Precipitation (Insoluble Salts)

Introduction

Many metal ions form compounds that are insoluble in water. We call them insoluble salts (duhhh) or precipitates. Common precipitates are carbonates, hydroxides, sulfates, and sulfides. Ions that we consider spectator ions when discussing acid-base equilibria will form insoluble salts.

An insoluble salt in contact with water maintains an equilibrium with the ions. In simple cases where there are no common ions or competing equilibria, the ion concentrations depend only on the equilibrium constant for the particular precipitate. When we talk about solubility equilibria we always write the equilibrium with the solid on the left. For example:

Ba(IO₃)_{2 (s)} Ba²⁺_(aq) + 2 IO₃^-_(aq)

The equilibrium constant expression for an insoluble salt is written following the same rules as for any other equilibrium. The equilibrium constant is called the solubility product, K_sp. The K_sp expression for the above equilibrium is:

K_sp = [Ba²⁺][IO₃^-]²

K_sp Values for Some Precipitates

Formula	Name	Ksp
AgCl	silver chloride	1.8x10-10
Al(OH)3	aluminum hydroxide	2x10-32
BaCO3	barium carbonate	5x10-9
Ba(IO3)2	barium iodate	1.6x10-9
BaSO4	barium sulfate	1.3x10-10
Fe(OH)2	iron(II) hydroxide	8x10-16
Fe(OH)3	iron(III) hydroxide	4x10-38
FeS	iron sulfide	6x10-18
PbCrO4	lead chromate	1.8x10-14
Pb(OH)2	lead hydroxide	2.5x10-16
PbS	lead sulfide	7x10-28
PbSO4	lead sulfate	1.6x10-8

The equilibria of insoluble salts is described in more detail in the document on solubility.