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Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is a mineral acid with the chemical formula H3PO4. Alternatively, orthophosphoric acid molecules can combine with themselves to form a variety of compounds referred to as phosphoric acids in a more general way. For a discussion of these, see Phosphoric acids and Phosphates. This article covers Orthophosphoric acid.
Phosphoric acid may be used by direct application to rusted iron or steel tools or surfaces to convert iron(III) 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 it 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 coating that may be scrubbed off. Multiple applications may be required to remove all rust. The resultant black compound can provide further corrosion resistance (such protection is somewhat provided by the superficially similar Parkerizing and blued electrochemical conversion coating processes.) 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. The low cost and bulk availability is unlike more expensive natural seasonings that give comparable flavors, such as ginger for tangyness, or citric acid for sourness, obtainable from lemons and limes.
Biological effects on bone calcium
Template:Disputed-section 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.Template:Fact 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.Template:Fact Other chemicals such as caffeine (also a significant component of popular common cola drinks) are also suspected as possible contributors to low bone density, which is now seen in increasing prevalence in men of late middle age.Template:Fact.
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. Phosphoric acid is also an ingredient in over the counter anti-nausea medications which also contain high levels of sugar (glucose and fructose). It should not be used by diabetics without consultation with a doctor.
Preparation of phosphoric acid
There are two distinct kinds of phosphoric acid: Thermal phosphoric acid and Wet phosphoric acid.
Thermal phosphoric acid This very pure phosphoric acid is obtained by burning elemental phosphorus and reacting the product with water. This is the cleanest way of producing phosphoric acid, since most impurities present in the rock have been removed when extracting Phosphorus from the rock in a furnace. The end result is food grade, thermal phosphoric acid.
Wet phosphoric acid Green phosphoric acid is prepared by adding sulfuric acid to calcium phosphate rock. While phosphoric acid has the potential to release three hydrogen ions, in aqueous solution the third requires a high pH because PO43− is almost as strong a base as hydroxide ion.
Through modern filtering techniques the wet process acid can be cleaned up significantly but still isn't as pure as thermal phosphoric acid.
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).
Phosphoric acid is also used as a cleaner by construction trades to remove mineral deposits, cementitous smears, and hard water stains.
- International Chemical Safety Card 1008
- National Pollutant Inventory - Phosphoric acid fact sheet
- NIOSH Pocket Guide to Chemical Hazards