Title: Useful to know how the concentration of a multiprotic acid and its various anionic forms vary with p
1Composition of Polybasic Acids vs. pH
- Useful to know how the concentration of a
multiprotic acid and its various anionic forms
vary with pH - Consider EDTA - ethylene diamine tetracetic acid
- Teta-protic acid - H4Y
- Kas
- H4Y H3Y- H
- H3Y- H2Y2- H
- H2Y2- HY3- H
- HY3- Y4- H
- Mass Balance CEDTA CT H4Y H3Y-
H2Y2- HY3- Y4-
2Composition of Polybasic Acids vs. pH
ai represents the fraction of each species in
solution
- Examine Figure 14-2, FAC7 p. 280
3Composition of Polybasic Acids vs. pH
Composition of EDTA solutions as a function of pH
4Complexometric Methods EDTA
- Some terms
- Lewis acid - base interactions involve the
donation of an electron pair by a Lewis base to
an electron pair acceptor, the Lewis acid - Lewis bases often contain electron rich donor
atoms such as amine nitrogens and oxygen atoms - Ligand molecules contain the electron donor atoms
- Ligand molecules having one donor atom are called
monodentate - Other ligand molecules have 2 donor atoms
(bidentate), 3 donor atoms (tridentate), 4
donor atoms (tetradentate), 5 donor atoms
(pentadentate) or 6 donor atoms (hexadentate) - Complexes formed from polydentate ligands are
called chelates - Chelate from the greek chele meaning claw
giving the English word chela meaning the
pincerlike organ or claw borne by certain of
the limbs of crustaceans and arachnids
5Complexometric Methods EDTA
- Some terms
- Lewis acids can metal ions, neutral metal atoms
and electron deficient atoms in molecules - The coordination number of the metal is the
number of ligand donor atoms connected to the
metal Lewis acid - Common coordination numbers are 2, 4 and 6
although 5 is not uncommon and 7 or 8 is known
for some large metal ions such as W - Consider several types of complexation reactions
for a metal with coordination number 4 - Formation of a 11 complex with a tetradentate
ligand - M D MD
- Formation of a 21 complex with 2 bidentate
ligands - M B MB
- MB B MB2
- M 2B MB2
6Complexometric Methods EDTA
- Consider several types of complexation reactions
for a metal with coordination number 4 - Formation of a 41 complex with 4 monodentate
ligands - M A MA
- MA A MA2
- MB2 A MA3
- MB3 A MA4
- M 4A MA4
- If its assumed Kf b2 b4 1020 and
- For the bidentate case, Kf1 1012 and Kf2 108
- For the tetradentate case, Kf1 108, Kf2 106,
Kf3 104 and Kf4 102 - Examine the titration curves of 60.00 mL of a
0.0200 M solution of a metal with - 0.0200 M D
- 0.0400 M B, and
- 0.0800 M A
See Figure 14-1, FAC7 p. 279
7Composition of Polybasic Acids vs. pH
Titration Curves for complex formation titrations
of 60.0 mL 0.0200 Metal ion with
A 0.020 M soln tetradentate ligand to give
MD B 0.040 M soln of bidentate ligand to give
MB2 C 0.080 M soln of monodentate ligand to
give MA4 (Overall Kf 1.0 x 1020)
8Complexometric Methods EDTA
- Titration curves of a metal with various ligands
- Its obvious the tetradentate ligand gives a
superior titration curve - Its also the case that the formation constants
of metal complexes with polydentate ligands
having the same kind of donor atoms are larger - The entropy effect
9Complexometric Methods EDTA
An (EDTA) complex of a metal ion
10Complexometric Methods EDTA
- Titration curves for EDTA complex formation
- The reaction between a metal ion and EDTA depends
on the pH because the protonated EDTA species
present depends on pH - Remember
- At pH 8.00, the major species is HY3- and the
reaction between a metal ion is - Mn HY3- MY(n-4) H
- But we wish to calculate the titration curve from
Kf which involves the reaction Mn Y4-
MY(n-4) - and since Y4- a4CT,
11Complexometric Methods EDTA
- Titration curves for EDTA complex formation
- The text lists Kfs for 18 metal ion - EDTA
complexes in Table 14-1, FAC7 p. 282 - The strategy is to calculate Kf and pretend CT
behaves like Y4-, calculate Mn and CT then
calculate Y4- from a4CT - Example Calculate the titration curve for 50.00
mL 0.01000 M Ca2 with 0.01000 M EDTA at pH
10.00. - a4 as a function of pH is given in Table 14-2,
FAC7 p. 284 - at pH 10.00, a4 0.35
- Kf(CaY2-) 5.08 x 1010 Kf Kf a4 1.75
x 1010 - At 0.00 mL added EDTA, Ca2 0.01 pCa 2.00
- Y4- 0 pY ?
- At 10.00 mL added EDTA
12Complexometric Methods EDTA
- Example Calculate the titration curve for 50.00
mL 0.01000 M Ca2 with 0.01000 M EDTA at pH
10.00 - At the equivalence point
- At 55.00 mL
13Complexometric Methods EDTA
- Effect of pH on the shape of the titration curve
of Ca2 with EDTA - As pH decreases, a4 decreases rapidly, so a4Kf
Kf drops rapidly - Thus Ca2 at the equivalence point will be
higher and pCa lower - And Ca2 after the equivalence point will be
higher and pCa lower - See Figure 14-6, FAC7 p. 289
14Complexometric Methods EDTA
- Effect of Kf on shape of the titration curve
- As Kf decreases, pM at the equivalence point
decreases - As Kf decreases, pM after the equivalence point
decreases - See Figure 14-7, FAC7 p. 289
15Complexometric Methods EDTA
- There is a minimum pH at which any metal ion
may be titrated so as to give a satisfactory
change in pM with change in Volume of titrant at
the equivalence point - See Figure 14-8, FAC7 p. 290
- Consider the titration of Fe3 with EDTA
- If pH too high, Fe(OH)3 will precipitate, since
Ksp(Fe(OH)3) 4 x 10-38 - If Fe30.01, Fe(OH)3 precipitates at
OH-1.59 x 10-12, pH2.20 - Minimum pH for EDTA titration is 1.8, so the
solution must be buffered very carefully