Coplexometric Titration

     

Complexometric (Chelatometry) titration

It is a form of volumetric analysis in which coloured complex are formed and they indicate the end point of titration.

is useful for the determination of mixture of different metal ion solution.

 An indicator capable to producing an unambiguous colour (clear) change so they denoted end point of the titration.

This method are also called chelatometry titration, chilatometry and EDTA titrations.

Reactions used

1.   When reaction reaches on equilibrium point they show each portion of titrant is added and mix well.

2.   Any types of Interference situations are not arise. If (for concentration), the stepwise formation are used to several different complexes of the metal ion with the titrant. Then the presence of more than one complex in solution during the titration process.

3.   A complexometric indicator capable to locate equivalence point with accuracy are available.

    By the use of EDTA (ethylene diamine tetra-acetic acid) as a titrant is well established.

Principle:-

In this titration metal ions are titrated with complex / chelating agent (ligand).

This method are involved to transfer simple ion into complex ion and they determined the equivalence point by use of metal indicators (Eriochrome black T, xylenol orange, murexide, fast sulphon black, methyl thymol blue etc).

Ligands:- A ligand is an ion or molecule which donates a pair of electrons to the central metal atom or ion to form a coordination complex. The word ligand is from to Latin, which means “tie or bind”.  Ligands have electron donating ability & bind to the metal ion to form a complex.

Types of ligands- there are mainly two types of ligands.

1.     Unidentate ligands:- This ligands are bound to metal ion at only one place. Eg- NH3 are bound to cupric ions, halide ions, cyanide ions.

2.     Bidentate & multidentate ligands- these ligands are contain more than one group and they capable to bind with metal ions

3.     Also they include in two donor atoms eg- ethylene diamine

4.     Tridentate ligands have three donor atoms

5.     Quadridentate ligands have four donor atoms.

Classification of complexometric titration:- They are classified into four types

       i.          Direct titration

      ii.          Back titration

    iii.          Replacement titration

    iv.          Indirect titration

 

      I.          Direct titration- This titration method are similar to acid base titration. In this titration standard chelon solution added into the metal ion solution till the end point.

Limitations:

·       It’s slow reaction

·       It may be affected due to presence of other ions.

    II.          Back titration:-In this method standard solution of EDTA are added into metal solution which are analysed and they back titrated with standard solution of second another metal solution.

   III.          Replacement Titration- In this method metal ions are analysed quantitatively to displace the metal ions from the complex.

  IV.          Indirect Titration (Alkalimetric Titration):- This method are used to determination of anions which are do not react with EDTA chelate.

So, the protons of EDTA are displaced by any heavy metal and titrated with sodium alkali.

Masking & Demasking Reagents

Masking Reagents:-

It is a complexing agent which prevents interference in solution by determination of selectively reactant.

This agent are act with precipitation / formed complexes with more stability of ion – EDTA complex.

Method of masking:-

v Masking by Precipitation:- heavy metals like copper, lead and carbon mono oxide are can be separated by using insoluble sulphides like- sodium sulphide  or thioacetamide.

These separated compounds are filtered, decomposed, and titrated with disodium EDTA.

Ø Masking by complex formation- high stability complex of metal ions are used as masking agent. These masking agent should not form complexes with metal ions being analysed.

·       Eg: Ammonium fluoride- they used to mask aluminium, iron, & titanium.

·       Ascorbic acid- for reducing iron

·       Potassium iodide- mercury ion, mercury chloride.

·       Potassium cyanide- silver, copper, mercury, iron , zinc, cadmium, cobalt, nickel etc.

v Demasking agent- The demasking agent are enable to masked substance which involve into a particular reaction.

Ø Methods of Demasking –

·       PH Control Method- The metal chelate is dependent on the pH of reaction medium. In weak acid solution the chelates (Bi, Fe++, Cr) of many metals completely dissociate.

·       Use of Selective Metals Indicators- These indicators are the metal complexing agent which react with different different metal ions. For metal indicator specific ion are used.

·       Classical separation- Cations that can be determined by complexometric method are calcium carbonate, nickel dimethylglyoximate.

·       Kinetic Masking-  in any case if  metal ion does not form any complex reaction because of its kinetic inertness so, slow reaction of chromium with EDTA make possible titrate with other metal ions they react rapidly without interference from Cromium.

Estimation (Assay) of magnesium sulphate- It can be determined by complexometric titrations.

Materials required:

·       Magnesium sulphate heptahydrate: 0.3gm

·       Strong ammonia ammonium chloride solution: (6.75gm NH4₄Cl){ 74.0ml strong ammonia solution + 100 ml water}

·       0.05M disodium edetate

·       Mordant black 2 mixture of 0.1gm

Procedure:-

·       Take 0.3 gm of magnesium sulphate hepta-hydrate dissolved in 50ml distilled water in beaker.

·       Now 10 ml of strong ammonia ammonium chloride solution are added.

·       This solution are titrated with 0.5M EDTA using 0.1gm of mordant black 2 mixture as indicator.

·       This titration are continued till the pink colour is converted into the blue colour. 

NOTE: Each ml of 0.05M disodium EDTA = 0.00602 gm of MgSO₄

 Estimation (Assay) of Calcium Gluconate-        

Calcium are determined in calcium gluconate by complexometric titration by the use of standard solution of EDTA

Materials required-

·       AR grade calcium carbonate (Standard calcium chloride)

·       Calcium gluconate tablet

·       Ammonia ammonium chloride buffer

·       Eriochrome black T indicator

·       Standard EDTA solution

Procedure:-

·       Firstly we take calcium gluconate crushed tablet with blade of stainless steel knife and powder mixed with spatula.

·       Crushed powder are dried at 100⁰C on oven for 1 hour.

·       Now 0.4gm sample is weighed and transfer it into three each 250ml conical flask.

·       Each sample is dissolved into 50ml deionised worm water.

·       In first sample 5ml of ammonia-ammonium chloride buffer and 5 drops of eriochrome black T indicator is add.

·       Now this solution is titrated with standard EDTA solution till the colour change from wine to pure blue.

·       Like this procedure repeated with remaining two other samples.