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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.
7
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+
15
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
17
[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
24
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
27
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
..
..
28
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
35
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.
39
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
40
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
41
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+
45
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.
47
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:
49
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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