscopic, which is one of the reasons it is often used in pyrotechnics.
Also, 'hygroscopic' materials don't necessarily have a tendency to absorb water from the atmosphere. Some hygroscopic materials don't absorb, but instead adsorb water from the atmosphere. --BGriffin