Transcript complexometric reactons and titration

COMPLEXOMETRIC REACTIONS
AND TITRATIONS
DR. A.K.M. SHAFIQUL ISLAM
Complexes
• Complexation reactions are widely applied through
complexometric titration in order to determine the metal
ions, present in the solution
• Metals ions, especially transition metals, act as Lewis
acids, because they accept electrons from Lewis bases
• When metal cations combine with Lewis bases, the
resulting species is called a complex ion
• This also called coordination complex
• The base is called a ligand
Complexes
• When the metals are covalently bonded
with surrounding ions or molecules the
resulting species are called metal
complexes or coordinate complex
• The surrounding ions or molecules are
called ligands
Coordination Number
• Coordination number = the
•
•
number of ligands
surrounding a central cation
in a transition metal
complex.
Common coordination
numbers are 2, 4 and 6
The geometries of the
ligands about the central
atom are as shown
• For example, copper (II) has coordination
•
•
•
number of four. The species formed from such
coordination or complexing, can be electrically
positive, neutral or negative.
Copper when complexed with ammonia results
in a cationic complex, Cu(NH3)42+,
when complexed with glycine, a neutral
complex, Cu(NH2CH2COO)2,
when complexed with chloride, an anionic
complex, CuCl42-.
Metal Cations Form Complex Ions
• Complex Ion = transition metal cation surrounded by
•
•
LIGANDS
Ligand = molecule or ions that have nonbonding electron
pairs
Bonding is called “coordination”
Terms Defined
• Complex formation – the process whereby a
species with one or more unshared electron
pairs forms coordinate bonds with metal
ions.
• Ligand – an ion or molecule that forms a
covalent bond with a cation or a neutral
metal atom by donating a pair of electrons
that are then shared by the two.
Terms Defined
• Chelating agent – substance with multiple
sites available for coordination bonding
with metal ions. Such bonding typically
results in the formation of five or six
member rings
• Dentate – (Latin) having toothlike
projections
• Some common inorganic ligands are ammonia,
•
•
•
water, and halides.
A ligand that has one donor group such as
ammonia, is called unidentate.
Glycine, which has two groups available for
covalent bonding, (the carbonyl oxygen and the
aminal nitrogen), is called bidentate.
As titrants, multidentate ligands, particularly
tetradentate and hexadentate chelating agents,
those having four or six donor groups, have two
advantages over their unidentate titrants.
• When a metal cation is complexed to ligands
•
forming a neutral compound, the complex is
called coordinated compound.1
A chelate is produced when a metal ion
coordinates with two or more donor groups of a
single ligand to form a five or six membered
heterocyclic ring. The copper complex of glycine,
is an example of a simple chelate
Metal Chelate Complex
O
Cu2+
+ 2
H2N CHC OH
H
O
O
O
Cu
N
H2
N
H2
O
Chelon Effect
• Chelating is the ability of multidentate ligands to
form more stable metal complexes than those
formed by monodentate or bidentate ligands.
• These reactions happen over the monodentate
because of favored thermodynamics.
• This results a larger Kf value for multidentate
complexes. This is known as chelon effect or
chelate effect.
Thermodynamic favorable
• The delta H’s for mono and multidentates
are generally comparable.
• However, the delta S’ s (entropy) favors a
reaction with the multidentate.
• ΔG° = ΔH° - TΔS°
• The chemical reaction is spontaneous when the free energy
change, G is negative, and d G=H – TS.
• The enthalpy change for legands with similar groups is often
similar. For example, four ammonia molecules complexed to
Cu2+ and four amino group from two ethylenediamine
molecule complexed to Cu2+ will result in about the same
release of heat.
• However, more disorder or entropy is created by the
dissociation of the Cu(NH3)42+ complex in which five species
are formed than in the dissociation of the
Cu(H2NCH2CH2NH2)22+ complex, in which three species are
formed.
• Hence, S is greater for former dissociation, creating a
more negative G and a greater tendency for dissociation.
• Thus, multidenate complexes are more stable (have large Kf
values), largely because of the entropy effect.
• First, these multidentate titrants, generally react
more completely with cations, thereby providing
sharper more accurately end points.
• Second, they ordinarily react with metal ions in a
single-step process,
• whereas with unidentate ligands usually involves
two or more intermediate species.
Ligand
• An example of a hexadendate ligand is
EDTA (Ethylenediaminetetraacetic Acid). It
has six potential sites for complex
formation – the electron pairs on the two
nitrogen atoms and the four electron-rich
carboxyl groups.
EDTA Structure
Neutral EDTA is a tetrabasic acid
H4 Y  H  H3 Y 
H3 Y   H  H2 Y 2 
H2 Y 2   H  HY 3 
HY 3   H  Y 4 
K a1  1.0  10 - 2
[H  ] [H3 Y  ]

