Can the hydroxide ion deprotonate water

The Protolysis (or protolytic reaction) is a chemical reaction in which a proton (H.+) between two respondentstransferbecomes. The term Protolysis often leads to the erroneous mention of the splitting off of a proton. Protolysis is the decisive process according to the important Brønsted acid-base theory. Then an acid transfers a proton (H.+) to a reaction partner. The compound called acid acts as a proton donor (Proton donor), the base (often water) absorbs the protons and is therefore called Proton acceptor designated. A chemical equilibrium is established between the reactants.

Protolytic reactions

If the gas hydrogen chloride (HCl) is introduced into water, hydrochloric acid is formed with protolysis. In this equilibrium reaction, the molecule is HCl and the ion is H.3O+Proton donors, so to Brønsted Acids. H2O and Cl act as proton acceptors, so they are according to Brønsted Bases.


For example, if pure acetic acid (H.3C – COOH) given in water form H with protolysis3O+ and the acetate anion (H.3C-COO). Here are CH3COOH and H3O+Proton donors, while H3C-COO and H2O are proton acceptors.

Protolysis of the biprotonic compound sulfuric acid in water:

In this reaction equation, the molecules are H.2SO4 and the ion H3O+Proton donors, so to Brønsted Acids. H2O and SO42− act as proton acceptors, so they are according to Brønsted Bases. HSO plays a special role4which, depending on the direction of the reaction, can react as a proton acceptor or proton donor. Substances with such properties are called ampholytes.

If the gas is ammonia (NH3) introduced into water, ammonium ions (NH4+) and hydroxide ions (OH). Proton donors are NH here4+ and H2O, while OH and NH3 Are proton acceptors.


Pure water is subject to a so-called Autoprotolysis. This creates oxonium ions H3O+ and hydroxide ions (OH). H2O can react both as a proton donor (as an acid) or as a proton acceptor (as a base). One therefore speaks of an ampholyte here as well.

The equilibrium is very much on the water side. The ionic product for this reaction is 10 at 298 K (25 ° C)−14 mol2 l−2. The autoprotolysis of water is the reason why chemically pure water also has at least a low electrical conductivity. The autoprotolysis is clearly dependent on the temperature. The equilibrium constant (in mol2 l-2):

  • at 0 ° C 0.11 x 10−14, around 10−15
  • at 60 ° C 9.61 x 10−14, around 10−13

Accordingly, the pH value of water is also temperature-dependent. This results in

  • pure water at 0 ° C a pH of 7.5
  • pure water at 25 ° C a pH of 7.0
  • pure water at 60 ° C a pH of 6.5

Protolysis in non-aqueous solutions

In addition to water, other sufficiently polar solvents can also serve as reactants in Brønsted acid-base reactions, for example methanol or ethanol. A good example is the autoprotolysis of liquid ammonia. The ions ammonium and amide are formed.

Analogous reactions are also known in concentrated sulfuric acid:

Category: acid-base reaction