Oxyanion

An oxyanion or oxoanion is a chemical compound

Chemical compound

A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 with the generic formula A_xO_y^z− (where A represents a chemical element

Chemical element

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

 and O represents an oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 atom). Oxoanions are formed by a large majority of the chemical element

Chemical element

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

s. The formulae of simple oxoanions are determined by the octet rule

Octet rule

The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low (The octet rule is a chemical rule of thumb that states that atoms of low (...

. The structures of condensed oxoanions can be rationalized in terms of AO_n polyhedral units with sharing of corners or edges between polyhedra. The phosphate and polyphosphate esters AMP

Adenosine monophosphate

Adenosine monophosphate , also known as 5'-adenylic acid, is a nucleotide that is used as a monomer in RNA. It is an ester of phosphoric acid and the nucleoside adenosine. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine...

, ADP

Adenosine diphosphate

Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....

 and ATP

Adenosine triphosphate

Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 are important in biology.

Monomeric oxoanions

The formula of monomeric oxoanions, AO_n^m−, is dictated by the oxidation state

Oxidation state

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

 of the element A and its position in the periodic table

Periodic table

The periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...

. Elements of the first row are limited to a maximum coordination number of 4. However, none of the first row elements has a monomeric oxoanion with that coordination number. Instead, carbonate

Carbonate

In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

 (CO₃²⁻) and nitrate

Nitrate

The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...

 (NO₃⁻) have a trigonal planar

Trigonal planar

In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of a triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°. Such species belong to...

 structure with π bonding between the central atom and the oxygen atoms. This π bonding is favoured by the similarity in size of the central atom and oxygen.

The oxoanions of second-row elements in the group oxidation state are tetrahedral

Tetrahedral molecular geometry

In a tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1 ≈ 109.5° when all four substituents are the same, as in CH4. This molecular geometry is common throughout the first...

. Tetrahedral SiO₄ units are found in olivine

Olivine

The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

 minerals, [Mg,Fe]SiO₄, but the anion does not have a separate existence as the oxygen atoms are surrounded tetrahedrally by cations in the solid state. Phosphate

Phosphate

A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...

 (PO₄³⁻), sulfate

Sulfate

In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...

 (SO₄²⁻), and perchlorate

Perchlorate

Perchlorates are the salts derived from perchloric acid . They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders. They are used extensively within the pyrotechnics industry, and ammonium perchlorate is also a...

 (ClO₄⁻) ions can be found as such in various salts. Many oxoanions of elements in lower oxidation state obey the octet rule

Octet rule

The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low (The octet rule is a chemical rule of thumb that states that atoms of low (...

 and this can be used to rationalize the formulae adopted. For example, chlorine(V) has two valence electrons so it can accommodate three electron pairs from bonds with oxide ions. The charge on the ion is +5 −3×2 = −1, and so the formula is ClO₃⁻. The structure of the ion is predicted by VSEPR theory to be pyramidal, with three bonding electron pairs and one lone pair. In a similar way,
the oxyanion of chlorine(III) has the formula ClO₂⁻, and is bent with two lone pairs and two bonding pairs.

Oxidation state	Name	Formula	Image
+1	The hypochlorite ion	ClO−
+3	The chlorite ion	ClO2−
+5	The chlorate ion	ClO3−
+7	The perchlorate ion	ClO4−

In the third and subsequent rows of the periodic table, 6-coordination is possible, but isolated octahedral oxoanions are not known because they would carry too high an electrical charge. Thus molybdenum(VI) does not form MoO₆⁶⁻, but forms the tetrahedral molybdate

Molybdate

In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, although the latter are only found in the solid state.The...

 anion, MoO₄²⁻. MoO₆ units are found in condensed molybdates. Fully protonated oxoanions with an octahedral structure are found in such species as Sn(OH)₆²⁻ and Sb(OH)₆⁻.

Naming

The naming of monomeric oxoanions follows some very simple rules.

