Complexometric Titrations
Complexometric formation reaction:
It is the combination of metal ion with electron
donating group to form a complex. The molecule
bound to the metal ion is called the ligand. The
complex has different properties from that metal
ion.
M2+
+
nL
[MLn]2+
electron acceptor + electron donor
(ligand)
Complex
The metal ion acts as Lewis acid (electron acceptor)
and the ligand is used as a titrant or a complex
forming agent which acts as a Lewis base
(electron donor).
• Although the reaction between a metal and a
ligand can be considered as an acid base
interaction, but it was found that metal ions
cannot be titrated with simple ligands as
ammonia or cyanide, because of lack of a sharp
end point.
Note:
Complexometric titrations are only possible if:
The complex formation reaction occurs in one step
with a high stability constant.
Factors affecting the stability of the
complex:
I) Ability of central metal ion to
form complex
II) Ability of ligand to form complex
I) Ability of central metal ion to form
complex
1-Acidity of Mn+: Acidity means electron accepting power
As the intensity of +ve charge increases electron accepting
power increases The most stable complex.
2- Ionic size:
The smallest ion (radius) The greatest electric field The most
stable complex.
3- Ionic charge :
For ions of equal size, the one with higher charge forms more
stable complex eg.: Ferricyanide [Fe(CN)6]-3 is more stable than
ferrocyanide [Fe(CN)6]-4
Therefore, a large (charge/radius) ratio leads to stable complex.
II) Ability of ligand to form complex:
1-Basicity of ligand:
Basicity of ligand is its electron donating ability. As the basicity of ligand
increase
its tendency to donate electrons increase
The most stable
complex. Therefore ligand must contain electron donating atom. Ñ, Ŏ, Š.
2- Size of ligand:
The biggest the ligand (radius)
The greatest electron donating ability
The most stable complex.
3- Steric effect:
Large, bulky or branched ligands form less stable complexes.
e.g: Ethylene diamine form more stable complex than its tetramethyl
derivative.
Remember that:
The coordination number (n) of the metal ion represents the
number of coordinate bonds around the central metal.
e.g: Ag+ + 2CN[Ag(CN)2]n=2
Cu2+ + 4NH3[Cu(NH3)4]2+
n=4
Fe3+ + 6SCN[Fe(SCN)6]3n=6
Types of Ligands:
According to the number of points of attachments to the metal ion
1- Monodentate Ligands:
are ligands which are bound to the metal ion at only one point.
Example: NH3 , CN- , FCu2+ + NH3
Cu(NH)32+
2- Bidentate Ligands:
are ligands which have two atoms, each of which has a lone pair of electrons
and form two coordinate bonds with the metal ion.
Example: Ehtylene diamine H2N-CH2-CH2-NH2
3- Multidentate Ligands:
are ligands which contain more than two coordinating atoms per molecule.
Example: Ethylene diamine tetra acetic acid (EDTA).
Complexometric titration Cont.
Metal Indicators
These indicators are dyes that can form a soluble
complex with the metal ion, resulting in a color
that is different from the dye itself. The metal-dye
complex must be less stable than the metal-EDTA
complex.
MgIn- + H2Y2MgY2- + HIn2- + H+
Indicator complex
free indicator
Eriochrome black T(E.B.T)
This indicator is blue at about pH 10 (using ammonia
buffer) and most of its complexes are reddish. It is used
at pH 10 for the determination of Ca2+, Mg2+, Zn2+ ,
Cd2+.
E.P: : wine red
blue.
Structure of E.B.T indicator (free indicator is blue)
MurexideMurexide
This indicator is violet at pH 12 (using 8% NaOH)
and most of its complexes are reddish. Since
magnesium-murexide is less stable than the
calcium- murexide, murexide indicator can be
used for the determination of calcium in
presence of magnesium.
