The self-ionization of water (also autoionization of water, and autodissociation of water) is the chemical reaction in which two water molecules react to produce a hydronium (H₃O⁺) and a hydroxide ion (OH⁻):

It is an example of autoprotolysis, and relies on the amphoteric nature of water. In Computer science, ACID ( Atomicity Consistency Isolation Durability) is a set of properties that guarantee that Database transactions are In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and 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 homeostasis is the part of Human homeostasis concerning the proper balance between Acids and bases, in other words the PH. An acidity function is a measure of the Acidity of a medium or solvent system usually expressed in terms of its ability to donate protons to (or accept protons from a For an individual weak acid or weak base component see Buffering agent. pH is the measure of the acidity or alkalinity of a Solution. The proton affinity, E pa of a Anion or of a neutral Atom or Molecule is a measure of its gas-phase basicity. In Computer science, ACID ( Atomicity Consistency Isolation Durability) is a set of properties that guarantee that Database transactions are A mineral acid is an Acid derived by Chemical reaction from inorganic Minerals as opposed to Organic acids These have Hydrogen An organic acid is an Organic compound with Acidic properties A Strong acid is an Acid that Ionizes completely in an Aqueous solution (not in the case of Sulfuric acid as it is diprotic A superacid is an Acid with an Acidity greater than that of 100% Sulfuric acid, which has a Hammett acidity function ( H 0 A weak acid is an Acid that does not completely donate all of its hydrogens when dissolved in water In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and An organic base is an Organic compound which acts as a base. Organic bases are usually but not always proton acceptors In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and In Chemistry, a superbase is an extremely strong base. There is no commonly accepted definition for what qualifies as a superbase but most chemists would accept As the name suggests a non-nucleophilic base is an organic base that is a very Strong base but at the same time a poor Nucleophile. In chemistry a weak base is a Chemical base that does not Ionize fully in an Aqueous solution. In Chemistry, hydronium is the obsolete name for the Cation H 3 O + derived from Protonation of Water In Chemistry, hydroxide is the most common name for the diatomic Anion OH− consisting of Oxygen and Hydrogen

Water, however pure, is not a simple collection of H₂O molecules. Even in "pure" water, sensitive equipment can detect a very slight electrical conductivity of 0. Electrical conductivity or specific conductivity is a measure of a material's ability to conduct an Electric current. 055 µS·cm^-1. The siemens (symbol S is the SI derived unit of Electric conductance. According to the theories of Svante Arrhenius, this must be due to the presence of ions. Svante August Arrhenius ( February 19, 1859 &ndash October 2, 1927) was a Swedish Scientist, originally a Physicist

Concentration and frequency

The preceding reaction has a chemical equilibrium constant of K_eq = ([H₃O⁺] [OH⁻]) / [H₂O]² = 3. For a general Chemical reaction \alpha A +\beta B. \rightleftharpoons \sigma S+\tau T. 23 × 10⁻¹⁸. So the acidity constant which is K_a = K_eq × [H₂O] = ([H₃O⁺] [OH⁻]) / [H₂O] = 1. 8 × 10^−16[1]. For reactions in water (or diluted aqueous solutions), the molarity (a unit of concentration) of water, [H₂O], is practically constant and is omitted from the acidity constant expression by convention. In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance The resulting equilibrium constant is called the ionization constant, dissociation constant, or self-ionization constant, or ion product of water and is symbolized by K_w.

K_w = K_a [H₂O] = K_eq [H₂O]² = [H₃O⁺] [OH⁻]

where

[H₃O⁺] = molarity of hydrogen or hydronium ion, and

[OH⁻] = molarity of hydroxide ion.

At Standard Ambient Temperature and Pressure (SATP), about 25 °C (298 K), K_w = [H₃O⁺][OH⁻] = 1. In Physical sciences standard conditions for temperature and pressure are Standard sets of conditions for experimental measurements to allow comparisons to be made 0×10⁻¹⁴. Pure water ionizes or dissociates into equal amounts of H₃O⁺ and OH⁻, so their molarities are equal:

[H₃O⁺] = [OH⁻].

At SATP, the concentrations of hydroxide and hydronium are both very low at 1. 0 × 10⁻⁷ mol/L and the ions are rarely produced: a randomly selected water molecule will dissociate within approximately 10 hours^[2]. Since the concentration of water molecules in water is largely unaffected by dissociation and [H₂O] equals approximately 56 mol/l, it follows that for every 5. 6×10⁸ water molecules, one pair will exist as ions. Any solution in which the H₃O⁺ and OH⁻ concentrations equal each other is considered a neutral solution. Absolutely pure water is neutral, although even trace amounts of impurities could affect these ion concentrations and the water may no longer be neutral. K_w is sensitive to both pressure and temperature; it increases when either increases.

