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1/7/2016
1
Ch 13 - EDTA Titrations
EthyleneDiamineTetraacetic Acid
Formation Constants, Kf
[M][L]
[ML]K MLLM f
4
32
23
12
332 1.9x10
]][NH[Cu
])[Cu(NHK )Cu(NHNHCu
3
32
3
223
22233
23 3.6x10
])][NH)[Cu(NH
])[Cu(NHK )Cu(NHNH)Cu(NH
2
3223
233
32333
223 7.9x10
]][NH)[Cu(NH
])[Cu(NHK )Cu(NHNH)Cu(NH
2
3233
243
42433
233 1.5x10
]][NH)[Cu(NH
])[Cu(NHK )Cu(NHNH)Cu(NH
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2
EDTA Formation Constants
EDTA is a hexaprotic weak acid that complexes 1:1 with metal cations -
Notice that the first 4 protons are much more acidic than the last two, so the dominant form of EDTA in solution will be H2Y
2- ....
Mn+ + H2Y2- = MYn-4 + 2H+
By Le Chatelier's Principle, the complex will dissociate at low pH's, and it will be more stable at high pH's.
EDTA titrations are therefore pH dependent and analyte solutions must be buffered to the optimum pH.
low pH
high pH
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3
from "Fundamentals of Analytical Chemistry", Skoog and West, 4th ed.
Minimum pH for Titration
The pH-Dependent Metal-EDTA Equilibrium (Sec. 12-5 and Sec 13-5)
Fractional Composition Equations - the fraction (percentage) of each species of an acid or base existing at a given pH
HA = H+ + A- CT = [HA] + [A-]
A- = fraction of HA dissociated to A-
TC
][A
][A[HA]
][A
HA = fraction of HA still existing as HA
TC
[HA]
][A[HA]
[HA]
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4
[HA]
]][A[HK ,AHHA for a
[HA]
]][A[HK ,AHHA for a
the mass balance equation is -
rearranging and substituting into Ka -
T-
HA C ][A [HA] C
[HA]C ][A T
[HA]
[HA])-](C[HK T
a
][HA][H-]C[HK [HA] Ta
solve for [HA] -
][HA][H-]C[HK [HA] Ta Ta ]C[H][HA][HK [HA] so
][HK
][H
C
[HA]
aT
][HK
][Hα so
a
HA
to solve for A- substitute [HA] = CT - [A-] into Ka
][A-C
]][A[H
[HA]
]][A[HK
-T
a
]][A[H])K[A-(C so a-
T
]][A[H]K[A-KC a-
aT a
-aT ]K[A]][A[HKC and
)aaT K ]]([H[AKC
Ta
a
C
][A
K ][H
K so
][HK
Kαfinally so
a
a-A
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5
Fractional Composition Diagram for Monoprotic Acids
cross where pH = pKa
][HK
Kα
a
a-A
][HK
][Hα
a
HA
Fractional Composition Diagrams for Polyprotic Acids: General Forms
diprotic triprotic
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6
Fractional Composition Diagram H2CO3
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 2 4 6 8 10 12 14
pH
alp
ha
Fractional Composition Diagram H3PO4
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 2 4 6 8 10 12 14
pH
alp
ha
Ka1 = 4.46 x 10-7
Ka2 = 4.69 x 10-11
Ka1 = 7.11 x 10-3
Ka2 = 6.34 x 10-8
Ka3 = 4.50 x 10-13
Fractional Composition Diagrams for EDTA
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7
Conditional (Effective) Formation Constant, K'f
For EDTA titration curves, it's convenient to base calcuations on the Y4- form of EDTA and derive a new, pH-dependent formation constant K'f
]][Y[M
][MYK MYYM
4n
4n
f4n4n
CT = all forms of EDTA (Y4-, HY3- etc)
][YCα therefore and C
][Yα 4
TY4
T
4
Y4
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8
TY4n
4n
4n
4n
fC]α[M
][MY
]][Y[M
][MYK into ngsubstituti
Tn
4n
Y4f
TY4n
4n
f]C[M
][MYαK or
C]α[M
][MYKfinally and
Tn
4n
Y4f'f
]C[M
][MYαK K constant formation lconditiona the
All EDTA equilibrium calculations will use K'f at the pH of the titration. The value of Y4- at this pH is taken from Table 13-3.
Example, p. 300
The formation constant for FeY- is 1.3 x 1025
(Fe3+). Calculate the concentration of free Fe3+
in a solution of 0.10 M FeY- at pH = 4.00 and pH = 1.00.
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9
EDTA Titration Curves, Sec 13-6
A complex formation titration curve plots pM (analogous to pH) vs. volume of titrant (see next slide). To save time, we will only calculate the pM = -log[Mn+] at the equivalence point in order to select the correct indicator.
8
4n
4n
f2-42 10 x 6.2
]][Y[M
][MYK MgYYMg e.g.
Y4f'f αK K
p.302 - Calculate the titration curve for the reaction of 50.0 mL of 0.0500 M Mg2+, buffered to a pH of 10.0, with 0.0500 M EDTA. The equivalence pt. volume is 50.0 mL.
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10
At the equivalence pt. virtually all of the Mg is in the form MgY2-.
2-42 MgY Y Mg
Initial:
Change:
Equilibrium:
[MgY2-] at the eq. pt. =
The ICE table for the reaction is:
Kf CaY2- > Kf MgY2- so the endpoint is sharper for Ca2+
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11
The equiv. pt. becomes sharper as the pH of the titration approaches the optimal value for the analyte, e.g. for Ca2+…..
The equiv. pt. becomes sharper as the Kf of the EDTA-metal complexes becomes larger…..
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12
H2In- + H2O = HIn2- + H3O+ pKa2 = 8.1
red blue
HIn2- + H2O = In3- + H3O+ pKa3 = 12.4
blue orange
end point reaction with metal cation…..
MIn- + Y4- = MY2- + In3- Kf < Kf analyte
red blue
In the titration of Ca, Mg is added to the titrant in order to sharpen the end point.