This document provides information about complexometric titration using EDTA. It defines complexometric titration and complexation involving the formation of complexes between metal ions and ligands. It describes the chemistry of EDTA, including its acid-base properties and ability to form stable complexes with most metal ions. The document discusses factors that affect the stability of metal-EDTA complexes and provides examples of different titration methods that can be used including direct titration, back titration, indirect titration and masking-demasking techniques.
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group 2 analytical chemistry.pptx
1. MUNI UNIVERSITY
FACULTY OF SCIENCE
DEPARTMENT OF BIOLOGY
CHM 2201 INTRODUCTORY TO ANALYTICAL CHEMISTRY
GROUPASSIGNMENT
COURSE FACILITATOR; MR. MUHWEZI GODFREY
GROUP MEMBERS
IRIMAASO YONAH
KAYAGA SAM
LUBULWA HENRY
TAMALE YAHAYA
RIZUYO JOZEPHINE
ADRANI AMIDU
MUGISHA BERNAD
2. Definitions
Complexometric titration
Is a form of volumetric analysis in which the formation of a colored
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 solution.
An indicator capable of producing an unambiguous color change is usually
used to detect the end point of the titration.
3. Complexometry : is the type of volumetric analysis involving the
formation of complexes which are slightly ionized in solution, like weak
electrolyte and sparingly soluble salt.
Complex is formed by the reaction of metal ion (Mn+) with either an
anion e.g. [Ag(CN)2]- or neutral molecule e.g. [Ag(NH3)2]+
The metal ion is known as Central metal atom.
The anion or neutral molecule is known as Ligand (L)
4. M+ + L ML
Ag+ + 2 CN- [Ag(CN)2]-
Cu2+ + 4 CN- [Cu(CN)4]2-
Ag+ + 2 NH3 [Ag(NH3)2]+
Cu2+ + 4 NH3 [Cu(NH3)4]2+
Central metal atom = acts as Lewis acid (electron acceptor)
Ligand = acts as Lewis base (electron donor)
5. Coordinate bond (dative) = The bond formed between central metal atom (ion)
(acceptor) and the Ligand (donor)
Dative bond is similar to covalent bond (formed of two electrons) But in dative bond
the electrons pair are donated from one atom to the other. The atom gives electron pair
is known as donor, while the atom accept electron pair is known as acceptor.
The bond is represented by an arrow () from donor to acceptor.
NH3
NH3 Cu NH3
NH3
6. * Coordination number = The no. of coordinate bonds formed to a metal ion
by its ligands.
* Characters of coordination number *
1- It is even number: 2 e.g. Ag+ , 4 e.g. Ni2+ , Cu2+ , 6 e.g. Fe3+ , Cr3+
2- It is usually double the charge of the metal.
The charge of a complex is the algebraic sum of the charges of the central ion
and ligand .. e.g.
[Ag(CN)2] - Ag+ + 2 CN -
1 (+ve) + 2 (-ve) = 1 (-ve)
e.g. [Fe(CN)6]3- Fe3+ + 6 CN -
3 (+ve) + 6 (-ve) = 3 (-ve)
The higher the valence of metal ion the more stable the complex e.g.
Ferricyanide is more stable than Ferrocyanide
7. Types of complexing agents (( Classification of ligands according to the no. of sites of
attachment to the metal ion ))
• Unidentate (Monodentate) Ligand or "Simple Ligand"
• The ligand attached to metal at one site e.g. H2O , NH3 , CN - , Cl - , I - ,
Br - , (i.e. forming one coordinate bond, or capable of donating one
unshared pair of electrons)
9. • Tridentate Ligand:
The Ligand attached to metal at 3 sites
• Tetradentate Ligand:
The Ligand attached to metal at 4 sites
Diethylene triamine
Triethylene tetramine
10. Chelation
• Chelate : It is a complex formed between the ligand containing
two or more donor groups and metal to form ring structure.
(heterocyclic rings or chelate rings).
• Chelating agents: organic molecules containing two or more
donor groups which combine with metal to form complex
having ring structure.
• Chelates are usually insoluble in water but soluble in organic
solvent.
11. Sequestering agents
This is an organic compound capable of linking metal ions or molecules
together to form complex ring-like structures known as chelates.
Sequestering agents are used to link undesirable metal ions together to
form a stable structure that does not readily decompose.