If central atom is not in Group VII:

Central atom oxidation number	Naming scheme	Examples
= Group number	*-ate	Borate Borate Borates are chemical compounds which contain oxoanions of boron in oxidation state +3. The simplest borate ion, BO33−, has a trigonal planar structure. Other borates are made up of trigonal BO3 or tetrahedral BO4 structural units, sharing oxygen atoms...  (BO33−), Carbonate Carbonate In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....  (CO32−), Nitrate Nitrate The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...  (NO3−), Phosphate Phosphate A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...  (PO43−), Sulfate Sulfate In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...  (SO42−), Chromate Chromate Chromate salts contain the chromate anion, CrO42−. Dichromate salts contain the dichromate anion, Cr2O72−. They are oxyanions of chromium in the oxidation state +6. They are moderately strong oxidizing agents.- Chemical properties :...  (CrO42−), Arsenate Arsenate The arsenate ion is AsO43−.An arsenate is any compound that contains this ion. Arsenates are salts or esters of arsenic acid.The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As[V]....  (AsO43−)
= Group number − 2	*-ite	Nitrite Nitrite The nitrite ion has the chemical formula NO2−. The anion is symmetric with equal N-O bond lengths and a O-N-O bond angle of ca. 120°. On protonation the unstable weak acid nitrous acid is produced. Nitrite can be oxidised or reduced, with product somewhat dependent on the oxidizing/reducing agent...  (NO2−), Phosphite Phosphite A phosphite is a salt of phosphorous acid. The phosphite ion is a polyatomic ion with a phosphorus central atom where phosphorus has an oxidation state of +3...  (PO33−), Sulfite Sulfite Sulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...  (SO32−), Arsenite Arsenite In chemistry an arsenite is a chemical compound containing an arsenic oxoanion where arsenic has oxidation state +3.The different forms of the anion are the next ones:* ortho-arsenite: AsO33-* meta-arsenite: AsO2-...  (AsO33−)
= Group number − 4	hypo-*-ite	Hypophosphite Hypophosphite The hypophosphite ion is H2PO2−, hypophosphorous acid minus one hydrogen ion.Hypophosphites are the compounds containing this ion, with phosphorus in oxidation state +1, or similar covalent ones....  (PO23−), Hyposulfite Hyposulfite Hyposulfite, SO22−, was reported as a sulfur oxyanion. However salts containing SO22− and the corresponding acid, hyposulfurous acid are not believed to exist.However, there are also some reports about hyposulfites...  (SO22−)

If central atom is in Group VII:

Central atom oxidation number	Naming scheme	Examples
= Group number	per-*-ate	Perchlorate Perchlorate Perchlorates are the salts derived from perchloric acid . They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders. They are used extensively within the pyrotechnics industry, and ammonium perchlorate is also a...  (ClO4−), Perbromate Perbromate In chemistry, the perbromate ion is the anion having the chemical formula . It is an oxyanion of bromine, the conjugate base of perbromic acid, in which bromine has the oxidation state +7. Unlike its chlorine and iodine analogs , it is difficult to synthesize...  (BrO4−), Periodate Periodate Periodate is an anion composed of iodine and oxygen. Periodates include two forms. The metaperiodate ion is IO4−; the orthoperiodate ion is IO65−. Usually both forms are called periodates. Periodate is the conjugate base of periodic acid....  (IO4-), Permanganate Permanganate A permanganate is the general name for a chemical compound containing the manganate ion, . Because manganese is in the +7 oxidation state, the permanganate ion is a strong oxidizing agent. The ion has tetrahedral geometry...  (MnO4−)
= Group number − 2	*-ate	Chlorate Chlorate The chlorate anion has the formula ClO. In this case, the chlorine atom is in the +5 oxidation state. "Chlorate" can also refer to chemical compounds containing this anion; chlorates are the salts of chloric acid. "Chlorate", when followed by a roman numeral in parentheses, e.g...  (ClO3-), Bromate Bromate The bromate anion, BrO, is a bromine-based oxoanion. A bromate is a chemical compound that contains this ion. Examples of bromates include sodium bromate, , and potassium bromate, .Bromates are formed many different ways in municipal drinking water...  (BrO3−), Iodate Iodate An iodate is a conjugate base of iodic acid. In the iodate anion, iodine is bonded to three oxygen atoms and the molecular formula is IO3−. The molecular geometry of iodate is trigonal pyramidal....  (IO3-)
= Group number − 4	*-ite	Chlorite Chlorite The chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.-Oxidation states:...  (ClO2−), Bromite (BrO2−)
= Group number − 6	hypo-*-ite	Hypochlorite Hypochlorite The hypochlorite ion, also known as chlorate anion is ClO−. A hypochlorite compound is a chemical compound containing this group, with chlorine in oxidation state +1.Hypochlorites are the salts of hypochlorous acid...  (ClO−), Hypobromite Hypobromite The hypobromite ion, also called alkaline bromine water, is BrO−. Bromine is in the +1 oxidation state. Hypobromite is the bromine compound analogous to hypochlorites found in common bleaches, and in immune cells...  (BrO−)

