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Phosphoric acid

Phosphoric(V) acid is a weak mineral acid with the chemical formula H3PO4.


Rust removal

Phosphoric acid may be used by direct application to rusted iron or steel tools or surfaces to convert iron oxide (rust) to a water soluble phosphate compound. It is usually available as a greenish liquid, suitable for dipping (acid bath), but is more generally used as a component in a gel, commonly called Naval jelly. As a thick gel, it may be applied to sloping, vertical, or even overhead surfaces. Care must be taken to avoid acid burns of the skin and especially the eyes, but the residue is easily diluted with water. When sufficiently diluted can even be nutritious to plant life, containing the essential nutrients phosphorus and iron. It is sometimes sold under other names, such as "rust remover" or "rust killer". It should not be directly introduced into surface water such as creeks or into drains, however. After treatment, the reddish-brown iron oxide will be converted to a black iron phosphate compound that may be scrubbed off. Multiple applications may be required to remove all rust. The resultant black compound does not provide any further corrosion resistance (such protection is somewhat provided by the superficially similar Parkerizing process). After application and removal of rust using phosphoric acid compounds the metal should be oiled (if to be used bare, as in a tool) or appropriately painted, most durably by using a multiple coat process of primer, intermediate, and finish coats.

Processed food use

It is also used to acidify foods and beverages such as various colas, but not without controversy as to its health effects. It provides a tangy taste, and being an agro-industrial chemical, is available cheaply and in large quantities (much like common salt, also used in excess in many processed foods). The low cost and bulk availability is unlike more expensive and even healthful natural seasonings that give comparable flavors, such as ginger for tangyness, or citric acid for sourness, readily obtainable from lemons and limes.

Effects on bone calcium

Phosphoric acid, used in many soft drinks (primarily so in cola drinks, such as the U.S. market dominating Coca Cola and Pepsi Cola), has become suspect in the increase of the bone deficiency disease osteoporosis in young women. Traditionally, osteoporosis (literally "porous bones") has been seen mostly in post-menopausal women, particularly those who did not build high bone density in youth, typically due to insufficient calcium intake. An excess of phosphorus may lead to poor bone density, however. Nutritionists point out that the body will attempt to maintain a balance between ions of phosphorus and calcium in the blood. When an excess of phosphorus (through phosphoric acid, for example) is introduced, the body's chemical balance mechanisms will attempt to maintain the proper calcium-phosphorus ionic ratio by extracting calcium from the bones. The excess phosphorus and calcium are eventually excreted. Other chemicals such as caffeine (also a significant component of popular common cola drinks) are also suspect as possible contributors to low bone density, which is now seen in increasing prevalence in men of late middle age.

Medical use

Another specialized use is the provision of local anesthesia. Phosphoric acid is used in dentistry and orthodontics as an etching solution, to clean and roughen the surfaces of teeth where dental appliances or fillings will be placed.

Preparation of phosphoric acid

Phosphoric acid is prepared by adding sulphuric acid to calcium phosphate rock. In its anhydrous form the acid is a white solid. While phosphoric acid has the potential to release three hydrogen ions, in aqueous solution only two are available due to the fact that PO4-3 is a stronger base than hydroxide.

Chemical properties

The anion associated with phosphoric acid is called the phosphate ion, and is found pervasively in biology, especially in the compounds derived from phosphorylated sugars, such as DNA and RNA and adenosine triphosphate (e.g. ATP).

Other applications

Used as the electrolyte in Phosphoric-acid fuel cells.

Last updated: 10-16-2005 00:08:04
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