A phosphoric acid, in the general sense, is a
phosphorus oxoacid in which each
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
(P) atom is in the
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
+5, and is
bonded to four
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
(O) atoms, one of them through a
double bond
In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...
, arranged as the corners of a
tetrahedron
In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all th ...
. Two or more of these tetrahedra may be connected by shared single-bonded oxygens, forming linear or branched chains, cycles, or more complex structures. The single-bonded oxygen atoms that are not shared are completed with
acidic
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a ...
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
atoms. The general formula of a phosphoric acid is , where ''n'' is the number of phosphorus atoms and ''x'' is the number of
fundamental cycles in the molecule's structure, between 0 and (''n''+2)/2.
Removal of
protons () from ''k''
hydroxyl groups –OH leaves
anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s generically called phosphates (if ''k'' = ''n''−2''x''+2) or hydrogen phosphates (if ''k'' is between 1 and ''n''−2''x''+1), with general formula
''n''−2''x''+2−''k''P''n''O3''n''+1−''x''">''n''−2''x''+2−''k''P''n''O3''n''+1−''x''sup>''k''−. The fully dissociated anion (''k'' = ''n''−2''x''+2) has formula
''n''O3''n''−''x''+1">''n''O3''n''−''x''+1sup>(''n''−2''x''+2)− is The term is also used in
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...
for the
functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the re ...
s that result when or more of the hydrogens are replaced by bonds to other groups.
These acids, together with their
salts
In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...
and
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...
s, include some of the best-known compounds of phosphorus, of high importance in
biochemistry
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
,
mineralogy,
agriculture
Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people t ...
,
pharmacy,
chemical industry, and
chemical research.
Acids
Orthophosphoric acid
The simplest and most commonly encountered of the phosphoric acids is
orthophosphoric acid
Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...
, H
3PO
4. Indeed, the term phosphoric acid often means this compound specifically (and this is also the current
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
nomenclature).
Oligophosphoric and polyphosphoric acids
Two or more orthophosphoric acid
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
s can be joined by
condensation into larger molecules by elimination of
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
. Condensation of a few units yields the oligophosphoric acids, while larger molecules are called polyphosphoric acids. (However, the distinction between the two terms is not well defined.)
For example,
pyrophosphoric,
triphosphoric and
tetraphosphoric acids can be obtained by the reactions
: 2 → +
: + → +
: + → +
The "backbone" of a polyphosphoric acid molecule is a chain of alternating P and O atoms. Each extra orthophosphoric unit that is condensed adds 1 extra H (
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
) atom, 1 extra P (
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
) atom, and 3 extra O (
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
) atoms. The general formula of a polyphosphoric acid is or HO(–P(O)(OH)–O–)
''n''H.
Polyphosphoric acids are used in
organic synthesis for
cyclization
A cyclic compound (or ring compound) is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where ...
s and
acylation
In chemistry, acylation (or alkanoylation) is the chemical reaction in which an acyl group () is added to a compound. The compound providing the acyl group is called the acylating agent.
Because they form a strong electrophile when treated with ...
s; an alternative is
Eaton's reagent.
Cyclic phosphoric acids
Condensation between two –OH units of the same molecule, on the other hand, eliminates two hydrogen atoms and one oxygen atom, creating a cycle, as in the formation of
trimetaphosphoric acid:
: → +
The general formula of a phosphoric acid is H
''n''−2''x''+2P
''n''O
3''n''−''x''+1, where ''n'' is the number of phosphorus atoms and ''x'' is the number of
fundamental cycles in the molecule's structure; that is, the minimum number of bonds that would have to be broken to eliminate all cycles.
File:Phosphoric-acid-3D-vdW.png, Orthophosphoric acid
Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...
H3PO4
File:Pyrophosphoric-acid-3D-vdW.png, Pyrophosphoric acid
Pyrophosphoric acid, also known as diphosphoric acid, is the inorganic compound with the formula H4P2O7 or, more descriptively, HO)2P(O)sub>2O. Colorless and odorless, it is soluble in water, diethyl ether, and ethyl alcohol. The anhydrous acid c ...
