Self-ion effect

The self-ion effect can the loosening of salts occur. It describes the phenomenon that the molar solubility of a salt is reduced in a solution in which a component of the salt is already present.

example

An example would be the dissolution of BaSO ₄ ( barium sulfate ) in a sulfate - ion containing solution. The solubility product of BaSO ₄ is 10 ⁻¹⁰ mol ² / l ² . The solubility product of a salt is defined e.g. B. for a simple AB or 1: 1 salt (e.g. sodium chloride ) as the molar solubility of Na ⁺ times the solubility of Cl ^- in moles per liter.

It follows that 10 ⁻⁵ mol of barium sulfate can be dissolved in one liter of water (i.e. the root of 10 ⁻¹⁰ mol ² / l ² ). 10 ⁻⁵ mol / l Ba ²⁺ times 10 ⁻⁵ mol / l SO ₄ ²⁻ = 10 ⁻¹⁰ = K _L

If the solution already contains Ba or sulfate ions, these are included in the calculation. For example, if we have a 10 ⁻² molar solution of Na ₂ SO _4, then there are already as many sulfate ions in the solution. Since the solubility product is constant at constant temperature, this means for the calculation: 10 ⁻¹⁰ (= solubility product) = 10 ⁻² mol / l (= SO ₄ ²⁻ ) times 10 ⁻⁸ mol / l Ba ²⁺ . Since 10 ^{−2 is} much larger than 10 ⁻⁵ , the proportion of sulfate that dissolves from the salt is neglected in this calculation.

Individual evidence

1. ^ Chemistry for Physicians , by Helmut Wachter, Arno Hausen, Gilbert Reibnegger