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ACIDS > Phosphoric Acid

Phosphoric acid is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way.

Formula: H3PO4, Density: 1.88 g/cm³, Molar mass: 98 g/mol, Melting point: 42.35 °C, Boiling point: 158 °C, IUPAC ID: trihydroxidooxidophosphorus phosphoric acid, Soluble in: Water

Uses :

Food additive

Food-grade phosphoric acid (additive E338) is used to acidify foods and beverages such as various colas, but not without controversy regarding its health effects.  It provides a tangy or sour taste, and being a mass-produced chemical is available cheaply and in large quantities. The low cost and bulk availability is unlike more expensive seasonings that give comparable flavors, such as citric acid which is obtainable from citrus, but usually fermented by Aspergillus niger mold from scrap molasses, waste starch hydrolysates and phosphoric acid. Various phosphates, e.g., monocalcium phosphate, are used as leavening agents

Niche uses

Phosphoric acid and its derivatives are pervasive and find many niche applications.

Rust removal

Phosphoric acid may be used as a "rust converter", by direct application to rusted iron, steel tools, or surfaces. The phosphoric acid converts reddish-brown iron(III) oxide, Fe2O3 (rust) to black ferric phosphate, FePO4. An empirical formula for this reaction is:

    2 H_3PO_4 + Fe_2 O_3 longrightarrow 2 FePO_4 + 3 H_2 O

"Rust converter" is sometimes a greenish liquid suitable for dipping (in the same sort of acid bath as is used for pickling metal), but it is more often formulated as a gel, commonly called "naval jelly". It is sometimes sold under other names, such as "rust remover" or "rust killer". As a thick gel, it may be applied to sloping, vertical, or even overhead surfaces.

After treatment, the black ferric phosphate coating can be scrubbed off, leaving a fresh metal surface. Multiple applications of phosphoric acid may be required to remove all rust. The black phosphate coating can also be left in place, where it will provide moderate further corrosion resistance (such protection is also provided by the superficially similar Parkerizing and blued electrochemical conversion coating processes).

In medicine

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 that also contain high levels of sugar (glucose and fructose). This acid is also used in many teeth whiteners to eliminate plaque that may be on the teeth before application.