Vi. Edta Titrations

C3100 - Winter 2011

VI. COMPLEXOMETRIC (EDTA) TITRATIONS

(Chapter 11, and L. G. Hargis: Analytical Chemistry: Principles and Techniques (1988) Prentice Hall, Inc.)

Introduction

Most metal ions can accept unshared pairs of electrons from an anion or molecule to form coordinate covalent bonds. Usually metals accept 2, 4 or 6 e- pairs (coordination number) Molecule containing donor atom = ligand Product resulting from reaction between metal ion and ligand = complex ion or (metal) complex Ligands with a single donor atom = monodentate (e.g. CN-, NH3) Multidentate ligands have multiple donor atoms and coordinate with a metal ion to form a ring structure = chelate ('claw') The geometry of the complex can be linear (2 ligands), square planar or tetrahedral (4 ligands), octahedral (6 ligands), or other.

1

Aminocarboxylic acids as synthetic chelating agents. When a carboxylate O atom binds to a metal ion, it loses its proton

ATP is a tetradentate ligand that binds to divalent ions (Mg2+, Mn2+, Ni2+) through 4 of the 6 coordination positions (5 & 6: H2O) The biologically active form generally is the Mg2+ complex

Fig. 11-2, Harris 8th Ed. 2

Dr. G. Van Biesen

C3100 - Winter 2011

Chelate Effect: multidentate ligands form thermodynamically more stable complexes than similar monodentate ligands

Explanation:

ΔG = ΔH - TΔS

ΔH is ~ same for both reactions, but the change in entropy for reactions involving multidentate ligands is more favourable than for monodentate ligands (net gain of species; increases entropy)

3

Note: complexation reactions in aqueous solution = ligand exchange reactions (no uncoordinated metal ions in aqueous solution!) Cu2+ + 4 NH3 Cu(H2O)42+ + 4 NH3 Applications of metal complexes: As indicators, for titrations (see further) or for direct determination of trace amounts of metal ions: 2+

N Fe

2+

Cu(NH3)42+ actually should be written as: Cu(NH3)42+ + 4 H2O

N Fe

+

3 N

Ferroin (red)

N

phenanthroline...