Complexometric
titrations
1
Complexometric titrations
Many analytical methods rely on formation of
complexes.
Gravimetry
Spectrophotometry
Titrations
Ni-dimethylglyoxime
Fe-phenanthroline
Metal- EDTA
2
Complexometric titrations
Determinations of metals
Titrate metal ions with titrant which is a
complexing agent
EDTA – very useful complexing agent
EDTA titration for determination of water
hardness (how much Ca and Mg?)
3
4
5
Lewis acid-base concept
Lewis acid => electron pair acceptor
metal
Lewis base = electron pair donor
ligand
coordinate covalent bond
ligand donates both electrons of the
electron pair bond
6
Formation of soluble complex between metal
ion and a complexing agent
Metal ion acts as a Lewis acid (accepts
electron pairs)
Complexing agent (ligand) acts a Lewis
base (donates electron pairs)
Covalent bond between metal and ligand.
7
Ligands
8
UJIAN 2
Date
Time
Venue
Topics: POLYPROTIC ACID COMPLEXOMETRY
9
10
11
12
13
14
15
16
17
Chelate Effect
the ability of multidentate ligands to
form more stable metal complexes than
those formed by similar monodentate
ligands
results from the formation of 5membered "ring" with metal and two
atoms on the ligand
18
Chelating agent
Has two/more groups that complex with
metal ion
Chelate – complex formed
EDTA – most widely used chelating
agent in titrations
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20
21
22
Co-EDTA
Complex
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Metal-ligand complex
24
Pt(en)2 Complex
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Formation constant, Kf
Ag+ + NH3
Ag(NH3)+
Kf1= [Ag(NH3)+] = 103.3
[Ag+][NH3]
Ag(NH3)+ + NH3
Ag(NH3)2+
Kf2 = [Ag(NH3)2+]
= 103.8
[Ag(NH3)+][NH3]
26
Overall formation constant
Ag+ + 2NH3
Ag(NH3)2+
Kf = Kf1 . Kf2
= [Ag(NH3)2+ ] = 107.1
[Ag+][NH3]2
27
Overall formation constant
Zinc ion forms the complex, Zn(NH3)4 2+
in a stepwise fashion.
M + nL
MLn
Kf1= Kf
Kf1Kf2 = Kf
Kf1Kf2Kf3 = Kf
Kf1Kf2Kf3Kf4 = Kf
MLn
Kf1Kf2Kf3Kf4..Kfn = Kf
28
Complexometric titrations
Monodentate ligands rarely used as titrants
Sharp end point generally difficult to achieve
Stepwise formation constants are frequently
close together, not very large, single
stoichiometric complex cannot be observed)
Most generally useful titrating agents
Aminocarboxylic acids – nitrogen and carboxylate
groups as ligands
1:1 metal-complex formed
29
EDTA
H4Y has v.low solubility in water
Disodium salt, Na2H2Y.2H2O is used to
prepare EDTA standard solutions;
dissociates to give predominantly
H2Y2pH EDTA will influence distribution of
H4Y, H3Y-, H2Y2-, Y4- .
30
EDTA, H4Y
H4Y
H+
+
H3Y-
Ka1 = 1.0 X 10-2
H3Y-
H+
+
H2Y2-
Ka2 = 2.2 X 10-3
H2Y2-
H+
+
HY3-
Ka3 = 6.9 X 10-7
HY3-
H+
+
Y4-
Ka4 = 5.5 X 10-11
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32
Fraction that exists as Y4-, α4
CH4Y = [Y4-] + [HY3-] + [H2Y2-] + [H3Y-] + [H4Y]
Y4- = [Y 4-] = α4
CH4Y
Ka1Ka2Ka3Ka4
αY4-=
[H+]4 +Ka1[H+]3 + Ka1Ka2[H+]2 +
Ka1Ka2Ka3[H+] + Ka1Ka2Ka3Ka4
Y4- predominant at pH > 11.0
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Metal ion complex with EDTA.
