Complex formation Titrations

Theories of Acid –Base Titrations

Arrhenius Theory

Bronsted-Lowry Theory

Lewis Theory

Arrhenius Theory

Aqueous Acid- Base Titration

(Neutralization Titration)

Acid has H+ Base has OH-

Theories of Acid –Base Titrations

Bronsted-Lowry Theory

Non Aqueous

Acid- Base titration

Acid

Proton H+

Donar

Base

Proton H+

Acceptor

Theories of Acid –Base titrations

Lewis Theory

Complexometric titration

Acid

Electron

Acceptor

Base

Electron

Donar

Theories of Acid –Base Titrations

Bonds

Types Of Bonds

Ionic Bond e.g. NaCl

Na+ Cl-

Covalent Bond e.g. O2

O : : O

Coordinate Bond e.g. Mn+ :L

Complexometry

Volumetric method involves reaction of metal with ligand to form complex

Mn+

Electron acceptor Ligand

Electron donor Coordinate bond

Complex formation is a type of acid base reaction according to lewis

Theory.

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where metal ion is lewis acid (electron acceptor) and ligand is lewis

base (electron donor)

: Contain Electron donating atom N , O , S : :

:

: :

lewis acid lewis base

(weakly dissociated stable compound )

Electron donar Electron acceptor

Ligand

Metal

2+ ,3+ , 4+

Coordinate Bond

Complex

weakly dissociated stable compound

_ _

ML

Type of Ligand

Neutral e.g.NH3

Charged e.g.CN-

The complex can form only when…

1. The central atom (a metal ion (or cation) in a complex) accepts an

electron pair from one or more ligands (ligand = electron-pair donating species).

2. The ligand possesses at least one electron pair to donate.

3. The bonding (coordinate bonding) occurs .

A number of common anionic and molecular ligands can form

complexes:

1. Neutral (Molecular) ligands include water, ammonia, RNH2 (amines) C5H5N (pyridine) H2NCH2CH2NH2 (ethylenediamine), etc

2. Anionic ligands include halides, SCN1-, CN1-, OH1-, RCOO1-, S2-, C2O42-

(oxalate), etc.

Metal Ligand

M-L Complex

: 2+

:

Cu2+

NH3

NH3

NH3

NH3

Valency of complex is algebric sum of valency of its components

CuH3N:

NH3

NH3

:NH3

2+

Cu2+

+ 4NH3

..

..

Co-ordination number

2 4 6

centralMetal

CuH3N:

NH3

NH3

:NH3

2+

Cu2+

+ 4NH3

..

..

Co-ordination number is, the number of bonds formed by the central metal ion with ligand.

Co-ordination number

Typical values of co-ordination number are 2,4 or 6.

+ Ag+ 2 CN- [Ag(CN)2]-

+ Ag+ 2 NH3 [Ag(NH3) 2]+

+ Cu2+ 4 CN- [Cu(CN)4]2-

Cu2+ + 4 NH3

[Cu(NH3)4]2+

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Usually double valency of Mn+

[M.H2Ox]n+ + L [M.H2Ox-1.L]n+

[M.H2Ox]n+ Heat

[M. H2Ox]

L

[M.H2Ox-1.L]n+

Labile

Non Labile

Or inert

e.g. Al3+,Co2+, Cr3+, Fe3+

Metal [M.H2Ox]n+

Metal sample

Inert complex

Very Slow

H2O H2Ox-1

Aquo complex

H2O: as a ligand

..

L

H - O - H

The tendency to form complex is inherent property in all metals

Therefore Metals form with water

Complexation reaction is the replacement of solvent molecules by ligand

Aquo complex

Some aquo complexes

undergo substitution

reaction very rapidly and

the complex is said to be

Labile

Others undergo substitution

reaction very Slowly and the

complex is said to be

non Labile

e.g. Al3+, Cr3+, Fe3+

Aquo-complex

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[M.H2Ox]n+ + L [M.H2Ox-1.L]n+

The replacement of solvent molecules by ligand can be repeated several times till the central metal ion takes an optimum number of ligand governed by the maximum co-ordination number of metal ion.

Aquo complex

(solvated metal ion) as oxygen of water donate electrons to metal ions

Ligand Complexing agent

Unidentate

Polydentate

Bidentate

Tridentate

Tetradentate

Ligands that have (or share) only one electron pair are called

unidentate.

1. "Dentate" = a tooth-like projection.

2. For example, ammonia is unidentate...

Cu2+ + 4 NH3 Cu(NH3)42+

Bidentate ligands share two electron pairs. Examples:

Ethylenediamine complexed with copper ion.

