Complex Formation Titrations

PURPOSE

The aims of this experiment are determining magnesium by direct titration and determining calcium by displacement titration and back titration.

THEORY

A complexation reaction involves a reaction between a metal ion (M) and another molecular or ionic entity (D), containing at least one atom with an unshared pair of electrons. Solvent or electron pair donors occupy that all the coordination positions on metal ion. 

Ligends are called unidentate when they can donate one pair of electron such as ammonia and multidentate when they can donate two or more electron pairs. The component is called chelate when a multidentate ligand forms two or more coordinate bonds with the same atom. A chelating agent bidentate, has two donor groups availablefor coordination bonding and tridentate has three groups. Tetradentate, pentadentate and hexadentate chelating agents are also known. Chelation process occurs in single step. Equilibrium constants in complexation reactions usually are expressed as formation or stability constats. Formation constants generally are larger than those involving unidentate ligands. Halides for analysis of mercury (II) and cyanide for analysis of Ag⁺ are some of the common unidentate ligands. Ethylene diamine tetraacetic acid (EDTA) is one of the most important polydentate ligands in analysis.

This hexadentate ligand forms very stable complexes (usually octahedral structures) with most of the transition metals.

One of the valuable properties of EDTA as a titrant, is that it combines with metal ions (except alkali metals) in 1:1 ratio regardless of the charge on the cation.

pH is important for M-EDTA formation. For example an alkaline medium is needed for titrations that involve Ca²⁺ and Mg²⁺, which form weak complexes. However some titrations require moderately acidic solutions to form more stable complexes such as Zn²⁺ and Ni²⁺.

End point can be seen with using a metal ion indicator. Eriochrome Black T (H₃In) is the common one.

There are four titration methods;

1.  Direct titration:     Determination with EDTA, using metal ion indicators for end point detection.

2.  Back titration:      It is useful for the analysis of cations, that form very stable EDTA complexes and for which a satisfactory indicator is not available.

3.  Displacement titration:    It is useful for introducing excess of a solution containing EDTA in the form of a Mg or Zn complex.

4.  Alcalimetric titration:     In this procedure, an excess of Na₂H₂Y is added to a neutral solution of the metal ion.

Reference

http://www.chem.purdue.edu/gchelp/cchem/edtalew.gif