Structure of murexide indicator (free indicator is
violet)
E.P: red
violet
EDTA Titration Methods
1.Direct titration
2. Back titration
3. Replacement or Substitution titration
4. Alkalimetric titration
5. Miscellaneous methods
• 1.Direct titration:
The solution containing the metal ion to be determined
is buffered to the desired pH (e.g. to pH 10 with
ammonia buffer) and titrated directly with the
standard EDTA solution.
At the equivalent point, the concentration of the metal
ion being determined decreases abruptly. This is
generally, dtermined by the change in the colour of a
metal indicator.
2. Back titration:
Many metals can’t be titrated directly; because:
1) They may precipitate from the solution in the pH of
the titration such as Aluimium forms hydroxide
2) They may form inert complexes with the indicator
3) There is no suitable metal indicator available.
An excess standard EDTA solution is added, the resulting
solution is buffered to the desired pH, and the excess
EDTA is back titrated with a standard metal ion solution
(ZnCl2 or ZnSO4 or MgCl2 or MgSO4).
The end point is detected with the metal indicator colour
which responds, to the zinc or magnesium ions
introduced in the back titration.
3. Replacement or Substitution titration:
Used for:
1) Metal ions that do not react (or react unsatisfactory)
with a metal indicator,
2) Sharp end point is not obtained.
This method is suitable for metal ions which form EDTA
complexes that are more stable than those of other
metals such as magnesium and calcium.
The metal cation Mn+ to be determined may be treated
with the magnesium complex of EDTA, where the
following reaction occurs:
Mn+ + MgY2MY2- + Mg2+
The amount of free magnesium ion is equivalent to the
metal present and can be titrated with a standard
solution of EDTA and a suitable metal indicator.
Example:
In the direct titration of calcium ions, E.B.T gives
poor end point; if magnesium is present, it is
displaced from its EDTA complex by calcium
and an improved end point results.
Ca2+ + MgY2MY2- + Mg2+
4. Alkalimetric titration:
When a solution of disodium
ethylenediaminetetraacetate, Na2H2Y, is added to
a solution containing metallic ions, complexes are
formed with the liberation of two equivalents of
hydrogen ions:
M2+ + H2Y2-
MY2- + 2H+
The free hydrogen ions can be titrated with a
standard solution of sodium hydroxide using an
acid base indicator. The solution of the metal to
be determined must be accurately neutralized
before titration.
5. Miscellaneous methods:
Exchange reactions between the
tetracyanonickelate(II) ions [Ni(CN)4]2- and the
element to be determined, where nickel ions
are set free. Thus silver and gold, which
themselves can’t be titrated
complexometrically, can be determined in this
way.
[Ni(CN)4]2- + 2 Ag+
2 [Ag(CN)2]- + Ni2+
Kcomplex stability
EDTA Titration Curve
pM
Sample: 0.01M Metal (M)
Titrant: 0.01M EDTA
What we need?
Draw the titration curve i.e. plot the .1
volume of
EDTA added versus
CALCULATED pM
2. Estimate the End Point
3. Select the suitable indicator
4. What did you understand from the
shape
i.e. stable of the curve (inflexion)
or unstable
complex at the giving pH??????
1016
16
14
12
1010
10
8
106
6
104
4
2
2
4
6
8
10 12
0.01M EDTA, mL
20
•
Titration curves for 0.1 M Ca2+ versus 0.1 M EDTA at pH 7 and pH 10 are shown in
Figure 4. This figure indicates the effect of pH on apparent stability constants and
correspondingly on the shape of the titration curve.
More
Stable
pCa
More
inflexion
Less
Less
Stable inflexion
mL EDTA
Fig.4: Titration curves of Ca2+ with EDTA at pH 7 and pH 10.
22
Titration of Mixtures, Selectivity, Masking and
Demasking Agents
EDTA is a very unselective reagent because it
complexes with many divalent, trivalent,
tetravalent cations.
The following procedures will help to increase
the selectivity
a) Suitable control of the pH of the solution:
• Bismuth Bi3+ and Th4+ can be titrated in an acidic
solution (pH=2) with xylenol orange as indicator
and most divalent cations don’t interfere.