Condensation reactions

In aqueous solution, oxoanions with high charge can undergo condensation reactions, such as in the formation of the dichromate ion, Cr₂O₇²⁻:

2 CrO₄²⁻ + 2 H⁺ Cr₂O₇²⁻ + H₂O

The driving force for this reaction is the reduction of electrical charge density on the anion and the elimination of the H⁺ ion

Hydronium

In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...

. The amount of order in the solution is decreased, releasing a certain amount of entropy

Entropy (order and disorder)

In thermodynamics, entropy is commonly associated with the amount of order, disorder, and/or chaos in a thermodynamic system. This stems from Rudolf Clausius' 1862 assertion that any thermodynamic processes always "admits to being reduced to the alteration in some way or another of the arrangement...

 which makes the Gibbs free energy

Gibbs free energy

In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...

 more negative and favors the forward reaction. It is an example of an acid–base reaction with the monomeric oxoanion acting as a base and the condensed oxoanion acting as its conjugate acid

Conjugate acid

Within the Brønsted–Lowry acid-base theory , a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton. A conjugate acid can also be seen as the chemical substance that releases, or donates, a proton in the forward chemical...

. The reverse reaction is an hydrolysis

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 reaction, as a water molecule, acting as a base, is split. Further condensation may occur, particularly with anions of higher charge, as occurs with adenosine phosphates.

AMP	ADP	ATP

The conversion of ATP to ADP is an hydrolysis reaction and is an important source of energy in biological systems.

The formation of most silicate

Silicate

A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

 minerals can be viewed as the result of a de-condensation reaction in which silica reacts with a basic oxide, an acid-base reaction in the Lux-Flood sense.

CaO (base) + SiO₂ (acid) → CaSiO₃

Structures and formulae of polyoxoanions

A polyoxoanion is a polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

ic oxoanion in which multiple oxoanion monomers, usually regarded as MO_n polyhedra, are joined by sharing corners or edges. When two corners of a polyhedron are shared the resulting structure may be a chain or a ring. Short chains occur, for example, in polyphosphate

Polyphosphate

Triphosphates are salts or esters of polymeric oxyanions formed from tetrahedral PO4 structural units linked together by sharing oxygen atoms. When two corners are shared the polyphosphate may have a linear chain structure or a cyclic ring structure. In biology the polyphosphate esters AMP, ADP...

s. Inosilicates, such as pyroxene

Pyroxene

The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...

s, have a long chain of SiO₄ tetrahedra each sharing two corners. The same structure occurs in so-called meta-vanadates, such as ammonium metavanadate

Ammonium metavanadate

Ammonium metavanadate, NH4VO3, is a yellow crystalline solid which is water soluble inorganic acid that acts as insulin mimic. It functions as a catalyst to certain reactions and is known to have toxic effects in certain species. It has been found in slags and fly ash from coal burning which has...

, NH₄VO₃.

