3 —
pH, Complexation and Solubility Equilibria
Since so many analytical procedures involve solution chemistry an understanding of the principles is
essential. Complex formation, precipitation reactions and the control of pH are three aspects with
special relevance in analysis.
In the simplest situation where a solution of a weak acid is used the degree of dissociation of the acid
and hence the composition of the solution will depend on the pH of the solution. If, as is often the case,
a complexing agent is itself a weak acid, its complexing reactions will also be pH dependent.
The formation of a metal ion complex may be used to facilitate its precipitation or solvent extraction,
whilst modification of its charge will influence its electrochemical properties.
Definitions
Acid
A species which donates protons in a reaction (Lowry–Brønsted) or accepts electrons (Lewis).
Base
A species which accepts protons in a reaction (Lowry–Brønsted) or donates electrons (Lewis).
Complex
A compound which is formed in a complexation reaction between two or more species which are
capable of independent existence. Most complexes of analytical importance involve dative bonds
between Lewis bases and metal ions.
Ligand
A species with at least one Lewis basic site which can participate in complex formation.
Strong Electrolyte
A compound which is almost completely dissociated in solution.