Amphoterism

In chemistry, an amphoteric compound is a molecule or ion that can react both as an acid and as a base.[1] Many metals (such as copper, zinc, tin, lead, aluminium, and beryllium) form amphoteric oxides or hydroxides. Amphoterism depends on the oxidation states of the oxide. Al2O3 is an example of an amphoteric oxide.

The prefix of the word 'amphoteric' is derived from a Greek prefix amphi-, which means both. In chemistry, an amphoteric substance is a substance that has the ability to act either as an acid or a base. Remember that acids donate protons (or accept electron pairs) and bases accept protons. Amphoteric substances can do either.

Metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. Amphoteric oxides include lead oxide and zinc oxide, among many others.

One type of amphoteric species are amphiprotic molecules, which can either donate or accept a proton (H+). Examples include amino acids and proteins, which have amine and carboxylic acid groups, and self-ionizable compounds such as water.

Ampholytes are amphoteric molecules that contain both acidic and basic groups and will exist mostly as zwitterions in a certain range of pH. The pH at which the average charge is zero is known as the molecule's isoelectric point. Ampholytes are used to establish a stable pH gradient for use in isoelectric focusing.

Etymology

Amphoteric is derived from the Greek word amphoteroi (ἀμφότεροι) meaning "both". Related words in acid-base chemistry are amphichromatic and amphichroic, both describing substances such as acid-base indicators which give one colour on reaction with an acid and another colour on reaction with a base.[2]

Amphiprotic molecules

According to the Brønsted-Lowry theory of acids and bases: acids are proton donors and bases are proton acceptors.[3] An amphiprotic molecule (or ion) can either donate or accept a proton, thus acting either as an acid or a base. Water, amino acids, hydrogen carbonate ion (bicarbonate ion) and hydrogen sulfate ion (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

A common example of an amphiprotic substance is the hydrogen carbonate ion, which can act as a base:

HCO3 + H3O+ → H2CO3 + H2O

or as an acid:

HCO3 + OH → CO32− + H2O

Thus, it can effectively accept or donate a proton.

Water is the most common example, acting as a base when reacting with an acid such as hydrogen chloride:

H2O + HCl → H3O+ + Cl,

and acting as an acid when reacting with a base such as ammonia:

H2O + NH3 → NH4+ + OH

Not all amphoteric substances are amphiprotic

Although an amphiprotic species must be amphoteric, the converse is not true. For example, the metal oxide ZnO contains no hydrogen and cannot donate a proton. Instead it is a Lewis acid whose Zn atom accepts an electron pair from the base OH. The other metal oxides and hydroxides mentioned above also function as Lewis acids rather than Brønsted acids.

Amphoteric oxides and hydroxides[4]

Amphoteric oxides

Zinc oxide (ZnO) reacts with both acids and with bases:

  • In acid: ZnO + H2SO4 → ZnSO4 + H2O
  • In base: ZnO + 2 NaOH + H2O → Na2[Zn(OH)4]

This reactivity can be used to separate different cations, such as zinc(II), which dissolves in base, from manganese(II), which does not dissolve in base.

Lead oxide (PbO):

  • In acid: PbO + 2 HCl → PbCl2 + H2O
  • In base: PbO + 2 NaOH + H2O → Na2[Pb(OH)4]

Aluminium oxide (Al2O3)

  • In acid: Al2O3 + 6 HCl→ 2 AlCl3 + 3 H2O
  • In base: Al2O3 + 2 NaOH + 3 H2O → 2 Na[Al(OH)4] (hydrated sodium aluminate)

Stannous oxide (SnO)

  • In acid : SnO +2 HCl ⇌ SnCl2 + H2O
  • In base : SnO +4 NaOH + H2O ⇌ Na4[Sn(OH)6]

Some other elements which form amphoteric oxides are gallium, indium, scandium, titanium, zirconium, vanadium, chromium, iron, cobalt, copper, silver, gold, germanium, antimony, bismuth, and tellurium.