• A mixture of bismuth and lead ions can be
successfully titrated by first titrating the bismuth
at pH 2 with xylenol orange as indicator, and then
adding hexamine to raise the pH to about 5, and
titrating the lead.
Q. How to analyse a mixture of Bismuth and
Lead?
A: By suitable control of the pH: by titrating Bi3+ #
EDTA at pH 2 with X.O as indicator and then
adding hexamine to raise pH to 5 and titrate Pb2+
# EDTA.
b) Use of masking agents:
Masking: Means a process in which a substance, without
physical separation, is transformed that it does not enter
into a particular reaction.
Demasking: It is a process in which the masked substance
regains its ability to enter a particular reaction.
By the use of masking agents, some of the cations in a mixture
can often be masked, so they can no longer react with EDTA
or with the indicator.
Example of masking agent is the cyanide ion; which forms
very stable cyanide complex with Cd2+, Zn2+, Hg2+, Cu2+,
Co2+, Ni2+, Ag+, Pt, whereas, manganese and lead form weak
complex, meanwhile Ca2+, Mg2+ do not form complex at all
with cyanide.
M2+ + CN[M (CN)4]2It’s possible to determine cations such as Ca2+, Mg2+, Pb2+, and
Mn2+ in presence of the above mentioned metals by
masking with an excess of potassium or sodium cyanide.
c) Selective demasking
The cyanide complexes of zinc and cadmium
may be demasked with formaldehyde-acetic
acid solution, which will give Zn2+ & Cd2+free
again.
[Zn(CN)4]2-+ 4H+ +4HCHO
Zn2++ HO.CH2.CN
A solution containing Mg2+, Zn2+ and Cu2+ can be
titrated as follows:
1- By back titration # standard Mg2+ solution using
E.B.T as indicator
Total metals.(1)
2- Treat another portion with excess KCN and
titrate # EDTA
Mg2+
(2)
3- Add excess of formaldehyde-acetic acid solution
to the above titrated solution in order to liberate
Zn2+ from the cyanide complex, and titrate with
EDTA tell the indicator turns blue
Zn2+ only. (3)
4- Cu2+ content can be found by difference.
Q. How to analyse a mixture of Mg2+, Zn2+ and
Cu2+?
How to analyse a mixture of Ca2+ and Mg2+?
1- By titration # EDTA in ammonia buffer with
E.B.T as indicator
total.
2- By titration another portion # EDTA in 8%
NaOH with murexide indicator
Ca2+ only.
Q. How to increase selectivity of EDTA? ‫هام‬
By control pH and masking & demasking, with
examples. ‫نشرح االمثله‬
• Titrations involving unidentate ligands
1- Cyanometric titrations
Liebeg's & Denige’s method
• 2- Mercurimetric titrations
•
1- Cyanometric titrations
(AgNO3 titrant)
• Liebeg's method
+
Ag
+ 2CN
-
Ag+ + Ag(CN)2-
-
Ag(CN)2
Ag[Ag(CN)2]
Deniges modification of Liebeg’s method
Indicator: IEP: AgI
• 2- Mercurimetric titrations
(Hg(NO3)2 titrant)
• 4I- + Hg2+ = (HgI4)2• (HgI4)2- + Hg2+ = 2 HgI2
Yes
I do not know
No
Mark(√) for the correct statement and (x) for the wrong one
•
EDTA is used as a precipitating agent for metal ions (N)
•
Coordination numbers of EDTA with Cu2+ at pH 10.0 is equal to three (N)
•
Copper-EDTA complex is more stable than copper ammine complex (Y)
•
More coordination number the more stable complex (Y)
•
Only one ionization form of EDTA at pH 13. (Y)
•
EDTA chemically is considered as strong acid. (N)
•
Metal:EDTA complex ratio is 1:1 Stoichiometry (Y)
•
The stability EDTA – Metal complexes are sensitive to pH change. (Y)
•
The apparent Stability constant of EDTA is considered one at pH 13. (Y)
•
Cd-EDTA complex is more stable at pH 2 than pH 11 (N).
32