Instant Notes: Analytical Chemistry

Section C – Analytical reactions in solution

C6 COMPLEXATION, SOLUBILITY

AND REDOX EQUILIBRIA

Complexation The formation of stable compounds and complexes is important in analytical
chemistry, since many species may be formed in a real sample. The amounts
and nature of the species present are analyzed to study speciation. For example,
in a natural water sample, the metal ions may form complexes with water
molecules, carbonate species, plant acids or pollutants. Complexes may be used
for titrations, both directly and for masking unwanted reactions.
The formation of a complex compound between an acceptor species, most
usually a metal ion, M, and a coordinating species, or donor ligand, L, involves
the formation of coordinate bonds, for example, hexamino cobalt (III)

Co^3 ++6NH 3 =Co (NH 3 ) 63 +

The formation of such complexes involves interactions between the orbitals of

the central atom and suitable orbitals or lone pair electrons of the ligands. The

structure and stability of the complexes are discussed more fully in textbooks of

inorganic chemistry.

The formation constant for the equilibrium may be represented in two

ways. The stepwise formation constants, Kf, relates to each addition of a ligand

molecule:

M +L =ML Kf1=c(ML)/(c (M). c(L))

ML +L =ML 2 Kf2=c (ML 2 )/(c(ML). c (L))

or, generally:

MLn- 1 +L =MLn Kfn=c(MLn)/(c (MLn–1). c (L))

The overall formation constant, b, relates to the formation of the entire complex

in one equation, so that for the complex with n ligands:

Key Notes

A complex is formed by reactions between two or more species that are

capable of independent existence. Often this is between a metal ion, M

and a coordinating molecule L.

Ionic reactions producing a compound that is insoluble in the chosen

solvent used may be used for analysis.

Where one species is reduced while the other is simultaneously oxidized,

the reactions are termed redox reactions and are useful in many

analytical methods.

Related topics Titrimetry II (C7) Other topics in this Section (C1-C5,

C8-C10)

Complexation

Solubility

Redox equilibria