Self-ionization of water

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

<math>H_2O_{(l)}+H_2O_{(l)} \Leftrightarrow H_3O^+_{(aq)} + OH^-_{(aq)}<math>

The reaction is also known as the autoionization or autodissociation of water. It is an example of autoprotolysis, and relies on the amphoteric nature of water.

Water, however pure, is not a simple collection of H₂O molecules. Even in "pure" water, sensitive equipment can detect a very slight electrical conductivity. According to the theories of Svante Arrhenius, this must be due to the presence of ions.

Concentration and Frequency

The ionic product of water at 25°C (298K) is K_w = [H₃O⁺][OH^-] = 1.0 × 10^-14mol².l^-2. At that temperature the concentrations of hydroxide and hydronium are both 1.0 × 10^-7mol.l^-1. Since the concentration of water molecules in water is largely unaffected by dissociation and approximately 56 mol.l^-1, it follows that for every 5.6 &times 10⁸ water molecules, one pair will exist as ions. A water deionizer can recombine these naturally occurring ions into pure deionized water temporarily.

Hydroxide and hydronium ions have low concentrations in water, and they are rarely produced: a randomly selected water molecule will dissociate within approximately 10 hours (1).

Acidity

Self-ionization is the process that determines the pH of water. Since the concentration of hydronium at 25°C is 1.0 × 10^-7mol.l^-1, the pH of liquid water at this temperature is 7. K_w is sensitive to both pressure and temperature; it increases with both. As a result, hot water has a higher concentration of hydronium than cold water, and is more acidic.

Mechanism

Geissler et al. have determined that electric field fluctuations in liquid water cause molecular dissociation (2). They propose the following sequence of events that takes place in about 150 fs: the system begins in a neutral state; the solvent's electric field breaks a hydrogen bond between two water molecules, creating a hydroxide and hydroxonium ion; the proton of the hydroxonium 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.

Within 1 ps, however, a second reorganization of the hydrogen bond network allows rapid proton transfer down the electric potential difference and subsequent recombination of the ions. This timescale is consistent with the time it takes for hydrogen bonds to reorient themselves in water (3,4,5).

See Also

• SATP
• Chemical equilibrium

References

1. Eigen, M. & de Maeyer, L. (1955) Untersuchungen über die Kinetik der Neutralisation I. Z. Elektrochem. 59 986.
2. Geissler, P. L.; Dellago, C.; Chandler, D.; Hutter, J. & Parrinello, M. (2001) Autoionization in liquid water. Science 291 2121-2124.
3. Stillinger, F. H. (1975) Adv. Chem. Phys. 31 1.
4. Rapaport, D. C. (1983) Mol. Phys. 50 1151.
5. Chen, S.-H. & Teixeira, J. (1986) Adv. Chem. Phys 64 1.