The formula of the oxoanion, SiO₃²⁻ is obtained as follows: each nominal silicon ion (Si⁴⁺) is attached to two nominal oxide ions (O²⁻) and has a half share in two others. Thus the stoichiometry and charge are given by:

Stoichiometry: Si + 2 O +2×½ O = SiO₃

Charge: +4 + (2 × − 2) + (2 × (½ × −2)) = −2.

A ring can be viewed as a chain in which the two ends have been joined. Cyclic triphosphate

Phosphoric acids and Phosphates

There are various kinds of phosphoric acids and phosphates. Of the many phosphorus oxoacids, the phosphoric acids constitute the largest and most diverse group. The simplest phosphoric acid series begins with monophosphoric acid, continues with many oligophosphoric acids such as diphosphoric acid...

, P₃O₉³⁻ is an example.

When three corners are shared the structure extends into two dimensions. In amphibole

Amphibole

Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...

s, (of which asbestos

Asbestos

Asbestos is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties. They all have in common their eponymous, asbestiform habit: long, thin fibrous crystals...

 is an example) two chains are linked together by sharing of a third corner on alternate places along the chain. This results in an ideal formula [Si₄O₁₁]⁶⁻ and a linear chain structure which explains the fibrous nature of these minerals. Sharing of all three corners can result in a sheet structure, as in mica

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

, [Si₂O₅]²⁻, in which each silicon has one oxygen to itself and a half-share in three others. Crystalline mica can be cleaved into very thin sheets.

The sharing of all four corners of the tetrahedra results in a 3-dimensional structure, such as in quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

. Aluminosilicate

Aluminosilicate

Aluminosilicate minerals are minerals composed of aluminium, silicon, and oxygen, plus countercations. They are a major component of kaolin and other clay minerals....

s are minerals in which some silicon is replaced by aluminium. However, the oxidation state of aluminium is one less than that of silicon, so the replacement must be accompanied by the addition of another cation. The number of possible combinations of such a structure is very large, which is, in part, the reason why there are so many aluminosilicates.
Octahedral MO₆ units are common in oxoanions of the larger transition metals. Some compounds, such as salts of the chain-polymeric ion, Mo₂O₇²⁻ even contain both tetrahedral and octahedral units. Edge-sharing is common in ions containing octahedral building blocks and the octahedra are usually distorted to reduce the strain at the bridging oxygen atoms. This results in 3-dimensional structures called polyoxometalate

Polyoxometalate

In chemistry, a polyoxometalate is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form a large, closed 3-dimensional framework....

s. Typical examples occur in the Keggin structure

Keggin structure

Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α–Keggin anions, which have a general formula of [XM12O40]n−, where X is the heteroatom , M is the addenda atom , and O represents oxygen...

 of the phosphomolybdate

Phosphomolybdic acid

Phosphomolybdic acid, also known as dodeca molybdophosphoric acid or PMA is a component of Masson's trichrome stain. It is a yellow-green compound, freely soluble in water and polar organic solvents such as ethanol...

 ion. Edge sharing is an effective means of reducing electrical charge density, as can be seen with the hypothetical condensation reaction involving two octahedra:

2 MO₆ⁿ⁻ + 4 H⁺ → M₂O₁₀⁽ⁿ⁻⁴⁾⁻ + 2 H₂O

Here, the average charge on each M atom is reduced by 2. The efficacy of edge-sharing is demonstrated by the following reaction, which occurs when an alkaline aqueous solution of molybdate is acidified.

7 MoO₄²⁻ + 8 H⁺ Mo₇O₂₄⁶⁻ + 4 H₂O

The tetrahedral molybdate ion is converted into a cluster of 7 edge-linked octahedra giving an average charge on each molybdenum of 6/7. The heptamolybdate cluster is so stable that clusters with between 2 and 6 molybdate units have not been detected even though they must be formed as intermediates.

Heuristic for acidity

The pKa of the related acids can be guessed from the number of double bonds to oxygen. Thus perchloric acid is a very strong acid while hypochlorous acid is very weak. A simple rule usually works to within about 1 pH unit.