Ca2+ + Y4CaY2Kf = [CaY2-]
[Y4-] influenced by pH.
[Ca2+][ Y4-]
K f = [CaY2-]
[Ca2+] α4CH4Y
Kfα4 = K’f = [CaY2-]__
[Ca2+]CH4Y
K’f - conditional formation constant,
dependent on pH
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35
TEST 2
Date:
Time:
Venue:
Topics: Acid-Base Equilibria
36
Titration of 50.0 mL 0.100 M Ca2+
with 0.100 M EDTA standard solution
Ca
2+
+ Y4- → CaY2- Kf = 5.0 x 1010
Ca2+ solution is buffered at pH 10.0.
Use K’ because CH4Y can be easily
determined compared to [Y4-].
K’ = α4 Kf = 5.0 x 1010 x 0.35
= 1.8 x 1010
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Before equivalence point
0.00 mL EDTA
pCa = - log [Ca2+] = - log (0.100) =
1.000
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Before equivalence point
25.00 mL EDTA
2+ = uncomplexed Ca2+ +
Amount of Ca
Ca2+ from dissociation of CaY2-.
Ca2+ + Y4- → CaY2Initial
5.00
∆
2.50
≡m
2.50
2.50
[Ca2+] = 2.50 mmol = 0.0333 M pCa = ?
75.00 mL
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Equivalence point, 50.00 mL
EDTA
Initial
∆
≡m
[CaY 2-]
[Ca2+]CH4Y
Ca2+ + Y45.00
5.00
xM
xM
→ CaY2-
{(5.00/100.00) –x}M
≈ 0.0500 M
(K’ v.large)
= (0.0500) = 1.8 x 1010
(x)(x)
x= [Ca2+] = ?
pCa = ?
40
After equivalence point
75.00 mL EDTA
Ca2+ + Y4- →
CaY2Initial
5.00
∆
7.50
≡m
x
(2.50 + x)
5.00 – x
125.00
125.00
[CaY2-]
= (0.0400) = 1.8 x 1010
[Ca2+]CH4Y (x)0.0200
neglect x, K’ v. large
x = [Ca2+] = ?
pCa =
41
After equivalence point
100.00 mL EDTA
Millimoles EDTA added = 100.00 x 0.100
= 10.00 mmol
Millimoles EDTA in excess = 5.00 mmol
[CaY2- ] = 5.00/150.00 M = 0.033 M
CH4Y = 5.00/150.00 = 0.033 M
Amount of CaY2- which dissociates is negligible.
[CaY 2- ]
= (0.033) = 1.8 x 1010
[Ca 2+ ]CH4Y
(x)0.033
x = [Ca2+] =?
pCa =?
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43
Minimum pH
for effective
titration of
various metal
ions
44
Detection of end point
Organic dye
Eriochrome Black T, triprotic acid, H3In
H2In-
H+
+
red
HIn2-
blue
HIn2-
Ka2 = 5.0 X 10-7
blue
H+
+
In3-
Ka3 = 2.8 X 10-12
orange
EDTA titrations at pH 8 and 10 – HIn2- anion (blue)
is predominant.
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Detection of end point
Mn+ + HIn2-
blue
MIn(n-3) + H+
red
Use a small amount of indicator
Indicator forms red complex with
metal ion
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Titration of zinc(II) with EDTA
Before equivalence point, red color is due
to formation of zinc-indicator complex.
Addition of EDTA titrant will complex free
zinc ions.
After free zinc ions has been titrated, EDTA
will react with zinc from zinc-indicator
complex
ZnIn+
HY3- → ZnY2+
HIn2
red
blue
47
Color of solution will gradually change
from red to blue.
Equivalence point reached when all ZnInhas been changed to ZnY2- and the
solution is a pure blue color.
