• Classification of Complexometric titration
Complexometric titrations are of the following four types:
1) Direct titration,
2) Back titration,
3) Replacement titration,
4) Indirect titration.
1). Direct Titration
This method is similar to acid-base titrations, and involves adding the standard chelon solution to the metal ion solution till the end point is attained. For example, calcium gluconate injection, calcium lactate tablets, and compound sodium lactate injection for the assay of calcium chloride
Limitations
1) It is a slow complexation reaction, and
2) It may suffer interference due to the presence of other ions.
2). Back Titration
In this method, excess of a standard EDTA solution is added to the metal solution (to be analysed), and the excess is back titrated with a standard solution of a second metal ion.
For example, Mn cannot be directly titrated with EDTA because of precipitation of Mn(OH)2. An excess of known volume of EDTA is added to an acidic solution of Mn salt and then by using ammonia buffer the pH is adjusted up to 10.
The excess EDTA remaining after chelation is back titrated with a standard Zn solution using Eriochrome black T as an indicator. This method is similar to back titration method in acidimetry, e.g., ZnO.
3). Replacement Titration
In this method, the metal to analysed quantitatively displaces the metal from the. complex. When direct or. back titrations do not give sharp end points, the metal may be determined by the displacement of an equivalent amount of Mg or Zn from a less stable EDTA complex:
Mn+2 + Mg EDTA-2 -..—>Mg+2 + MnEDTA-2
Mn displaces Mg from Mn-EDTA solution. The freed Mg metal is then directly titrated with a standard EDTA solution. In this method, excess quantity of Mg-EDTA chelate is added to Mn solution. Mn quantitatively displaces Mg from Mg-EDTA chelate. This displacement takes place because Mn forms a more stable complex with EDTA. By this method Ca, Pb, and Hg may be determined using Eriochrome black T indicator.
4). Indirect Titration
This is also known as alkalimetric titration. It is used for the determination of anions which do not react with EDTA chelate. Protons from disodium EDTA are displaced by a heavy metal and titrated with sodium alkali.
For example, barbiturates do not react with EDTA but are quantitatively precipitated from alkaline solution by mercuric ions as 1: 1 complex.
Barbiturate to be analysed is taken in a flask and heated with excess of mercury in alkaline solution. When precipitated, Hg-barbiturate complex is fonned which is filtered and dissolved in excess of standard EDTA solution. The unreacted EDTA solution is then back titrated with a standard Zn solution.
