Complexometric titration involves converting simple metal ions into stable, water-soluble complexes via ligand interactions, with examples including unidentate, bidentate, and multidentate ligands like EDTA. Various titration types, such as direct, back, and indirect titrations, are employed based on the stability of the metal-ligand complex. Applications include measuring water hardness and detecting metal ion concentrations in different contexts.

1. COMPLEXOMETRIC
TITRATION
Kencha swathi
Assistant professor
ABIPER
Bangalore

2. INTRODUCTION
 The reaction in which simple metal ion is converted into
complex ion by the addition of the reagent.
 The complex formed is stable and is water soluble.
 In complexometric titration ,metal ion accepts electrons and
ligand donates electron.
 The ligand molecule is having atleast one lone pair of electrons
and forms a linkage with metal ion.

3. LIGAND: A species which donates
lone pair of electrons and forms a
coordinate linkage with metal ion.

4. CLASSIFICATION OF LIGANDS
1.UNIDENTATE: If a single site is present in ligand to form a
complex. Eg: Cyanide ion (CN-)
2.BIDENTATE: Two sites are present in a ligand
Eg: Glycine and oxalic acid
3. MULTIDENTATE: When more than 2 or 3 sites are present.
Eg: EDTA

5. EDTA
 Ethylene diamine tetra acetic acid (EDTA) and its disodium salts
are versatile complexometric agents.
 EDTA form complexes with all metal ions.
Structure of EDTA:

6.  The complexes formed by using EDTA are of chelate type.
 The complex with metal ion & EDTA is as follows.

7.  Alkaline medium is required for complex ions becoz EDTA will
ionize more in an alkali solution.

8. TYPES OF
COMPLEXOMETRIC
TITRATIONS:
1. Direct titration
2. Back titration
3. Displacement or substitution titration
4. Indirect titration
5. Alkalimetric titration
6. Miscellaneous titration

9. DIRECT TITRATION
Metal ion + Buffer + indicator Titrate with EDTA solution
It is a simplest and most convenient method in which the standard
solution of EDTA is slowly added to the metal ion solution till the end
point is achieved. The solution containing the metal ion is buffered to the
desired pH and titrated directly with the standard EDTA solution.A
blank titration may be performed by omitting the sample to check the
presence of impurities in reagents.
Eg: ca²⁺,Mg²⁺ & Zn²⁺ion are determined by direct titration with EDTA.

10. BACK TITRATION
 A measured amount of EDTA is added in excess to react with the
metal to be titrated. An indicator is added and the excess of EDTA is
back titrated with a suitable salt solution.
Eg: A solution of ZnSO₄ or MgSO₄ is used for back titration.

11. SUBSTITUTION &
DISPLACEMENT TITRATION
 This type of titration is used for metal ions which form EDTA
complexes which are more stable than other metals such as Mg²⁺ and
Ca²⁺.
Eg: To the calcium salt solution,ammonia-ammonium buffer is
added.To this, a standard known volume of Mg-EDTA solution is added.
In the rection,stable Ca-EDTA complex is formed and Mg ions are
liberated which may be titrated with a standard EDTA solution.

12.  Ca ²⁺ + Mg –EDTA Ca – EDTA + Mg ²⁺
( complex)
 Mg ²⁺ + EDTA Mg - EDTA

13. INDIRECT TITRATION
 Indirect titration method is applicable to compounds having
aluminium or bismuth or mercury.
Eg: Barbiturates are estimated by this method.
Barb + Hg²⁺ Hg – Barb
Hg – Barb + EDTA Hg- EDTA + Excess of EDTA
EDTA + Pb ²⁺ Pb - EDTA

14. ALKALIMETRIC TITRATION
 The solution of disodium EDTA is added to a solution
containing metallic ions ,complexes are formed with liberation of
hydrogen ions.The Hydrogen ions are titrated against alkali
solution (standard) using an acid indicator.

15. MISCELLANEOUS METHODS
 A number of anions have been determined indirectly with EDTA
by reactions based upon complex formation with selected cations.

16. DETECTION OF END POINT
1. Metal ion indicator: It is a compound whose colour changes when
it binds to metal ion
M + Indicator Metal ion indicator
M⁻ + EDTA M⁻ - EDTA complex
2. ERIOCHROME BLACK T

17. APPLICATIONS
 Hardness of water : The hardness is due to both Ca & Mg salts
but the two are determined together in the titration.
 To determine the concentration of metal ions in solution.
 To determine calcium present in a food product.
 To determine concentration of bad metals in the environment.

18. MASKING
 Masking prevents one element from interfering in the analysis of
another element
Al ᶾ⁺ + Mg²⁺ + F⁻ Al F₆ᶾ⁺ + Mg ²⁺
Only Mg²⁺ can be react with EDTA
Note: Masking Al ᶾ⁺ with F ⁻
 Masking agent : CN⁻ , F ⁻

19. DEMASKING
 It is a reagent that release of a metal ion from a masking agent.