48
Distribution of metal species in
polyligand complexes
CAg = [Ag+] + [Ag(NH3)+] + [Ag(NH3)2+]
Rewrite as :
CAg = [Ag+] +Kf1[Ag+][NH3] +
Kf1Kf2[Ag+][NH3]2
49
βAg+ = [Ag+]
CAg
β
Ag(NH3)+
= [Ag(NH3)+] = Kf1[Ag+][NH3]
CAg
β
Ag(NH3)2+
CAg
= [Ag(NH3)2+]
CAg
=Kf1Kf2[Ag+][NH3]2
CAg
The sum of all β must be 1.
50
=
1_________________
1 +Kf1[NH3] + Kf1Kf2[NH3]2
β Ag(NH3)+ =
Kf1[NH3]_______________
1 +Kf1[NH3] + Kf1Kf2[NH3]2
β Ag(NH3)2+ =
Kf1Kf2[NH3]2__________
1 +Kf1[NH3] + Kf1Kf2[NH3]2
β
Ag+
Fraction of each species depends on
ammonia concentration.
51
Application of EDTA titrations
EDTA does not have high selectivity
For samples with mixture of metal ions,
require separation of ions, e.g. by
pH control
precipitation
or extraction
52
Minimum pH
for effective
titration of
various metal
ions
53
Analytical Applications
Determination of water hardness titration of magnesium and calcium in the
presence of cadmium, cobalt, copper,
nickel, zinc ions –
use masking agents such as cyanide ion.
Cyanide ions complexes interfering ions.
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55
Fig 1-f2, p.15
Analytical applications
As masking agent – form complexes
with unwanted/interfering metal ions.
As spectrophotometric reagent, e.g.,
determination of
iron by 1,10-phenanthroline,
copper by dithizone.
56
Water Test Kit from Hach
Semiquantitative determination of metal ions in
water samples based on complexometry
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Crown ethers – complexing agents of high selectivity
58
Food Industry
Use in products such as mayonnaise to
extend the shelf-life
EDTA will complex with cations,
preventing them from acting as catalyst
in oxidation process.
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Medical applications..
Detoxification in heavy metal poisoning
cases.
Na2CaY is administered.
Heavy metals (e.g. Pb) form more stable
EDTA complex than calcium, and will
displace calcium from EDTA.
The chelated lead is excreted via the
kidneys.
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CHELATION THERAPY
AND THALASSEMIA
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Drugs Used for Iron Therapy
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Structure of desferrioxamine B
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Oxygen is carried in the
circulatory system by the
iron-containing protein,
hemoglobin, When there
is not enough hemoglobin
in the red blood cells,
oxygen cannot get to all
parts of the body. Organs
then become starved for
oxygen and are unable to
function properly.
Hemoglobin
consists of two pairs of
subunits, designated as α
or β.
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β -thalassemia major is a genetic
disease in which the β subunits
of hemoglobin are not
synthesised in sufficient amount.
Children afflicted with this
disease can survive only with
frequent transfusion of normal
red blood cells.
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People with Thalassemia
disease are not able to make
enough hemoglobin, which
causes severe anemia.
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The problem with transfusions is that
the patient accumulates 4-8 g of iron
per year. The body has no
mechanism for excreting large
quantities of iron, so iron builds up in
all tissues.
Most victims die by the age of 20
from the toxic effects of iron
overload.
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Chelation therapy
Chelators are small molecules that can
bind selectively with iron and make it
soluble. The chelated iron is readily
excreted from the body.
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Drugs Used for Iron Therapy
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Structure of desferrioxamine B
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CHELATION THERAPY AND
THALASSEMIA
Deferoxamine is poorly absorbed
if taken orally. It is also so
rapidly eliminated from the body
that it must be administered by
a slow infusion to have the
appropriate therapeutic
effect. A needle is attached to a
small subcutaneous pump five to
seven times a week for up to 12
hours. It is a difficult and
painful procedure.
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IDEAL DRUG FOR TOMORROW
Water soluble
High specificity
Cheap
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END
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