Phosphoric acid is a phosphorus-containing inorganic acid made up of phosphorus,
oxygen, and hydrogen. In a broardened term, it includes the monomeric
(orthophosphoric acid), dimeric (pyrophosphoric acid), and polymeric
(metaphosphoric acid) forms of the acid. Phosphoric acid commonly indicates the
monomeric form orthophosphoric acid. On heating to about 225 C, it
dehydrates to form pyrophosphoric acid and to metaphosphoric acid (2HPO3
= P2O5.H2O) at higher
temperatures. Pyrophosphates are salts of pyrophosphoric acid and metaphosphates
are salts of metaphosphoric acid. Two molecules of orthophosphoric acid are formed when
three molecules of water are added to one
molecule of phosphorus pentoxide (2H3PO4
= P2O5.3H2O). The pyrophosphates are formed by the loss of 1
molecule of water from 2 moles of an orthophosphate. Pure orthophosphoric acid
is a crystalline solid; melting point 42 C; soluble in alcohol, and very soluble
in water. Phosphoric Acid
violently polymerizes under the influence of azo
compounds, epoxides and other polymerizable compounds. It forms toxic fumes of
phosphorous oxides when combusted. It is a medium
strong acid and attacks metals to release flammable hydrogen gas. Decomposition
may occur on contacting with alcohols, aldehydes, cyanides, ketones, phenols, esters,
sulfides, halogenated organics compounds.
Phosphoric acid is essential in the body organism as the constituent
of bones and teeth as well as in many metabolic process of carbohydrate, fat and
protein. Phosphoric acid is abundant in natural foods as the form of free
phosphoric acid itself or as the mineral salts (potassium, sodium or calcium).
Phosphoric acid is used to acidify foods and beverages. But the continuous and
excessive absorption of beverages particularly Coca Cola and Pepsi Cola which
contain large amount of phosphoric acid should be limited. Phosphate excretion
takes place in the form of calcium phosphate. The excessive amounts of
phosphoric acid in the body may cause calcium deficiency which causes poor teeth
and weak bone density (osteoporosis). Phosphoric acid is used in pharmaceutical
preparations as a solvent and as a gastric acidifier orally. Phosphoric acid is
important raw material in industrial field. It is a tribasic acid (having three replaceable hydrogen
atoms) which can
forms phosphates with either one, two, or all three of the hydrogen by
replacing with some other positive ion. The structure of the phosphate anion consists of phosphorus at the center of a tetrahedron defined by
a solid bond between one oxygen atom and phosphorus and single bonds for the
other oxygen atoms. In this very formal depiction, negative charge is localized on the O atoms of
the single P-O bonds. It is used in making fertilizers,
electrolytes, electroplating and derusting solutions. It is used in the
manufacture of industrial cleaning products, other inorganic and organic
phosphoric chemicals, foundry resins, paints, enamels and refractory, antifreeze
productions, and textile process materials. It is used in water treatment. Food
grade phosphoric acid is used; as a acidulation in soft drink (particularly
cola); ph control in imitation jellies; nutrient in production of yeast;
bacteria growth control in selected processed foods; flocculating agent for
clarification of sugar juices after liming process.
- Phosphorous acid: a
diprotic acid which contains one hydrogen bonded directly to the central
phosphorus atom and two hydrogens bonded to oxygen
- Phosphite:any salt, ester
or anion of phosphorous acid
- Phosphate: any salt, ester or anion of
phosphoric acid
- Phosphide:
any binary compound of phosphorus with another
element or radical
- Phosphine: binary compound of phosphorus with hydrogen or
organic compounds derived from this
One of the important phosphates
is calcium phosphate, Ca3(PO4)2.
The most important phosphorus fertilizer is monobasic
calcium phosphate, called superphosphate[Ca(H2PO4)2],
derived by treating rock phosphate with sulfuric acid
(or phosphoric acid). Superphosphate is soluble
in water and acts as a carrier of phosphorus needed
by plants. Dibasic calcium phosphate (CaHPO4)
is used in pharmaceuticals, animal feeds, and toothpastes.
Tribasic calcium phosphate, [Ca3(PO4)2]
is used as a fertilizer and as a plastics stabilizer. Other
important inorganic phosphates include ammonium phosphate
[(NH4)2HPO4] used as
a fertilizer and fire retardant; trisodium phosphate
used in detergents and for softening water; mono and
disodium phosphate used for dispersion, sequestration,
scale inhibition and preparing baking powders. The salt
forms of phosphate polymers is used as a sequestering
agent. As phosphate polymers themselves are hydrated
in water at high temperature or high pH, and thereby
revert to a more simple and stable phosphate form, which
can no longer sequester metal ions. Industrial
phosphates are used in many applications including
dispersion, sequestration and scale inhibition.
Detergent application is important also, with
products such as sodium tripolyphosphate
being key ingredients in detergents. Phosphates
are used in a wide range of food products
to perform several functions like retention
of natural fluids in the muscle that would otherwise
be lost in the aging, cooking, or freeze-thaw
processes. Polyphosphates also act in solubilization
of myofibrillar proteins that aid in the binding
of meat particles. Their presence results in
stabilised texture, flavour, and colour.
Mono and dibasic phosphate esters produced by the reaction of either alcohols,
alcohol ethoxylates or phenyl ethoxylates with polyphosphoric acid or
phosphorous pentoxide are anionic surfactants which have the main feature of the
stability in alkaline conditions over other surfactants. They have a wide range
of properties including outstanding wetting, emulsification, lubrication,
coupling activity and detergency. They exhibit the properties of anti-wear and
corrosion inhibition either as free esters or in the forms of metal and amine
salts. They are used in:
- Heavy
duty alkaline cleaning products
- Laundry
Detergents
- Emulsion polymerisation
- Textile auxiliaries
- Acid
cleaners
- Metal
working fluids
- Emulsifiers
- Wetters
- Dispersants
- Water based lubricants
- Intermediate for
the synthesis of other anionic surfactants
Phosphoric acid
alkyl esters are also used as alkylation agent for nitrogen heterocyclic compounds
and as catalysts to produce phenolic and urea resins. They are used flame
retarding plasticizers for cellulose esters, lacquers, plastic and vinyl resins
as well as as dispersing agents in plastisols. They are used as solvents in
liquid-liquid extractants or separation agent of metals. They are is used as
heat exchange media and as pigment grinding assistants and antifoam agents.
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