Download Ch. 13 EDTA Titrations

Ch. 13 EDTA Titrations
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Chelation in Biochemistry
Chelating ligands can
form complex ions with
metals through multiple
ligands. This is
important in many areas,
especially biochemistry.
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Metal-Chelate Complexes
Metals are Lewis acids that accept electron pairs from
donating ligands that act as Lewis bases
–CN- is a common monodentate ligand, binding to a
metal ion through one atom (C)
–Metals can bind to multiple ligands (usually 6)
A ligand that can attach to a metal by more than one atom
is multidentate or a chelating ligand
Chelating agents can be used for titration of metals to form
complex ions (complexometric titration)
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Chelating Agents in Analytical Chemistry
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Ethylenediamenetetraacetic acid (EDTA)
EDTA forms 1:1 complexes with metal ions by with 6 ligands:
4 O & 2N. EDTA is the most used chelating agent in analytical
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chemistry, e.g. water hardness.
Acid/Base Properties of EDTA
EDTA is a hexaprotic system (H6Y2+) with 4 carboxylic
acids and 2 ammoniums:
O
pK1 0.0
OH
pK 2 1.5
O
pK 3 2.0
+
O
NH
NH
+
OH
pK 4 2.66
pK 5 6.16
OH
O
OH
pK 6 10.24
We usually express the equilibrium for the formation of
complex ion in terms of the Y4- form (all six protons
dissociated). You should not take this to mean that only
the Y4- form reacts
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Fraction of EDTA in Y4- Form
Similar to acids and bases, we can define fractional
compositions, α, defined as the fraction of “
free”EDTA in a
particular form.
–“
Free”means uncomplexed EDTA
–So, for Y4-:
H Y
Y 
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
Y 

H Y 

H Y

H Y 
H Y 
HY 
Y 
4-
Y 4 
2

5
-
4
2-
3
3-
4-
2
4-

EDTA 
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EDTA Complexes
The equilibrium constant for a reaction of metal with
EDTA is called the formation constant, Kf, or the
stability constant:
M nY 4  MY n 4
MY 
M Y 
Kf 
n 4
n
4
Again, Kf could have been defined for any form of EDTA,
it should not be understood that only the Y4- reacts to form
complex ion.
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pH Dependence of αY4-
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Formation Constants for M-EDTA Complexes
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Some Metals Form 7 or 8 Coordinate Complexes
The rings formed in the
M-EDTA complex can
become strained. If the
oxygen atoms pull back
toward the nitrogen
atoms, the strain is
relieved. This opens up
the metal to other
ligands. Water
molecules frequently
occupy these sites.
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Conditional Formation Constant
We saw from the fraction plot that most of the EDTA is not in
the form of Y4- below a pH ~10.
We can derive a more useful equilibrium equation by
rearranging the fraction relationship:
Y   Y  EDTA

EDTA 
4-
Y 4 
4-
Y 4
MY   MY 
M Y  M  EDTA
Kf 
n 4
n
4
n 4
n
Y 4-
If we fix the pH of the titration with a buffer, then αY4- is a
constant that can be combined with Kf
MY 
M EDTA
K f' Y 4- K f 
n 4
n
M nEDTA  MY n 4
K f' Y 4K f
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Example
Calculate the concentration of free Ca2+ in a solution of
0.10 M CaY2- at pH 10 and pH 6. Kf for CaY2- is 4.9x1010
(Table 13-2)
2
2
'
Ca
K f Y 4K f
EDTA  CaY
at pH 10.00, K f' Y 4K f (0.36)(4.9 1010 ) 1.8 1010
at pH 6.00, K f' Y 4K f (2.3 105 )(4.9 1010 ) 1.1106
Ca 2EDTA  CaY 2
Conci
0
0
0.1
Concf
x
x
0.1 - x
CaY  0.1 x
Ca EDTA x
K f' 
2
2
2
 
