In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, an amphoteric
compound () is a
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 b ...
or
ion that can react both as an
acid
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 se ...
and as a
base. What exactly this can mean depends on
which definitions of acids and bases are being used.
One type of amphoteric species are amphiprotic molecules, which can either
donate
A donation is a gift for Charity (practice), charity, humanitarian aid, or to benefit a cause. A donation may take various forms, including money, alms, Service (economics), services, or goods such as clothing, toys, food, or vehicles. A donat ...
or
accept
Accept may refer to:
* Acceptance, a person's assent to the reality of a situation etc.
* Accept (band), a German heavy metal band
** ''Accept'' (Accept album), their debut album from 1979
* ''Accept'' (Chicken Shack album), 1970
* ACCEPT (org ...
a
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 mass ...
(). This is what "amphoteric" means in
Brønsted–Lowry acid–base theory
The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction theory which was proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. The fundamental concept of this theory ...
. Examples include
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
s and
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s, which have
amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent ...
() and
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxyli ...
() groups, and
self-ionizable compounds such as
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 ...
.
Ampholytes are amphoteric molecules that contain both acidic and basic
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. For example, an amino acid has both a basic group and an acidic group , and exists as several structures in
chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which both the reactants and products are present in concentrations which have no further tendency to change with time, so that there is no observable change in the properties of the ...
:
:
H2N-RCH-CO2H + H2O<=> H2N-RCH-COO- + H3O+<=> H3N+-RCH-COOH + OH-<=> H3N+-RCH-COO- + H2O
In approximately neutral aqueous solution (pH ≅ 7), the basic amino group is mostly protonated and the carboxylic acid is mostly deprotonated, so that the predominant species is the
zwitterion . The pH at which the average charge is zero is known as the molecule's ''
isoelectric point
The isoelectric point (pI, pH(I), IEP), is the pH at which a molecule carries no net electrical charge or is electrically neutral in the statistical mean. The standard nomenclature to represent the isoelectric point is pH(I). However, pI is also ...
''. Ampholytes are used to establish a stable pH gradient for use in
isoelectric focusing.
Metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. Many metals (such as
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
,
tin,
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...
,
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
, and
beryllium
Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to for ...
) form amphoteric oxides or hydroxides.
Aluminium oxide () is an example of an amphoteric oxide. Amphoterism depends on the
oxidation states of the oxide. Amphoteric oxides include
lead (II) oxide and
zinc (II) oxide, among many others.
Etymology
Amphoteric is derived from the Greek word () meaning "both". Related words in acid-base chemistry are amphichromatic and amphichroic, both describing substances such as
acid-base indicators
A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hence ...
which give one colour on reaction with an acid and another colour on reaction with a base.
Amphiprotic molecules
According to the
Brønsted-Lowry theory of acids and bases, acids are proton donors and bases are proton acceptors. An amphiprotic molecule (or ion) can either donate or accept a
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 mass ...
, thus acting either as an
acid
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 se ...
or a
base.
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 ...
,
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
s,
hydrogencarbonate ion (or bicarbonate ion) ,
dihydrogen phosphate ion , and
hydrogensulfate ion (or bisulfate ion) are common examples of amphiprotic species. Since they can donate a proton, all amphiprotic substances contain a hydrogen atom. Also, since they can act like an acid or a base, they are amphoteric.
Examples
The water molecule is amphoteric in aqueous solution. It can either gain a proton to form a
hydronium
In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid ...
ion , or else lose a proton to form a
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. ...
ion .
Another possibility is the
molecular autoionization
In chemistry, molecular autoionization (or self-ionization) is a chemical reaction between molecules of the same substance to produce ions. If a pure liquid partially dissociates into ions, it is said to be self-ionizing. In most cases the oxida ...
reaction between two water molecules, in which one water molecule acts as an acid and another as a base.
:
H2O + H2O <=> H3O+ + OH-
The
bicarbonate
In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula .
Bicarbonate serves a crucial biochemi ...
ion, , is amphoteric as it can act as either an acid or a base:
:As an acid, losing a proton:
HCO3- + OH- <=> CO3^2- + H2O
:As a base, accepting a proton:
HCO3- + H+ <=> H2CO3
Note: in dilute aqueous solution the formation of the
hydronium ion, , is effectively complete, so that hydration of the proton can be ignored in relation to the equilibria.
Other examples of inorganic polyprotic acids include anions of
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
,
phosphoric acid,
EDTA, and
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The under ...
that have lost one or more protons. In organic chemistry and biochemistry, important examples include
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
s and derivatives of
citric acid
Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in ...