Multidentate ligands complexed to metal ions are called

chelates. Chelates always have a "chelate ring." For example,

the magnesium -8-hydroxyquinolate complex.

Metal Ligand

M-L Complex

: 2+

:

Unidentate Ligand

Metal

Ligand

Chelate (ring Structure) Weakly dissociated (insoluble in water)

::

2+

::

Metal

Ligand

Ligand Metal

Ligand

Ligand

Chelating agent

N.B. If soluble in water called complex and ligand called Complexing agent

Examples

H2N-CH2-CH2-NH2 + Cu2+

H2C

H2C

H2N:

NH2

..Cu

CH2

CH2

:NH2

H2N..•Ethylene diamine:

• Bidentate ligand.

•8-hydroxyquinoline N

OH

+ Mg2+2

N

O:

+ 2H+

:

Mg

•1,10-o-phenanthrolene: N

N

+ Fe2+

N

N

2+

Fe

322

2

2+

Six membered ring

e.g. Diethylene triamine H2C CH2

NHNH2

H2C CH2

NH2

H2N-CH2-CH2-NH-CH2-CH2-NH2.. .. ..

e.g. Triethylene tetramine (Trien)

H2C CH2

NHNH2

H2C CH2

NHNH2 CH2

CH2

• Tridentate ligand.

• Tetradentate ligand

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Stability of complex

2 4 6

centralMetal

As the points of attachment increases , the stability of the complex increases

Which one is more stable ?

Metal Stability Constant

[Cu (NH3)4]2+ 6

[Cu (en)2]2+

9

[Cu (Trien)2]2+

10.5

Cu Edtate(hexadentate) 18.8

•Stepwise formation of complex with unidentate ligands give no sharp end point.

Chelates are more stable than complexes formed by unidentate ligands, due to the presence of more than one point of attachment of ligand to Mn+

Polydentate ligands are preferable as titrant if compared with unidentate ligands because:

•Polydentate ligands react with metal completely (in one step and rapid) forming stable complex leading to sharp end point

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Types of complexing agents. (ligand) •Unidentate ligands

Attached to the central Mn+ from one point of attachment

e.g. NH3, H2O, CN- ,halide.

Example : Formation of copper amine complex

CuH3N:

NH3

NH3

:NH3

2+

Cu2+

+ 4NH3

..

..

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Polydentate ligand

Attached to the central Mn+ from more than one point of attachment forming

Soluble complex Ring structure with metal

(Chelate).

(inorganic molecules)

Ligands are called chelating agents and the complexes are called chelates.

Chelating agents are organic molecules

usually water insoluble,(gravimetry) except EDTA(water soluble) (Complexometric Titrimetry)

called Sequestering agent

Unidentate Polydentate Type of molecule

Inorganic Organic

Point of attachment

One point More than one

Reaction stepwise One step

Inflection gradual Sudden

End point gradual

Sharp

Stability Less stable More Stable

Titrimetric reaction

non applicable Used as titrant

Example NH3 Disodium salt of EDTA

-Compare unidentate and polydentate -Ammonia couldn’t use as titrant -Polydentate ligand used as titrant -Polydentate ligand form more stable complexes - than unidentate

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Complex

Ligand

Metal Metal : Acid Ligand : Base

Factors Affecting Stability of the Complex

Metal

Electronegativity

Ionic Size

Ionic Charge

Ligand

Electronegativity

Size of Ligand

Structure of final chelate

Extent of chelation

Electronegativity : Ability to attract the outer shell electrons

Factors affecting stability of complex

The higher the acidity (electro-negativity) of central metal ion (Mn+) the higher the stability of the formed complex.

Acidity of Mn+ depends on:

•Intensity of + ve charge on Mn+

m

QF

radius

Mn+ of small radius and high + ve charge has high ability to form stable complex.

1- Ability of central metal ion to form complex.

intensity of + ve charge.

Charge

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2+

2+

m

QF F = intensity of + ve charge Q = Charge ,

m = radius

3+ 2+

Fe3+ [Fe(CN)6]3

more stable Fe2+

[ Fe(CN)6]4 -

Acidity of central Metal

Ligand

.

.

.

.

.

-

-

-

-

Mn+

M L

[ML]n+

2- Ability of ligand to form complex.

• Basicity of ligand.

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Basicity of ligand is its electron donating ability. Therefore ligand must contain electron donating atom e.g.

N , O , S , X- (I- , Br- , Cl- , F-)

Organic ligands(must contain co-ordinate or acidic groups or both of them)

Co-ordinate groups Acidic groups

Amino group - NH2 Carboxylic acid – COOH

Alcoholic – OH,

Phenolic – OH

Cyclic nitrogen - N =

oxime –NOH

Co-ordinate groups : Able to loss electrons Acidic group : H+ could be easily substituted

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Stablest chelates are formed by saturated ligand that forms five-

membered ring or by unsaturated ligand forming six-membered ring.