x Ca 2 2.4 106 M @pH 10
3.0 10-4 M @pH 6
At low pH, the metal-complex is less stable
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Calcium/EDTA Titration Curve
For calcium, the end point
becomes hard to detect below
~pH=8. The formation
constant is too small below
this point. This can be used
to separate metals. At pH=4,
Ca does not perform
significant complexaion with
EDTA. However, Fe can still
form the complex, so it can be
titrated without interference
from Ca.
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Generic Titration Curve
Like a strong acid/strong
base titration, there are three
points on the titration curve of
a metal with EDTA: before,
at, and after the equivalence
point.
We’
ll consider a titration
where we have 50.0 mL of
0.040 M Ca2+ (buffered at
pH=10) with 0.080 M EDTA.
Ve=25.0 mL
K f' (0.36)(4.9 1010 ) 1.8 1010
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Before the Equivalence Point
What’
s pCa2+ when we have added 5.0 mL of EDTA?
25.0 - 5.0 
50.0 
Ca 

(0.040) 0.0291 M
25.0
55.0
2

Fraction
Remaining

Initial
Concentration


Dilution
Factor
pCa 2 log(0.0291) 1.54
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At the Equivalence Point
What’
s pCa2+ when we have added 25.0 mL of EDTA?
–At the equivalence point almost all the metal is in the
form CaY250.0 
CaY (0.040)
 0.0267 M
75.0
2-


Dilution
Factor
Initial
Concentration
–Free Calcium is small and can be found w/ algebra
CaY  0.0267 x 1.8 10
Ca EDTA x
Ca 2EDTA  CaY 2
2
K f' 
Conci
0
0
0.0267
Concf
x
x
0.0267 - x
10
2
2
x 1.2 10 6 M
pCa 2 log(1.2 106 ) 5.91
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After the Equivalence Point
What’
s pCa2+ when we have added 26.0 mL of EDTA?
–We have 1.0 mL excess EDTA
1.0

EDTA 
(0.080) 

3
1.05 10 M
76.0
 
Initial
Concentration
Dilution
Factor
50.0 
CaY (0.040) 
 2.63 10
76.0
2-


Initial
Concentration
CaY   2.63 10
Ca EDTA Ca (1.05 10
K f' 
2
2
3
M
Dilution
Factor
2
2
2
)
1.8 1010
Ca 2  1.4 10 9 M
pCa 2 8.86
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Auxiliary Complexing Agents
In aqueous solution, metal-hydroxide complexes or
precipitates can form, especially at alkaline pH
We often have to use an auxiliary complexing agent
–This is a ligand that binds strongly enough to the metal
to prevent hydroxide precipitation, but weak enough to
be displaced by EDTA
Ammonia is a common auxiliary complex for transition
metals like zinc
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Metal Ion Indicators
To detect the end point of EDTA titrations, we usually use a
metal ion indicator or an ion-selective electrode
Metal ion indicators change color when the metal ion is
bound to EDTA:
MgEbT EDTA  MgEDTA EbT
(Red) (Clear)
(Clear)
(Blue)
–Eriochrome black T is an organic ion
The indicator must bind less strongly than EDTA
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Metal Ion Indicator Compounds
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EDTA Titration Techniques
Direct titration: analyte is titrated with standard EDTA
with solution buffered at a pH where Kf’is large
Back titration: known excess of EDTA is added to
analyte. Excess EDTA is titrated with 2nd metal ion.
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EDTA Titration Techniques (2)
Displacement titration: For metals without a good
indicator ion, the analyte can be treated with excess
Mg(EDTA)2-. The analyte displaces Mg, and than Mg
can be titrated with standard EDTA
Indirect titration: Anions can be analyzed by
precipitation with excess metal ion and then titration of
the metal in the dissolved precipitate with EDTA.
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Example Titration
25.0 mL of an unknown Ni2+ solution was treated with
25.00 mL of 0.05283 M Na2EDTA. The pH of the
solution was buffered to 5.5 and than back-titrated with
17.61 mL of 0.02299 M Zn2+. What was the unknown
Ni2+ M?
Zn 2  Y 4-  ZnY 2mol EDTA (25.00 mL)(0.05283 M) 1.32 mmol EDTA
mol Zn 2 (17.61 mL)(0.02299 M) 0.4049 mmol Zn 2
Ni 2 Y 4-  NiY 2mol Ni 2  1.321 mmol EDTA - 0.4049 mmol Zn 2 0.916 mmol
M Ni 2 (0.916 mmol)/(25.00 mL) 0.0366 M
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