Amphoteric hydroxides

Aluminium hydroxide is also amphoteric:

  • As a base (neutralizing an acid): Al(OH)3 + 3 HCl → AlCl3 + 3 H2O
  • As an acid (neutralizing a base): Al(OH)3 + NaOH → Na[Al(OH)4]

Beryllium hydroxide

  • with acid: Be(OH)2 + 2 HCl → BeCl2 + 2 H2O
  • with base: Be(OH)2 + 2 NaOH → Na2[Be(OH)4].[5]

See also

References

  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "amphoteric". doi:10.1351/goldbook.A00306
  2. ^ Penguin Science Dictionary 1994, Penguin Books
  3. ^ R.H. Petrucci, W.S. Harwood, and F.G. Herring, "General Chemistry" (8th edn, Prentice-Hall 2002), p.669
  4. ^ Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. pp. 173–4. ISBN 978-0130399137.
  5. ^ CHEMIX School & Lab - Software for Chemistry Learning, by Arne Standnes (program download required)
Acid-base extraction

Acid-base extraction is a procedure using sequential liquid–liquid extractions to purify acids and bases from mixtures based on their chemical properties.Acid-base extraction is routinely performed during the work-up after chemical syntheses and for the isolation of compounds and natural products like alkaloids from crude extracts. The product is largely free of neutral and acidic or basic impurities. It is not possible to separate chemically similar acids or bases using this simple method.

Acid–base imbalance

Acid–base imbalance is an abnormality of the human body's normal balance of acids and bases that causes the plasma pH to deviate out of the normal range (7.35 to 7.45). In the fetus, the normal range differs based on which umbilical vessel is sampled (umbilical vein pH is normally 7.25 to 7.45; umbilical artery pH is normally 7.18 to 7.38). It can exist in varying levels of severity, some life-threatening.

Alkahest

Alkahest is a hypothetical "universal solvent": able to dissolve every other substance, including gold. The famous alchemist Philippus Paracelsus described alkahest in the 1500s.

Because of its perceived invaluable medicinal qualities, Alchemists of the time were concerned with its plausibility and existence.

Amphiphile

An amphiphile (from the Greek αμφις, amphis: both and φιλíα, philia: love, friendship) is a chemical compound possessing both hydrophilic (water-loving, polar) and lipophilic (fat-loving) properties. Such a compound is called amphiphilic or amphipathic. This forms the basis for a number of areas of research in chemistry and biochemistry, notably that of lipid polymorphism. Organic compounds containing hydrophilic groups at both ends of a prolate (in the aggregate) molecule are called bolaamphiphilic. Common amphiphilic substances are soaps, detergents and lipoproteins.

Buffer solution

A buffer solution (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. In nature, there are many systems that use buffering for pH regulation. For example, the bicarbonate buffering system is used to regulate the pH of blood.

Hammett acidity function

The Hammett acidity function (H0) is a measure of acidity that is used for very concentrated solutions of strong acids, including superacids. It ±± x၃ was proposed by the physical organic 'f 'chemist Louis Plack Hammettႎ,

,b' is the best-known acidity function used to extend the measure of Brønsted–Lowry acidity beyond the dilute aqueous solutions for which the pH scale is useful.

In highly concentrated solutions, simple approximations such as the Henderson–Hasselbalch equation are no longer valid due to the variations of the activity coefficients. The Hammett acidity function is used in fields such as physical organic chemistry for the study of acid-catalyzed reactions, because some of these reactions use acids in very high concentrations, or even neat (pure).

Imidazole

Imidazole is an organic compound with the formula C3N2H4. It is a white or colourless solid that is soluble in water, producing a mildly alkaline solution. In chemistry, it is an aromatic heterocycle, classified as a diazole, and has non-adjacent nitrogen atoms.

Many natural products, especially alkaloids, contain the imidazole ring. These imidazoles share the 1,3-C3N2 ring but feature varied substituents. This ring system is present in important biological building blocks, such as histidine and the related hormone histamine. Many drugs contain an imidazole ring, such as certain antifungal drugs, the nitroimidazole series of antibiotics, and the sedative midazolam.When fused to a pyrimidine ring, it forms a purine, which is the most widely occurring nitrogen-containing heterocycle in nature.The name "imidazole" was coined in 1887 by the German chemist Arthur Rudolf Hantzsch (1857–1935).

Mineral acid

A mineral acid (or inorganic acid) is an acid derived from one or more inorganic compounds. All mineral acids form hydrogen ions and the conjugate base when dissolved in water.

Non-nucleophilic base

As the name suggests, a non-nucleophilic base is a sterically hindered organic base that is a poor nucleophile. Normal bases are also nucleophiles, but often chemists seek the proton-removing ability of a base without any other functions. Typical non-nucleophilic bases are bulky, such that protons can attach to the basic center but alkylation and complexation is inhibited.