File:Tripolyphosphoric-acid-3D-vdW.png, Tripolyphosphoric acid
File:Tetrapolyphosphoric-acid-3D-vdW.png, Tetrapolyphosphoric acid
File:Trimetaphosphoric-acid-3D-vdW.png, Trimetaphosphoric acid
H3P3O9
File:Phosphorus-pentoxide-3D-vdW.png, Phosphoric anhydride
P4O10
The limiting case of internal condensation, where all oxygen atoms are shared and there are no hydrogen atoms (''x'' = (''n''+2)/2) would be an
anhydride
An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the pa ...
P
''n''O
5''n''/2, such as
phosphorus pentoxide
Phosphorus pentoxide is a chemical compound with molecular formula P4 O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydra ...
.
Phosphates
Removal of the hydrogen atoms as
protons turns a phosphoric acid into a phosphate anion. Partial removal yields various hydrogen phosphate anions.
Orthophosphate
The anions of orthophosphoric acid are
orthophosphate
A phosphoric acid, in the general sense, is a phosphorus oxoacid in which each phosphorus (P) atom is in the oxidation state +5, and is bonded to four oxygen (O) atoms, one of them through a double bond, arranged as the corners of a tetrahedron. ...
(commonly called simply "phosphate") ,
monohydrogen phosphate , and
dihydrogen phosphate
Dihydrogen phosphate or dihydrogenphosphate ion is an inorganic ion with the formula 2PO4sup>−. Phosphates occur widely in natural systems.
These sodium phosphates are artificially used in food processing and packaging as emulsifying agents, ...
.
Linear oligophosphates and polyphosphates
Dissociation of pyrophosphoric acid generates four anions,
''k''−, where the charge ''k'' ranges from 1 to 4. The last one is
pyrophosphate
In chemistry, pyrophosphates are phosphorus oxyanions that contain two phosphorus atoms in a P–O–P linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate (Na2H2P2O7) and tetrasodium pyrophosphate (Na4P2O7), among othe ...
. The pyrophosphates are mostly water-soluble.
Likewise, tripolyphosphoric acid yields at least five anions
''k''−, where ''k'' ranges from 1 to 5, including
tripolyphosphate . Tetrapolyphosphoric acid yields at least six anions, including tetrapolyphosphate , and so on. Note that each extra phosphoric unit adds one extra P atom, three extra O atoms, and either one extra hydrogen atom or an extra negative charge.
Branched polyphosphoric acids give similarly branched polyphosphate anions. The simplest example of this is
triphosphono phosphate 9− and its partially dissociated versions.
The general formula for such (non-cyclic) polyphosphate anions, linear or branched, is
''k''−, where the charge ''k'' may vary from 1 to ''n''+2. Generally in an aqueous solution, the degree or percentage of dissociation depends on the
pH of the solution.
Cyclic polyphosphates
The phosphoric acid units can be bonded together in rings (cyclic structures) forming metaphosphoric acid molecules. The simplest such compound is trimetaphosphoric acid or cyclo-triphosphoric acid having the formula H
3P
3O
9. Its structure is shown in the illustration. Since the ends are condensed, its formula has one less H
2O (water) than tripolyphosphoric acid. What are commonly called trimetaphosphates actually have a mixture of ring sizes. A general formula for such cyclic compounds is (HPO
3)
x where x = number of phosphoric units in the molecule. The hypothetical monomer monometaphosphoric acid (HPO
3), which would be
valence isoelectronic with
nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
, is not known to exist.
When these metaphosphoric acids lose their hydrogens as H
+, cyclic
anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s called metaphosphates are formed. An example of a compound with such an anion is
sodium hexametaphosphate
Sodium hexametaphosphate (SHMP) is a salt of composition Na6 PO3)6 Sodium hexametaphosphate of commerce is typically a mixture of metaphosphates (empirical formula: NaPO3), of which the hexamer is one, and is usually the compound referred to by t ...
