EDTA Titrations
Chelation in Biochemistry
Chelating ligands can form
complex ions with metals
through multiple ligands. This
is important in many areas,
especially biochemistry.
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)
Chelating Agents in Analytical
Chemistry
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 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
O
NH
pK 3  2.0
+
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
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-:
Y 4- 
Y 
H Y  H Y  H Y  H Y  H Y  HY  Y 
Y 
4
2
6

4-
EDTA 

5
-
4
3
2-
2
3-
4-
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 
n4
Kf
n
4
• Again, Kf could have been defined for any form of
EDTA, it should not be understood that only the Y4reacts to form complex ion.
pH Dependence of αY4-
Formation Constants for M-EDTA
Complexes
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.
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 
4
Y 


EDTA 


MY 
MY 
K 

M Y  M  EDTA 
4-
 Y 4-   Y 4 EDTA 
n4
n
f
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
K f'   Y4- K f

MY 

M EDTA 
n4
n
Mn  EDTA  MY n4
Kf'   Y4 Kf
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)
Kf'   Y4 Kf
Ca 2  EDTA  CaY2
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
Conc i
0
0
0.1
Conc f
x
x
0.1 - x

CaY 
0.1  x


Ca EDTA  x
2
K f'
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
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.
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
Before the Equivalence Point
• What’s pCa2+ when we have added 5.0 mL of
EDTA?
Ca 
2
 25.0 - 5.0 
 50.0 

 (0.040)
  0.0291 M
 25.0 
 55.0 
Fraction
Remaining
Initial
Concentration
Dilution
Factor
pCa 2   log( 0.0291)  1.54
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 CaY2-
CaY 
2-
 50.0 
 (0.040)
  0.0267 M
 75.0 
Dilution
Factor
Initial
Concentration
– Free Calcium is small and can be found w/ algebra
Ca 2  EDTA  CaY 2
Conc i
0
0
0.0267
Conc f
x
x
0.0267 - x

CaY 
0.0267  x


 1.8 10
Ca EDTA 
x
2
K
'
f
10
2
2
x  1.2 10 6 M
pCa 2   log( 1.2 106 )  5.91
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 
3
EDTA   (0.080) 
  1.05 10 M
 76.0 
Initial
Concentration
CaY 
2-
 50.0 
2
 (0.040) 
  2.63 10 M
 76.0 
Initial
Concentration
CaY   2.6310

Ca EDTA  Ca (1.05 10
2
K
'
f
2
2
2
Dilution
Factor
3
)
Dilution
Factor
 1.8 1010
Ca 2  1.4 10 9 M
pCa 2  8.86
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
Metal Ion Indicators
• To detect the end point of EDTA titrations, we
usually use a metal ion indicator or an ionselective electrode (Ch. 15)
• 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
Metal Ion Indicator Compounds
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.
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.
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)(0.0528 3 M)  1.32 mmol EDTA
mol Zn 2  (17.61 mL)(0.0229 9 M)  0.4049 mmol Zn 2
Ni2  Y 4-  NiY 2mol Ni2  1.321 mmol EDTA - 0.4049 mmol Zn 2  0.916 mmol
M Ni2  (0.916 mmol)/(25. 00 mL)  0.0366 M