3- Structure of the final chelate.

Large, bulky ligands form less stable complexes than smaller ones due to steric effect.

H2N - CH2 - CH2 - NH2

Ethylene diamine

Form more stable complexes

N CH2 CH2 N

H3C

H3CCH3

CH3

form less stable complexes

• Size of ligand

EthyleneDiamine Tetra-acetic Acid, EDTA

N CH2 CH2 N

CH2COOH

CH2COOHHOOC.H2C

HOOC.H2C

The di-sodium salt of EDTA which is soluble in water forms soluble chelates and called sequestering agent, therefore it is the most suitable titrant.

EDTA as titrant in complexometric titration

The disodium salt of EDTA is abbreviated by Na2H2Y

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N CH2 CH2 N

CH2 COO H

CH2COO NaNa OOC.H2C

H OOC .H2C

Y

-+ - +

Y

The following equilibrium is established when Na2H2y is dissolved in water

Na2H2Y 2Na+ + H2Y2-

, Complexon III

Complex Formation Titrations

The most commonly used chelon (or titrant) in metal ion titrations is

EDTA (ethylenediaminetetracetic acid).

EDTA

• Ethylene diamine tetraacetic acid. H4EDTA => H4Y

ethylenediaminetetraacetate anion

EDTA-4 => Y-4

N CH2 CH2 N

CH2 COO H

CH2COO NaNa OOC.H2C

H OOC .H2C

Y

-+ - +

EDTA is hexadentate, donating one electron pair from

each of the two amine group and one electron pair from

each of the four carboxylates to the bound metal ion.

• Every divalent element in the periodic table can be

determined by titrating with EDTA and forming complexes

while sodium and potassium forming salts.

• EDTA forms a "cage" around metal ions, like a spider

grasping a fly.

EDTA is used as a titrant for the determination of

water hardness ( calcium and magnesium).

•EDTA is a tetraprotic acid,

Standard solutions of EDTA are usually prepared by

dissolving the Na2H2Y.2H2O in a volumetric flask.

It has low price

The special structure which has 6 ligand atoms(Hexadentate ligand)

Very stable complexes

EDTA

Typical sequestering agent

1

2

3

4

5

6

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

M4+ + H2Y2- MY0 + 2H+

M3+ + H2Y2- MY- + 2H+

M2+ + H2Y2- MY2- + 2H+

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1) Mn+ displace (2H+) in H2Y2- producing 2 gm. mol. of hydrogen.

2) EDTA is not selective reagent

3) The reaction between EDTA and Mn+ is according to M.wt., Reacts in ratio Of 1:1 with all metals therefore EDTA is used as molar standard

4) Formation or dissociation of the formed complexes is affected by pH.

The reversibility of the reaction can be prevented by addition of buffer or alkali to shift the reaction forward and hence stability of chelate is increased.

It was found that, EDTA reacts with different Mn+ in the ratio of 1:1 irrespective to the valency of Mn+

Alkali

Or alkaline buffer

-EDTA standard prepared as molar solution -Alkaline Buffer was used in complexometric titration -EDTA isn't selective

Stability of a complex is evaluated by its stability constant (or formation constant). Assume a reaction between metal ion (Mn+) and EDTA (H2Y

2-)

Stability and stability constant of complex

According to the law of mass action:

]YH][M[

]H][My[K

22

n

2)4n(

eq

As Keq. represents the rate of formation of the complex, it is called as formation constant (Kf) or stability constant (Kst.).

]YH][M[

]My[K

22

n

)4n(

.st

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

Vf

Vb

In alkaline buffered medium; Kst. is represented as follows:

the greater the Kst the greater the rate of forward reaction and stability of the formed complex.

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MY(n-4) Mn+ + H2Y2-

[Mn+ ][ H2Y2-]

[MY(n-4)] K inst =

It was found that, reaction producing complex with Kst. ~ 108 is sufficiently complete at the equivalence point for a feasible titration.

Kinstability= 1

Kst

It is clear that, the greater the stability constant, the sharper the inflection in pM at the end point.

Complexometric titration curve:

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if pM (= -log [Mn+]) is plotted against the volume of EDTA solution added, an inflection in pM occurs at the equivalence point.

]YH][M[

]My[K

22

n

)4n(

.st

[Mn+] -log [Mn+].

Stability Constant

Effect of pH on complexometric titration

The effect of pH on the stability of complex, which represents titration of calcium salt at different pH values.

It is clear form the titration curves, that stability of metal - EDTA is increased by increase of pH.

Stability Constant

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