List of aqueous ions by element

This table lists the ionic species that are most likely to be present, depending on pH, in aqueous solutions of binary salts of metal ions. The existence must be inferred on the basis of indirect evidence provided by modelling with experimental data or by analogy with structures obtained by X-ray crystallography.

Introduction

When a salt of a metal ion, with the generic formula MXn, is dissolved in water, it will dissociate into a cation and anions.

M

X

n

→

M

n

+

(

a

q

)

+

n

X

−

(

a

q

)

{\displaystyle MX_{n}\rightarrow M^{n+}(aq)+nX^{-}(aq)}

(aq) signifies that the ion is aquated, with cations having a chemical formula [M(H2O)p]q+ and anions whose state of aquation is generally unknown. For convenience (aq) is not shown in the rest of this article as the number of water molecules that are attached to the ions is irrelevant in regard to hydrolysis. This reaction occurs quantitatively with salts of the alkali-metals at low to moderate concentrations.

With salts of divalent metal ions, the aqua-ion will be subject to a dissociation reaction, known as hydrolysis, a name derived from Greek words for water splitting. The first step in this process can be written as

M

n

+

+

O

H

−

{\displaystyle M^{n+}+OH^{-}}

⇌

M

(

O

H

)

(

n

−

1

)

+

{\displaystyle M(OH)^{(n-1)+}}

When the pH of the solution is increased by adding an alkaline solution to it, the extent of hydrolysis increases. Measurements of pH or colour change are used to derive the equilibrium constant for the reaction. Further hydrolysis may occur, producing dimeric, trimeric or polymeric species containing hydroxy- or oxy- groups.