Sequestering agents include chelants and threshold inhibitors
12. Factors affecting stability of complex
• [A]- Effect of central metal ion :
• (1)- Ionic size (metal radius):
• - Smaller an ion (small radius of metal) greater its electrical field
more stable complex
• (2)- Ionic charge (metal charge):
• - Metal of higher charge give more stable complexes. e.g. Ferricyanide
[hexacyanoferrate III] is more stable than Ferrocyanide [hexocyanoferrate
II].
13. • Electronegativity :
• The higher acidity (electronegativity) of metal (Mn+) the higher
stability of complex.
• (4)- Metal which has incomplete outer shell (has high acidity) have
more tendency to accept electrons more stable complex. e.g. Ca2+
, Ni2+ , Zn2+ , Mn2+ , Cu2+
14. • [B]- Effect of Ligand:
• [1]- Basic character:
• - The higher the basicity (strong base is good electron
donor) the higher the ability of ligand to form complex.
e.g. ligand contain electron donating atom.
• e.g. N > O > S > I- > Br- > Cl- > F-
• [2]- The extent of chelation:
• - Multidentate ligands form more stable complexes than
monodentate.
15. 3. Steric effect:
- Large, bulky ligand form less stable complexes than smaller ones
due to steric effect. e.g. ethylene diamine complexes are more stable
than those of the corresponding tetramethyl ethylene diamine.
16. Use of EDTA in complexometric
titration
EDTA replaces the indicator to form a more stable complex
with metal, and when the reaction is completed, the change
for the colour is observed.
The most common indicators in Complexometric titrations
are organic dyes which function by forming a coloured
complex with the metal ion being titrated
17. Metallochromic indicators
These are metal-ion indicators that have been developed for use in
Complexometric titrations with EDTA and other chelating reagents.
These are organic dyes exhibiting marked acido-basic colour changes and
containing a grouping capable of chelate bond formation directly joined to
the conjugate system.
The metallochromic indicators are organic compounds which are capable of
forming intensely Coloured complex with EDTA.
This metal indicator complex is weaker than the metal-EDTA complex and
it has different colour than uncomplexed indicator.
18. During the course of titration, the metal ion from metal-indicator
complex is replaced to form metal-EDTA complex.
. Examples of such indicators include
• Eriochrome black T.
• Calcon and
• Murexide.
19. Chemistry and properties of EDTA
• EDTA is a hexadentate ligand, containing 4 oxygen and 2 nitrogen
donor.
• It is a white powder
• It is soluble in sodium carbonate, ammonia and boiling water.
• It is insoluble in ethanol and organic solvents.
20. It reacts with most cations (divalent, trivalent, tetravalent) (except "Alkali
group.") forming freely soluble stable complexes.
The formed complexes contain the metal and EDTA in the ratio of 1:1
irrespective to the charge of the metal ion.
The general reactions for the formation of metal – EDTA complexes as follows
M2+ + H2Y2- MY2- + 2 H+
M3+ + H2Y2- MY- + 2 H+
M4+ + H2Y2- MYo + 2 H+
Mn+ + H2Y2- (MY)n-4 + 2 H+
21. 2 moles of hydrogen ions are formed in each case.
EDTA is not selective chelating agent.
Formation or dissociation of complexes is affected by pH.
i)- In acidic medium: i.e. [H+] ionization of EDTA
stability of metal – EDTA complex shift the reaction backward.
ii)- In slightly alkaline solution the reaction is forward
stability of complexes (chelates).
iii)- In strong alkali pptn. of metal as hydroxide.
23. Metal-EDTA Formation constants.
The equilibrium constant for the reaction of a metal with a ligand is called the formation
constant, Kf, or the stability constant.
1.) EDTA (Ethylenediaminetetraacetic acid)
One of the most common chelating agents used for complexometric titrations in
analytical chemistry.
EDTA has 6 nitrogens & oxygens in its structure giving it 6 free electron pairs
that it can donate to metal ions.
- High Kf values
- 6 acid-base sites in its structure
24. 2.) Acid-Base Forms
EDTA exists in up to 7 different acid-base forms depending on
the solution pH.
The most basic form (Y4-) is the one which primarily reacts
with metal ions.