.
Although an amphiprotic species must be amphoteric, the converse is not true. For example, a metal oxide such as
zinc oxide, ZnO, contains no hydrogen and so cannot donate a proton. Nevertheless, it can act as an acid by reacting with the hydroxide ion, a base:
:
ZnO_ + 2OH- + H2O -> Zn(OH)_^2-
This reaction is not covered by the
Brønsted–Lowry acid–base theory
The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction theory which was proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. The fundamental concept of this theory ...
. Because zinc oxide can also act as a base:
:
ZnO_ + 2H+ -> Zn^2+_ + H2O,
it is classified as amphoteric rather than amphiprotic.
Oxides
Zinc oxide (ZnO) reacts with both acids and with bases:
*
ZnO + \overset -> ZnSO4 + H2O
*
ZnO + \overset + H2O -> Na2n(OH)4
N, or n, is the fourteenth letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''en'' (pronounced ), plural ''ens''.
History
...
/chem>
This reactivity can be used to separate different cation
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 conven ...
s, for instance zinc(II), which dissolves in base, from manganese(II), which does not dissolve in base.
Lead oxide (PbO):
* PbO + \overset -> PbCl2 + H2O
* PbO + \overset + H2O -> Na2b(OH)4
B, or b, is the second letter of the Latin-script alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''bee'' (pronounced ), plural ''bees''. It rep ...
/chem>
Aluminium oxide ():
* Al2O3 + \overset -> 2 AlCl3 + 3 H2O
* Al2O3 + \overset + 3 H2O -> 2 Nal(OH)4
L, or l, is the twelfth letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''el'' (pronounced ), plural ''els''.
History
Lamedh ...
/chem> (hydrated sodium aluminate)
Stannous oxide
Tin(II) oxide (stannous oxide) is a compound with the formula SnO. It is composed of tin and oxygen where tin has the oxidation state of +2. There are two forms, a stable blue-black form and a metastable red form.
Preparation and reactions
Bl ...
(SnO):
* SnO + \overset <=> SnCl2 + H2O
* SnO + \overset + H2O <=> Na4 n(OH)6/chem>
Vanadium dioxide
Vanadium(IV) oxide or vanadium dioxide is an inorganic compound with the formula VO2. It is a dark blue solid. Vanadium(IV) dioxide is amphoteric, dissolving in non-oxidising acids to give the blue vanadyl ion, Osup>2+ and in alkali to give the b ...
():
*VO2 + \overset -> VOCl2 + H2O
*4 VO2 + \overset -> Na2V4O9 + H2O
Some other elements which form amphoteric oxides are gallium
Gallium is a chemical element with the Symbol (chemistry), symbol Ga and atomic number 31. Discovered by France, French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in boron group, group 13 of the periodic table and is similar to ...
, indium, scandium
Scandium is a chemical element with the symbol Sc and atomic number 21. It is a silvery-white metallic d-block element. Historically, it has been classified as a rare-earth element, together with yttrium and the Lanthanides. It was discovered in ...
, titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion i ...
, zirconium, chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hard ...
, iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
, cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, p ...
, copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
, silver
Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
, gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...
, germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors ...
, antimony
Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient ti ...
, bismuth, beryllium
Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to for ...
, and tellurium
Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionall ...
.
Hydroxides
Aluminium hydroxide
Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic ...
is also amphoteric:
*Al(OH)3 + \overset -> AlCl3 + 3 H2O
*Al(OH)3 + \overset -> Nal(OH)4
L, or l, is the twelfth letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''el'' (pronounced ), plural ''els''.
History
Lamedh ...
/chem>
Beryllium hydroxide:
*Be(OH)2 + \overset -> BeCl2 + 2 H2O
*Be(OH)2 + \overset -> Na2 e(OH)4/chem>CHEMIX School & Lab - Software for Chemistry Learning, by Arne Standnes
(program download required)
Chromium hydroxide:
*Cr(OH)3 + \overset -> CrCl3 + 3H2O
*Cr(OH)3 + \overset -> Na r(OH)4/chem>
See also
* Ate complex
* Isoelectric point
The isoelectric point (pI, pH(I), IEP), is the pH at which a molecule carries no net electrical charge or is electrically neutral in the statistical mean. The standard nomenclature to represent the isoelectric point is pH(I). However, pI is also ...
* Zwitterion
References
{{Reflist
Acid–base chemistry
Chemical properties
General chemistry