Acid-base properties

Most oxoanions are weak base

Base (chemistry)

For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...

s and can be protonated to give acids or acid salts. For example, the phosphate ion can be successively protonated to form phosphoric acid.

PO₄³⁻ + 3 H⁺ HPO₄²⁻ + 2 H⁺ H₂PO₄⁻ + H⁺ H₃PO₄

The extent of protonation in aqueous solution will depend on the acid dissociation constant

Acid dissociation constant

An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...

s and pH

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

. For example, AMP (adenosine monophosphate) has a pK_a value of 6.21, so at pH 7 it will be about 10% protonated. Charge neutralization is an important factor in these protonation reactions. By contrast, the univalent anions perchlorate

Perchlorate

Perchlorates are the salts derived from perchloric acid . They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders. They are used extensively within the pyrotechnics industry, and ammonium perchlorate is also a...

 and permanganate

Permanganate

A permanganate is the general name for a chemical compound containing the manganate ion, . Because manganese is in the +7 oxidation state, the permanganate ion is a strong oxidizing agent. The ion has tetrahedral geometry...

 ions are very difficult to protonate and so the corresponding acids are strong acids.

Although acids such as phosphoric acid are written as H₃PO₄, the protons are attached to oxygen atoms forming hydroxyl groups, so the formula can also be written as to better reflect the structure. Sulfuric acid may be written as ; this is the molecule observed in the gas phase.

The phosphite

Phosphite

A phosphite is a salt of phosphorous acid. The phosphite ion is a polyatomic ion with a phosphorus central atom where phosphorus has an oxidation state of +3...

 ion, PO₃³⁻, is a strong base, and so always carries at least one proton. In this case the proton is attached directly to the phosphorus atom with the structure HPO₃²⁻. In forming this ion, the phosphite ion is behaving as a Lewis base and donating a pair of electrons to the Lewis acid, H⁺.


As mentioned above, a condensation reaction is also an acid-base reaction. In many systems, both protonation and condensation reactions can occur. The case of the chromate ion provides a relatively simple example. In the predominance diagram

Predominance diagram

thumb|200px|Predominance diagram for chromateA predominance diagram purports to show the conditions of concentration and pH where a chemical species has the highest concentration in solutions in which there are multiple acid-base equilibria. The lines on a predominance diagram indicate where...

 for chromate, shown at the right, pCr stands for minus the logarithm of the chromium concentration and pH

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 stands for minus the H⁺ ion concentration. There are two independent equilibria. Equilibrium constants are defined as follows.

CrO42− + H+ HCrO4−		log K1=5.89
2 HCrO4− Cr2O72− + H2O		log K2=2.05

The predominance diagram is interpreted as follows.

• The chromate ion, CrO₄²⁻, is the predominant species at high pH. As pH rises the chromate ion becomes ever more predominant, until it is the only species in solutions with pH > 6.75
• At pH < pK₁ the hydrogen chromate ion, HCrO₄⁻ is predominant in dilute solution.
• The dichromate ion, Cr₂O₇²⁻, is predominant in more concentrated solutions, except at high pH.

The species H₂CrO₄ and HCr₂O₇⁻ are not shown as they are formed only at very low pH.

Predominance diagrams can become very complicated when many polymeric species can be formed, such as in vanadate

Vanadate

In chemistry, a vanadate is a compound containing an oxoanion of vanadium generally in its highest oxidation state of +5. The simplest vanadate ion is the tetrahedral, orthovanadate, VO43− anion, which is present in e.g. sodium orthovanadate and in solutions of V2O5 in strong base...

s, molybdate

Molybdate

In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, although the latter are only found in the solid state.The...

s, and tungstate

Tungstate

In chemistry a tungstate is a compound that contains an oxoanion of tungsten or is a mixed oxide containing tungsten. The simplest tungstate ion is WO42−, "orthotungstate"...

s. Another complication is that many of the higher polymers are formed extremely slowly, such that equilibrium may not be attained even in months, leading to possible errors in the equilibrium constants and the predominance diagram.