What are the applications of complexometric titration?

Answer 1

The Applications of Complexometric Titrations are as follows:

1. Direct Titration: It is the simplest and the most convenient method in which the

standard solution of EDTA is slowly added to the metal ion solution till the end

point is achieved. It is similar to simple acid-base titrations. For this method to

be useful the formation constant must be large and the indicator must provide a

very distinct color change as mentioned earlier. Further we need standardized

solution of EDTA and sometimes auxiliary complexing agents may be required.

Some important elements which could be determined directly by the

complexometric titration are Cu, Mn, Ca, Ba, Br, Zn, Cd, Hg, Al, Sn, Pb, Bi, Cr,

Mo, Fe, Co, Ni, and Pd, etc. However, the presence of other ions may cause

interference and need to be suitably handled.

2. Back Titration: In this method, an excess of a standard solution of EDTA is

added to the metal solution being determined so as to complex all the metal ions

present in the solution. The excess of EDTA left after the complex formation

with the metal is back titrated with a standard solution of a second metal ion.

This method becomes necessary if the analyte precipitates in the absence of

EDTA or reacts too slowly with EDTA, or it blocks the indicator. For example,

determination of Mn is done by this method because a direct titration is not

possible due to precipitation of Mn (OH)2. The excess EDTA remaining after

complexation, is back titrated with a standard Zn solution using Eriochrome

black T as indicator. However, one has to ensure the standard metal ion should

not displace the analyte ion from their EDTA complex.

3. Replacement Titration: When direct or back titrations do not give sharp end

points or when there is no suitable indicator for the analyte the metal may be

determined by this method. The metal to be analyzed is added to a metal-EDTA

complex. The analyte ion (with higher Kf′) displaces EDTA from the metal and

the metal is subsequently titrated with standard EDTA. For example, in the

determination of Mn an excess of Mg EDTA chelate is added to Mn solution.

The Mn ions quantitatively displace Mg from Mg-EDTA solution because Mn

forms a more stable complex with EDTA.

Mn+ + MgY2 - (MY)(n - 4)+ + Mg2+

The freed Mg metal is then directly titrated with a standard solution of EDTA

using Eriochrome black T indicator. Ca, Pb and Hg may also be determined by

this method.

4. Indirect Titration: Certain anions that form precipitate with metal cations and

do not react with EDTA can be analyzed indirectly. The anion is first

precipitated with a metal cation and the precipitate is washed and boiled with an

excess of disodium EDTA solution to form the metal complex.

Mn+ + H2Y2 - (MY)(n - 4)+ + 2H+

The protons from disodium EDTA are displaced by a heavy metal and titrated

with sodium alkali. Therefore, this method is also called alkalimetric titration.

For example, barbiturates can be determined by this method.