[H4 Y]
K a2  2.2  10 - 3
[H  ] [H2 Y 2  ]

[H 4 Y]
K a3  6.9  10 - 7
[H  ] [HY 3  ]

[H4 Y]
K a4  5.5  10 -1 1
[H  ] [Y 4  ]

[H4 Y]
Disodium EDTA
• Since Y4- is the ligand species in complex
formation, the complexation equilibria are
affected markedly by the pH.
• H4Y has a very low solubility in water, and so
that disodium salt Na2H2Y,2H2O is used.
• This salt dissociates in solution to give H2Y2-, pH
of this solution is approximately 4 to 5.
EDTA Complex with Metal Ions
(1) Forms strong 1:1 complexes regardless of the
charge on the cation
(2) Chelate with all cations
(3) Since the anion Y4- is the ligand species in complex
formation, the complexation equilibria are affected
markedly by the pH.
(4) The formation constant are in Table (next slide)
2
Ca
Y
4
 CaY
2
2
[CaY ]
Kf 
2
[Ca ] [Y4-]
Table of Formation Constants for EDTA Complexes
Cation
Kf
Log Kf
Ag+
2.1 x 107
7.32
Mg2+
4.9 x 108
8.69
Ca2+
5.0 x 1010
10.70
Sr2+
4.3 x 108
8.63
Ba2+
5.8 x 107
7.76
Mn2+
6.2 x 1013
13.79
Fe2+
2.1 x 1014
14.33
Co2+
2.0 x 1016
16.31
Ni2+
4.2 x 1018
18.62
Cu2+
6.3 x 1018
18.80
Zn2+
3.2 x 1016
16.50
Cd2+
2.9 x 1016
16.46
Hg2+
6.3 x 1021
21.80
Pb2+
1.1 x 1018
18.04
Al3+
1.3 x 1016
16.13
Fe3+
1.3 x 1025
25.1
V3+
7.9 x 1025
25.9
Th4+
1.6 x 1025
23.2
Effect of pH on EDTA equilibria
H+
CaY2
Ca2+ + Y4-
HY3-
H+
H+
H2Y2-
H3Y-
CH 4Y
CH4Y  [Ca 2 ]
From the overall equilibrium
Ca2+ + H4Y
CaY2- + 4H+
CH4Y  [H 4 Y]  [H3Y ]  [H 2 Y2 ]  [HY 3 ]  [Y 4 ]
H+
H4Y
Let us consider that CH4Y represent the total
uncomplexed EDTA
  
 
 

CH 4Y  Y 4  HY 3  H 2Y 2  H 3Y   H 4Y 
If we substitute the values of [HY3-], [H2Y2-], [H3Y-] and [H4Y]
derived from the Ka values to this equation and divide each term
with [Y4-], we will get the following equation:-
CH 4Y
Y 
4