It should be noted that deionized water (also called DI water) is water that has had most impurity ions common in tap water or natural water sources (such as Na⁺ and Cl⁻) removed by means of distillation or some other water purification method. Purified water is water from any source that is physically processed to remove impurities Distillation is a method of separating Mixtures based on differences in their volatilities in a boiling liquid mixture Water purification is the process of removing contaminants and other harmful microorganisms from a raw water source Removal of all ions from water is next to impossible, since water self-ionizes quickly to reach equilibrium.

Dependence on temperature and pressure

Temperature dependence of the water ionization constant at 25 MPa

Pressure dependence of the water ionization constant at 25 °C

By definition, pK_w = −log₁₀ K_w. At SATP, pK_w = −log₁₀ (1. 0×10⁻¹⁴) = 14. 0. The value of pK_w varies with temperature. As temperature increases, pK_w decreases; and as temperature decreases, pK_w increases (for temperatures up to about 250 °C). This means that ionization of water typically increases with temperature.

There is also a (usually small) dependence on pressure (ionization increases with increasing pressure). The dependence of the water ionization on temperature and pressure has been well investigated and a standard formulation exists^[3].

Acidity

pH is a logarithmic measure of the acidity (or alkalinity) of an aqueous solution. pH is the measure of the acidity or alkalinity of a Solution. In Mathematics, the logarithm of a number to a given base is the power or Exponent to which the base must be raised in order to produce By definition, pH = −log₁₀ [H₃O⁺]. Since [H₃O⁺] = [OH⁻] in a neutral solution, by mathematics, for a neutral aqueous solution pH = 7 at SATP.

Self-ionization is the process that determines the pH of water. pH is the measure of the acidity or alkalinity of a Solution. Since the concentration of hydronium at SATP (approximately 25 °C) is 1. 0×10⁻⁷mol/l, the pH of pure liquid water at this temperature is 7. Since K_w increases as temperature increases, hot water has a higher concentration of hydronium than cold water (and hence lower pH), but this does not mean it is more acidic, as the hydroxide concentration is also higher by the same amount.

Mechanism

Geissler et al. have determined that electric field fluctuations in liquid water cause molecular dissociation. ^[4] They propose the following sequence of events that takes place in about 150 fs: the system begins in a neutral state; random fluctuations in molecular motions occasionally (about once every 10 hours per water molecule) produce an electric field strong enough to break an oxygen-hydrogen bond, resulting in a hydroxide (OH⁻) and hydronium ion (H₃O⁺); the proton of the hydronium ion travels along water molecules by the Grotthuss mechanism; and a change in the hydrogen bond network in the solvent isolates the two ions, which are stabilized by solvation. To help compare Orders of magnitude of different Times this page lists times between 10&minus15 second and 10&minus12 second (1 Femto The Grotthuss Mechanism is the mechanism by which an 'excess' Proton or protonic defect diffuses through the Hydrogen bond network of water molecules or other

Within 1 picosecond, however, a second reorganization of the hydrogen bond network allows rapid proton transfer down the electric potential difference and subsequent recombination of the ions. To help compare Orders of magnitude of different Times this page lists times between 10&minus12 seconds and 10&minus11 seconds (1 Pico This timescale is consistent with the time it takes for hydrogen bonds to reorient themselves in water^[5][6][7].

See also

• Chemical equilibrium
• Acid
• Base
• Acid-base reaction theories

External links

• General Chemistry—Autoionization of Water
• [1] Calculation of Ionic Product of Water by IAPWS ([2]) Formulations

References

1. ^ McMurry, John. In a Chemical process, chemical equilibrium is the state in which the chemical activities or Concentrations of the reactants and products have no net change In Computer science, ACID ( Atomicity Consistency Isolation Durability) is a set of properties that guarantee that Database transactions are In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and (2004) Organic Chemistry, pg 44
2. ^ Eigen, M. & de Maeyer, L. (1955) Untersuchungen über die Kinetik der Neutralisation I. Z. Elektrochem. 59 986
3. ^ International Association for the Properties of Water and Steam (IAPWS)
4. ^ Geissler, P. L. ; Dellago, C. ; Chandler, D. ; Hutter, J. & Parrinello, M. (2001) Autoionization in liquid water. Science 291 2121-2124
5. ^ Stillinger, F. H. (1975) Adv. Chem. Phys. 31 1
6. ^ Rapaport, D. C. (1983) Mol. Phys. 50 1151
7. ^ Chen, S. -H. & Teixeira, J. (1986) Adv. Chem. Phys 64 1