Organic acid

An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are relatively stronger acids. Alcohols, with –OH, can act as acids but they are usually very weak. The relative stability of the conjugate base of the acid determines its acidity. Other groups can also confer acidity, usually weakly: the thiol group –SH, the enol group, and the phenol group. In biological systems, organic compounds containing these groups are generally referred to as organic acids.

A few common examples include:

Lactic acid

Acetic acid

Formic acid

Citric acid

Oxalic acid

Uric acid

Organic base

An organic base is an organic compound which acts as a base. Organic bases are usually, but not always, proton acceptors. They usually contain nitrogen atoms, which can easily be protonated. Amines and nitrogen-containing heterocyclic compounds are organic bases. Examples include:

pyridine

alkanamines, such as methylamine

imidazole

benzimidazole

histidine

guanidine

phosphazene bases

hydroxides of quaternary ammonium cations or some other organic cations

PH

In chemistry, pH () is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It is approximately the negative of the base 10 logarithm of the molar concentration, measured in units of moles per liter, of hydrogen ions. More precisely it is the negative of the base 10 logarithm of the activity of the hydrogen ion. At 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic. The neutral value of the pH depends on the temperature, being lower than 7 if the temperature increases. Pure water is neutral, pH 7 at (25 °C), being neither an acid nor a base. Contrary to popular belief, the pH value can be less than 0 or greater than 14 for very strong acids and bases respectively.Measurements of pH are important in agronomy, medicine, chemistry, water treatment, and many other applications.

The pH scale is traceable to a set of standard solutions whose pH is established by international agreement.

Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such as the silver chloride electrode.

The pH of aqueous solutions can be measured with a glass electrode and a pH meter, or an indicator.

There are three current theories used to describe acid–base reactions: Arrhenius, Bronsted-Lowry and Lewis when determining pH.

Proton affinity

The proton affinity (PA, Epa) of an anion or of a neutral atom or molecule is the negative of the enthalpy change in the reaction between above species and proton in the gas phase:

A− + H+ → HA

B + H+ → BH+These reactions are always exothermic in the gas phase, i.e. energy is released when the reaction advances in the direction shown and enthalpy is negative, while the proton affinity is positive. This is the same sign convention as is used for electron affinity. The property related to the proton affinity is the gas-phase basicity, which is the negative of the Gibbs energy for above reactions, i.e. the gas-phase basicity includes entropic terms in contrast to the proton affinity.

Solid acid

Solid acids are acids that do not dissolve in the reaction medium. They are often used in heterogeneous catalysts.

Superacid

According to the classical definition, a superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function (H0) of −12. According to the modern definition, a superacid is a medium in which the chemical potential of the proton is higher than in pure sulfuric acid. Commercially available superacids include trifluoromethanesulfonic acid (CF3SO3H), also known as triflic acid, and fluorosulfuric acid (HSO3F), both of which are about a thousand times stronger (i.e. have more negative H0 values) than sulfuric acid. Most strong superacids are prepared by the combination of a strong Lewis acid and a strong Brønsted acid. A strong superacid of this kind is fluoroantimonic acid. Another group of superacids, the carborane acid group, contains some of the strongest known acids.

Superbase

In chemistry, a superbase is an extremely basic compound or caustic substance that has a high affinity for protons. The hydroxide ion is the strongest base possible in aqueous solutions, but bases that exist with much greater strengths than the bases that could exist in aqueous solutions are possible. Such bases are valuable in organic synthesis and are fundamental to physical organic chemistry. Superbases have been described and used since the 1850s. Reactions involving superbases often require special techniques since they are destroyed by water and atmospheric carbon dioxide as well as oxygen. Inert atmosphere techniques and low temperatures minimize these side reactions.

Tin(IV) Oxide

Tin(IV) Oxide, also known as stannic oxide, is the inorganic compound with the formula SnO2. The mineral form of SnO2 is called cassiterite, and this is the main ore of tin. With many other names, this oxide of tin is an important material in tin chemistry. It is a colourless, diamagnetic, amphoteric solid.

Uranium trioxide

Uranium trioxide (UO3), also called uranyl oxide, uranium(VI) oxide, and uranic oxide, is the hexavalent oxide of uranium. The solid may be obtained by heating uranyl nitrate to 400 °C. Its most commonly encountered polymorph, γ-UO3, is a yellow-orange powder.

Weak base

In chemistry, a weak base is a base that does not ionize fully in an aqueous solution. As Brønsted–Lowry bases are proton acceptors, a weak base may also be defined as a chemical base in which protonation is incomplete. This results in a relatively low pH compared to strong bases.

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