A hydrolysis constant is an equilibrium constant for a hydrolysis

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 reaction.

For example, if a metal salt such as AlCl₃

Aluminium chloride

Aluminium chloride is the main compound of aluminium and chlorine. It is white, but samples are often contaminated with iron trichloride, giving it a yellow colour. The solid has a low melting and boiling point. It is mainly produced and consumed in the production of aluminium metal, but large...

 dissolves in an aqueous solution

Aqueous solution

An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...

, the metal cation behaves as a Lewis acid

Lewis acid

]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...

 and hydrolyzes the water molecules in the solvent.

Al³⁺ + 2H₂O → AlOH²⁺ + H₃O⁺

The hydrolysis constant for this reaction is as shown:

K_hydrolysis = [H₃O⁺] * [AlOH²⁺] / [Al³⁺]

In a more generalized form, the hydrolysis constant can be described as:

K_a = [H₃O⁺] * [A^-] / [HA]

where A^- represents any base

Base (chemistry)

For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...

, and HA represents any acid.