25. EDTA
2.) Acid-Base Forms
Fraction (a) of the most basic form of EDTA (Y4-) is defined by
the H+ concentration and acid-base equilibrium constants
EDTA
Y
Y
HY
Y
H
Y
H
Y
H
Y
H
Y
H
Y
4
Y
4
3
2
2
3
4
5
2
6
4
Y
4
4
]
[
]
[
]
[
]
[
]
[
]
[
]
[
]
[
]
[
a
a
Fraction (a) of EDTA in the form Y4-:
where [EDTA] is the total concentration of all free EDTA species in solution
}
]
[
]
[
]
[
]
[
]
[
]
{[ 2
3
4
5
6
6
5
4
3
2
1
5
4
3
2
1
4
3
2
1
3
2
1
2
1
1
6
5
4
3
2
1
Y
K
K
K
K
K
K
K
K
K
K
K
H
K
K
K
K
H
K
K
K
H
K
K
H
K
H
H
K
K
K
K
K
K
4
a
aY4- is depended on the pH of the solution
26. 3.) EDTA Complexes
The basic form of EDTA (Y4-) reacts with most metal ions to
form a 1:1 complex.
- Other forms of EDTA will also chelate metal ions
Recall: the concentration of Y4- and the total concentration of
EDTA is solution [EDTA] are related as follows:
]
][
[
]
[
4
n
4
n-
f
Y
M
MY
K
Note: This reaction only involves Y4-, but not the other forms of EDTA
EDTA
Y 4
Y
4
a
]
[
where aY4-is dependent on pH
27. 3.) EDTA Complexes
The basic form of EDTA (Y4-) reacts with most metal ions to
form a 1:1 complex.
28. EDTA
3.) EDTA Complexes
Substitute [Y4-] into Kf equation
If pH is fixed by a buffer, then aY4- is a constant that can be combined with Kf
]
][
[
]
[
4
n
4
n-
f
Y
M
MY
K
EDTA
Y 4
Y
4
a
]
[
]
[
]
[
]
[
-
4
Y
EDTA
M
MY
K n
4
n-
f
a
where [EDTA] is the total
concentration of EDTA added to
the solution not bound to metal
ions
]
][
[
]
[
-
4
Y
EDTA
M
MY
K
K
K n
4
n-
f
'
f
a
Conditional or effective formation
constant:
(at a given pH)
29. 2.) Example
Final titration curve for 50.0 ml of 0.0500 M Mg2+ with 0.0500 m EDTA at pH
10.00.
- Also shown is the titration of 50.0 mL of 0.0500 M Zn2+
Note: the equivalence point is sharper for Zn2+
vs. Mg2+. This is due to Zn2+ having a larger
formation constant.
The completeness of these reactions is
dependent on aY4- and correspondingly pH.
pH is an important factor in setting the completeness
and selectivity of an EDTA titration
30. EDTA titration methods
Complexometric titrations with EDTA is used for the determination of any metal ion
with the exception of alkaline metals (group I metals).
Displacement titrations
In displacement titrations an excess of a solution containing EDTA in
the form of a magnesium or zinc complex is introduced; if the metal
ions form a more stable complex than that of magnesium or zinc, the
following reactions occurs.
MgY2- + M2- = MY2- + Mg2-
The liberated magnesium is then titrated with a standard EDTA solution.
This technique is useful where no satisfactory indicators is available for
the metal ion being determined.
31. Back titration.
A known excess of standard solution of EDTA is added to the solution
containing the analyte.
When the reaction is complete, excess EDTA is back titrated to end
point using a magnesium or zinc standard solution with eriochrome
black T.
This procedure is useful for determining cations that form stable
complexes with EDTA and for which there is no effective indicator. It is
also often used for those species of ions which react very slowly with
EDTA.
32. Indirect titrations
This method is used to determine the ions such as halides, phosphate, and
sulphate that do not form complex with EDTA.
In the determination of sulphate ion, SO4
2- ion solution is treated with
excess of standard of barium ions.
The formed precipitate of barium sulphate (BaSO4) is filtered off and
unreacted barium ions present in the filtrate is titrated with EDTA.
In this we are able to indirectly determine the amount of sulphate ion
present in the sample solution.
33. Masking-demasking
In this technique, the one of the cation is first masked using a masking agent
and the remaining free cation is titrated with standard EDTA.
A masking agent is a reagent that protects some components of the analyte
from reaction with EDTA.
For example Al3+ in a mixture of Mg2+ and Al3+ can be measured by first
masking the Al3+ with F-, there by leaving only the Mg2+ to react with EDTA.
The cyanide anion is one of the most common masking agents, masking cd2+,
Zn2+, Hg+, Co2+, Cu2+, Ag2+, Ni2+ ,Pd2+ but not Mg2+ , Ca2+ .
Then previously masked cation is demasked by using suitable reagent to get
free cation. This free cation is then titrated by using standard EDTA.
