Complexation TitrationDepartment of physical sciences & technologyuniversity of sabaragamuwa srilanka(PST-Group 11)

1) What is a Titration?

2) What are the equipment Using Titration

3) Equipment cleaning technique

4) Titration type

5) How Complexation Titration differ from normal Titration

6) Indicators

7) EDTA Titration

8) Analyzing

The process of adding standard solution until the

reaction completes, it’s called a titration.

The reaction completion is identified by the colour

change of the indicator.

Main equipment when using titration

1. Titration flask

2. Pipette

3. Pipette bulb

4. Burette

Titration flask is only washed using distilled water.

Burette is first washed by using distilled water, then

it is washed by using titrant(solution having known

concentration).

Pipette is first washed by using distilled water, then

it is washed by using the analyte(solution

concentration that has to be checked).

1) Neutralization titrations

2) Complex formation titrations

3) Precipitation titrations

4) Oxidation-Reduction Titrations

Complexometric titration is a form of volumetric analysis in which

the formation of a coloured complex is used to indicate the end

point of a titration. Complexometric titrations are particularly

useful for the determination of a mixture of different metal ions in

a solution

Complexion Normal

At the end point colourful complex

formation forms.

At the end point precipitate, solution

may form.

Used by metal indicator & metal ion

indicator.

Normally acid base indicator can be

used or sometimes it is not necessary to

use the indicator.

Complex: metal + ligand

Ligand: has at least one pair of unshared electrons available for bond formation.

Complex formation has a specific colour.

(electron donor) Ex: H2O, NH3, Cl-, Br-, I-……

~ ~ An ion or a molecule that forms a covalent bond with a cation or a neutral metal atom by donating a pair of electrons, which is then shared by the two atoms.

Ex: [Cu(NH3)4]2+, [CuCl4]

2-, [Cr(NH3)6]3+

Metal ligand Complexes

Complexation Titrations are based on the reaction of a metal ion with a chemical agent to form a metal-ligand complex.

Complexation Titrations are essentially Lewis acid-base reactions, in which an electron pair is donated from one chemical to another.

The ligands used in complexometric titrations are also known as chelating agents.

- Ligand that attaches to a metal ion through more than one ligand atom

Most chelating agents contain N or O

- Elements that contain free electron pairs that may be donated to a metal

Metal Ligand Metal-Ligand Complex

Metal – Lewis Acid or Electron-pair acceptor

Ligand – Lewis Base or Electron-pair donor

• Complex formation reactions are reversible. So there have

formation constant(Kf)

The equilibrium constant for the reaction between a

metal ion (M+n) and a chelating agent (L-P) is known as

a formation constant or stability constant.

][Y ][M

]Y [MK

Y M YM

b

ab

f

abb

a

a

Properties of EDTA

1.) EDTA (Ethylenediaminetetraacetic acid)

One of the most common chelating agents used for complexometric titrations in

analytical chemistry.

EDTA has 6 hydrogen atoms. 2 atoms from ammine group & 4 atoms from

carboxylic group. So in its structure it gives 6 free electron pairs that it can donate

to metal ions.

- High Kf values

- 6 acid-base sites in its structure

2) EDTA is an example of a multidentate (many-toothed) ligand, which can bind metal ions through

multiple atoms.

3) EDTA is an amphoteric substance.

4) It can both donate and accept protons.

5)The four hydrogen atoms shown in the above line structure are acidic.

6) Because of this, the formula of EDTA is often abbreviated H4Y, where H4

represents the four acidic hydrogen atoms and Y represents the

remaining structure.

7)At very low pH, EDTA will be present in its completely protonated form

H6Y2+.

8)At very high pH, EDTA will be present in its completely deprotonated

form, Y4-.

EDTA can represent as different types

Compounds which having same

chemical properties like EDTA

EGTA (ethylene glycol tetraacetic acid)

Metal-Ion Indicator

Objective of using metal ion indicator

1) Detect the end point of EDTA titration.

2) Generally in an EDTA titration a pH meter with pH electrode is

necessary. But using an EDTA indicator ,the end point of a

titration can be detected.

3) Metal ion indicators change colour when they bind to a metal

ion. Useful indicators must bind to the metal. (must give up

their metal ions to EDTA) So the indicator change its colour

with different pH values.

Detection of the color change

1) The use of a metal ion indicator in an EDTA titration can be expressed as,

M-In + EDTA M-EDTA + In

2) The color change of indicator affected by hydrogen ion concentration of medium.

H2In- HIn2- In3-

pHpH

5.3 -

7.310.5 -

12.5

Yellow -

orangered blue

Type of EDTA titrations Direct titration

Indirect titration

Back titration

Displacement titration

Types of EDTA titrations

Direct Titration

Simply add an indicator to the solution of the metal ion and titrate

with EDTA. Before starting the titration, it is needed to check the

pH of the solution.

Indirect titration

EDTA can be used as titrant for anions. Anions can be

precipitated with suitable metal ion. Filter and wash the ppt with

proper solution. Then boil in excess EDTA to complex the metal

ion(ppt).

Back Titration

An excess of EDTA is added to the metal ion solution, and the excess EDTA is

titrated with a known concentration of a second metal ion. The second metal ion

must form a weaker complex with EDTA than the analyte ion.

Displacement titration

The analyte is treated with an excess of a second metal bound to EDTA. The

analyte ion displaces the second metal from the EDTA complex, and then the

second metal is titrated with EDTA.

• EDTA is able to form stable 1:1 complex ions with many different

metal ions, and can therefore be used in titrations to determine the

concentrations of these metal ions in a solution.

• Because the reactions used in these titrations involve the formation of

complex ions, they are called complexiometric Titrations.

1. L.P. Usgodaarachchi2. M.H.K. Madushani3. D.D.J.C.Thilakasiri4. A.P.N.M.Pathirana5. T.M. Perera6. M.K. Perera