1
4
H   H 
 1

Ka4
 2
K  3 K 4

H 
 3
K  2 K  3 K 4

H 
 4
K 1 K  2 K 3 K  4
Where α4 is the fraction of the total EDTA exists as Y4- .
Y 4
4 
C H 4Y
Y 4   4CH 4Y
Effect of pH on EDTA equilibria
From the distribution of EDTA species as function of pH we
can see that above pH 10, most of the EDTA exist as Y4form.
At lower pH values, the protonated species are
dominating, hydronium ions compete with EDTA for
binding the metal ions. Thus, at those pH using Kabs to
calculate the formation of EDTA metal complex will be
misleading.
Obviously as the pH goes down, there will be more
dissociation than formation. In this situation, the Kabs and
all the Ka values of EDTA will be involved for calculation.
If we consider the following chemical reaction of EDTA with any
metal, Mn+
Mn+ + Y4-= MY-(4-n)
Then, the formation constant or Kf will be
Kf = [MY-(4-n)] / [Mn+] [Y4-]
Now, substituting the [Y4-], we can rewrite the above equation as
follows:-
Kf = [MY-(4-n)] / [Mn+] α4 CH4Y
Kf ’ = Kf α4 = [MY-(4-n)] / [Mn+] CH4Y
Kf’ is called conditional solubility constant or effective solubility
constant.
Effect of pH on EDTA Titration of Ca2+
Less distinct
end point
EDTA Titration Curve
Region 1
Excess Mn+ left after each addition
of EDTA. Conc. of free metal
equal to conc. of unreacted Mn+.
Region 2
Equivalence point:[Mn+] = [EDTA]
Some free Mn+ generated by
MYn-4  Mn+ + EDTA
Region 3
Excess EDTA. Virtually all metal
in MYn-4 form.
EDTA Titration Curves for Ca2+ and Sr2+
(Buffered at pH 10)
*Ca2+ end point more distinct.
*Lower pH, Kf ’ decreases, &
End point less distinct.
*We cannot raised pH
arbitrarily:
Metal hydroxides might
precipitate.
Metal Ion Indicators
Compounds changing colour when binding to metal ion.
Kf for Metal-In < Kf for Metal-EDTA.
Before Titration:
•
Mg2+
+
(colourless)
In

(blue)
MgIn
(red)
During Titration: Before the end point
•
Mg2+
+
EDTA

MgEDTA
(free Mg2+ ions) (Solution red due to MgIn complex)
At the end point:
3. MgIn
+
EDTA
(red)
(colourless)

MgEDTA
(colourless)
+
In
(Blue)
Indicators for EDTA titration
Erichrome Black T (EBT)
The structure of Eriochrome Black T is as follows:-
Calmagite
Eriochrome Black T is, unfortunately, unstable in
solution and solutions must be freshly prepared in
order to obtain the proper color change. It is still
widely used, but another indicator of similar
structure, called calmagite, has been developed. Its
structure is as follows:-
EDTA Titration Techniques
1. Direct Titration
*Buffer analyte to pH where Kf’ for MYn-2 is large,
*and M-In colour distinct from free In colour.
*Auxiliary complexing agent may be used.
2. Back Titration
*Known excess std EDTA added.
*Excess EDTA then titrated with a std sol’n of a second
metal ion.
*Note: Std metal ion for back titration must not displace
analyte from MYn-2 complex.
2. Back Titration: When to apply it
*Analyte precipitates in the absence of EDTA.
*Analyte reacts too slowly with EDTA.
*Analyte blocks indicator
3. Displacement Titration
*Metal ions with no satisfactory indicator.
*Analyte treated with excess Mg(EDTA)2Mn+ + MgYn-2

MYn-4
* Kf’ for MYn-2 > Kf’ for MgYn-2
+
Mg2+
4. Indirect Titration
*Anions analysed: CO32-, CrO42-, S2-, and SO42-.
Precipitate SO42- with excess Ba2+ at pH 1.
*BaSO4(s) washed & boiled with excess EDTA at pH 10.
BaSO4(s) + EDTA(aq) 
BaY2-(aq)
+ SO42-(aq)
Excess EDTA back titrated:EDTA(aq) + Mg2+MgY2-(aq)
Alternatively: *Precipitate SO42- with excess
Ba2+ at pH 1.
*Filter & wash precipitate.
*Treat excess metal ion in filtrate with EDTA.
5. Masking
*Masking Agent: Protects some component of analyte
from reacting with EDTA.
*F- masks Hg2+, Fe3+, Ti4+, and Be2+.
*CN- masks Cd2+, Zn2+, Hg2+, Co2+, Cu+, Ag+, Ni2+, Pd2+,
Pt2+, Hg2+, Fe2+, and Fe3+,
but not Mg2+, Ca2+, Mn2+, Pb2+.
*Triethanolamine: Al3+, Fe3+, and Mn2+.
*2,3-dimercapto-1-propanol: Bi3+, Cd2+, Cu2+, Hg2+,
and Pb2+.
*Demasking: Releasing masking agent from analyte.
OH
M CN 
nm
m
Mn+

 mH 2 CO  mH  mH2C
Metal-Cyanide Formaldehyde
Complex
CN
*Oxidation with H2O2 releases Cu2+ from
Cu+-Thiourea complex.
*Thus, analyte selectivity:
1. pH control
2. Masking
3. Demasking