34. Sample calculations on EDTA titrations
1.) Titration Curve
The titration of a metal ion with EDTA is similar to the titration of a
strong acid (M+) with a weak base (EDTA)
The Titration Curve has three distinct regions:
- Before the equivalence point (excess Mn+)
- At the equivalence point ([EDTA]=[Mn+]
- After the equivalence point (excess EDTA)
-
4
Y
a
f
'
f K
K
]
[
n
M
log
pM
35. 2.) Example
What is the value of [Mn+] and pM for 50.0 ml of a 0.0500 M
Mg2+ solution buffered at pH 10.00 and titrated with 0.0500 m
EDTA when (a) 5.0 mL, (b) 50.0 mL and (c) 51.0 mL EDTA is
added?
Kf = 108.79 = 6.2x108
aY4- at pH 10.0 = 0.30
mL
V
M
mL
M
mL
V e
e 00
.
50
)
0500
.
0
(
00
.
50
0500
.
0
)
(
mL EDTA at equivalence point:
mmol of EDTA mmol of Mg2+
36. 2.) Example
(a) Before Equivalence Point ( 5.0 mL of EDTA)
Before the equivalence point, the [Mn+] is equal to the
concentration of excess unreacted Mn+. Dissociation of MYn-4 is
negligible.
]
[
)]
)(
(
-
)
)(
[(
]
[
L
0050
.
0
L
0500
.
0
L
0050
.
0
M EDTA
0500
.
0
L
0500
.
0
M Mg
0500
.
0
Mg
2
2
moles of Mg2+
originally present moles of EDTA added
Original volume
solution
Volume titrant
added
39
.
1
Mg
log
pMg
M
0409
.
0
Mg 2
2
2
]
[
]
[
Dilution effect
37. 2.)Example
(b) At Equivalence Point ( 50.0 mL of EDTA)
Virtually all of the metal ion is now in the form MgY2-
)
(
)
(
)
(
]
[
L
0500
.
0
L
0500
.
0
L
0500
.
0
M
0500
.
0
MgY2
Original [Mn+]
Original volume of
Mn+ solution
Original volume
solution
Volume titrant
added
Dilution effect
Moles Mg+ ≡ moles MgY2-
M
0250
.
0
MgY 2
]
[
38. 2.)Example
(b) At Equivalence Point ( 50.0 mL of EDTA)
The concentration of free Mg2+ is then calculated as
follows:
Initial Concentration (M) 0 0 0.0250
Final Concentration (M) x x 0.0250 - x
]
][
[
]
)
[
EDTA
Mg
EDTA
(
Mg
K
K 2
2
-
Y
f
'
4
f
a
)
x
)(
x
(
)
x
0250
.
0
(
)
30
.
0
)(
10
2
.
6
( 8
Solve for x using the quadratic equation:
94
.
4
pMg
10
16
.
1
EDTA
Mg
x 2
5
2
]
[
]
[
39. 2.) Example
(c) After the Equivalence Point ( 51.0 mL of EDTA)
Virtually all of the metal ion is now in the form MgY2- and
there is excess, unreacted EDTA. A small amount of free
Mn+ exists in equilibrium with MgY4- and EDTA.
)
(
)
)(
(
]
[
L
0510
.
0
L
0500
.
0
L
0010
.
0
M
0500
.
0
EDTA
Original [EDTA]
Volume excess
titrant
Original volume
solution
Volume titrant
added Dilution effect
Excess moles EDTA
M
10
95
.
4
EDTA 4
]
[
Calculate excess [EDTA]:
40. 2.) Example
(c) After the Equivalence Point ( 51.0 mL of EDTA)
Calculate [MgY2-]:
)
(
)
(
)
(
]
[
L
0510
.
0
L
0500
.
0
L
0500
.
0
M
0500
.
0
MgY2
Original [Mn+]
Original volume of
Mn+ solution
Original volume
solution
Volume titrant
added
Dilution effect
Moles Mg+ ≡ moles MgY2-
M
0248
.
0
MgY 2
]
[
Only Difference
41. 2.)Example
(c) After the Equivalence Point ( 51.0 mL of EDTA)
[Mg2+-] is given by the equilibrium expression using [EDTA] and [MgY2-]:
]
][
[
]
)
[
EDTA
Mg
EDTA
(
Mg
K
K 2
2
-
Y
f
'
4
f
a
)
M
10
95
.
4
)(
x
(
)
M
0248
.
0
(
)
30
.
0
)(
10
2
.
6
( 4
8
57
.
6
pMg
10
7
.
2
Mg
x 2
7
2
]
[
