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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 Ea ...
(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 bet ...
, 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 the ...
. 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 sequ ...
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-toxi ...
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
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mas ...
s () from ''k''
hydroxyl group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
s –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 convent ...
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''sup>''k''−. The fully dissociated anion (''k'' = ''n''−2''x''+2) has formula ''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, J. ...
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 r ...
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 cha ...
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. Glyceride ...
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 an ...
,
mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the pr ...
,
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 Pharmacy is the science and practice of discovering, producing, preparing, dispensing, reviewing and monitoring medications, aiming to ensure the safe, effective, and affordable use of medicines. It is a miscellaneous science as it links hea ...
,
chemical industry The chemical industry comprises the companies that produce industrial chemicals. Central to the modern world economy, it converts raw materials (oil, natural gas, air, water, metals, and minerals) into more than 70,000 different products. The ...
, and chemical research.


Acids


Orthophosphoric acid

The simplest and most commonly encountered of the phosphoric acids is orthophosphoric acid, H3PO4. 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 bioch ...
s can be joined by
condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...
into larger molecules by elimination of
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
. 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-toxi ...
) 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 Ea ...
) 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 Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
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 ...
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''O3''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
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 cr ...

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 p ...
P''n''O5''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
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mas ...
s turns a phosphoric acid into a phosphate anion. Partial removal yields various hydrogen phosphate anions.


Orthophosphate

The anions of orthophosphoric acid are orthophosphate (commonly called simply "phosphate") ,
monohydrogen phosphate Monohydrogen phosphate is the inorganic ion with the formula PO4sup>2-. Its formula can also be written as O3(OH)sup>2-, which shows the presence of a O-H bond. Together with dihydrogen phosphate, monohydrogen phosphate occurs widely in natura ...
, and dihydrogen phosphate .


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 ot ...
. 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 H3P3O9. Its structure is shown in the illustration. Since the ends are condensed, its formula has one less H2O (water) than tripolyphosphoric acid. What are commonly called trimetaphosphates actually have a mixture of ring sizes. A general formula for such cyclic compounds is (HPO3)x where x = number of phosphoric units in the molecule. The hypothetical monomer monometaphosphoric acid (HPO3), 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 convent ...
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 ...
(Na6P6O18), used as a sequestrant and a
food additive Food additives are substances added to food to preserve flavor or enhance taste, appearance, or other sensory qualities. Some additives have been used for centuries as part of an effort to preserve food, for example vinegar ( pickling), salt ( ...
.


Chemical properties


Solubility

These phosphoric acids series are generally
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
-
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 solubil ...
considering the polarity of the molecules.
Ammonium The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary ...
and
alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a s ...
phosphates are also quite soluble in water. The
alkaline earth The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are all ...
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 cha ...
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 Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...
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 ...
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
detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are mor ...
s (i. e. cleaners) formulations. For example, see
Sodium tripolyphosphate Sodium triphosphate (STP), also sodium tripolyphosphate (STPP), or tripolyphosphate (TPP),
. 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 an ...
. 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 group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
s of
alcohols In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a saturated carbon atom. The term ''alcohol'' originally referred to the primary alcohol ethanol (ethyl alcohol), which is ...
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. Glyceride ...
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. Glyceride ...
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 substituen ...
*
Adenosine diphosphate Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound in metabolism and is essential to the flow of energy in living cells. ADP consists of three important structural components: a sugar backb ...
*
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 form ...
* Adenosine tetraphosphate * Nucleoside triphosphate *
Organophosphate In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure , a central phosphate molecule with alkyl or aromatic substituents. They can be considered ...
* 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)( ...
* Ribonucleoside monophosphate *
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 ro ...


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