Composition of Polybasic Acids vs' pH

1

• Composition 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-

2

• Composition of Polybasic Acids vs. pH
• ai represents the fraction of each species in
  solution

• Examine Figure xxx, FAC7 p. xxx

3

• Complexometric 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

4

• Complexometric 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 tetradentat
  ligand
• M D MD
• Formation of a 21 complex with 2 bidentate
  ligands
• M B MB
• MB B MB2
• M 2B MB2

5

• Complexometric 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
6

• Complexometric 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

7

• Complexometric 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,

8

• Complexometric 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

9

• Complexometric 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

10

• Complexometric 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
• 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
• 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 precipitat, 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