(Na
6P
6O
18), used as a
sequestrant
A sequestrant is a food additive which improves the quality and stability of foods. A sequestrant forms chelate complexes with polyvalent metal ions, especially copper, iron and nickel, which can prevent the oxidation of the fats in the food. Se ...
and a
food additive.
Chemical properties
Solubility
These phosphoric acids series are generally
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
-
soluble
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubi ...
considering the
polarity of the molecules.
Ammonium and
alkali phosphates are also quite soluble in water. The
alkaline earth salts
In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...
start becoming less soluble and phosphate salts of various other metals are even less soluble.
Hydrolysis and condensation
In aqueous solutions (solutions of water), water gradually (over the course of hours)
hydrolyzes polyphosphates into smaller phosphates and finally into ortho-phosphate, given enough water. Higher temperature or acidic conditions can speed up the hydrolysis
reaction
Reaction may refer to a process or to a response to an action, event, or exposure:
Physics and chemistry
*Chemical reaction
*Nuclear reaction
* Reaction (physics), as defined by Newton's third law
*Chain reaction (disambiguation).
Biology and m ...
s considerably.
Conversely, polyphosphoric acids or polyphosphates are often formed by dehydrating a phosphoric acid solution; in other words, removing water from it often by heating and evaporating the water off.
Uses
Ortho-, pyro-, and tripolyphosphate compounds have been commonly used in
detergents (i. e. cleaners) formulations. For example, see
. Sometimes pyrophosphate, tripolyphosphate, tetrapolyphosphate, etc. are called diphosphate, triphosphate, tetraphosphate, etc., especially when they are part of
phosphate esters in
biochemistry
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
. They are also used for scale and
corrosion control by
potable water providers. As a corrosion inhibitor, polyphosphates work by forming a protective film on the interior surface of pipes.
Phosphate esters
The -OH groups in phosphoric acids can also condense with the
hydroxyl groups of
alcohols to form
phosphate esters. Since orthophosphoric acid has three -OH groups, it can esterify with one, two, or three alcohol molecules to form a mono-, di-, or triester. See the general structure image of an ortho- (or mono-) phosphate
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...
below on the left, where any of the R groups can be a hydrogen or an
organic radical. Di- and tripoly- (or tri-) phosphate esters, etc. are also possible. Any -OH groups on the phosphates in these ester molecules may lose H
+ ions to form anions, again depending on the pH in a solution. In the biochemistry of living organisms, there are many kinds of (mono)phosphate, diphosphate, and triphosphate compounds (essentially
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...
s), many of which play a significant role in metabolism such as
adenosine diphosphate (ADP) and
triphosphate (ATP).
See also
*
Adenosine monophosphate
Adenosine monophosphate (AMP), also known as 5'-adenylic acid, is a nucleotide. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine; it is an ester of phosphoric acid and the nucleoside adenosine. As a substit ...
*
Adenosine diphosphate
*
Adenosine triphosphate
Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms o ...
*
Adenosine tetraphosphate
*
Nucleoside triphosphate
*
Organophosphate
*
Phosphonic acid
*
Phosphoramidate
Phosphoramidates (sometimes also called amidophosphates) are a class of phosphorus compounds structurally related to phosphates (or organophosphates) via the substitution of an OR for a NR2. They are derivatives of phosphoramidic acids O=P(OH)(NR2 ...
*
Ribonucleoside monophosphate
In biochemistry, a ribonucleotide is a nucleotide containing ribose as its pentose component. It is considered a molecular precursor of nucleic acids. Nucleotides are the basic building blocks of DNA and RNA. Ribonucleotides themselves are basic ...
*
Superphosphate Triple superphosphate is a component of fertilizer that primarily consists of monocalcium phosphate, Ca(H2PO4)2. Triple superphosphate is obtained by treating phosphate rock with phosphoric acid. Traditional routes for extraction of phosphate roc ...
References
Further reading
*
External links
Determination of Polyphosphates Using Ion Chromatography with Suppressed Conductivity Detection, Application Note 71 by Dionex*
{{Phosphorus compounds
Dietary minerals
Inorganic compounds
Phosphates
Pyrophosphates
Reagents for organic chemistry