Complexation Reactions and Titrations

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CHAPTER
17
Complexation Reactions and Titrations
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Complexation reaction(?????????)
Complex adj.?????,??? n ???????
Complexes ??

• One of the first uses of these reactions was for
  the titrating cations the major topic
• Complexes are colored or absorb ultraviolet
  radiation the basis for spectrophotometric
  determine

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The Formation of Complexes
Cr(NH3)63

• Complex metal ligand
• Ligand have at least one pair of unshared
  electrons available for bond formation.
• (electron donor) Ex H2O, NH3, Cl-, Br-, I-
• An ion or a molecule that forms a covalent
  bond with a cation or neutral metal atom by
  donating a pair of electrons, which ate then
  shared by two. (p450)

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The Formation of Complexes

• Coordination number (???) the number of covalent
  bonds that a cation tends to form with electron
  donors
• Ex Cu(NH3)42, CuCl42-, Cr(NH3)63
• Complexometric method titrimetric methods based
  on complex formation (??????)
• Chelate (???) when a metal ion coordinates with
  two or more donor groups of a single ligand to
  form a five- or six-member heterocyclic ring.

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• Chelate when a metal ion coordinates with two or
  more donor groups of a single ligand to form a
  five- or six-member heterocyclic ring. (???)
• Unidentate (???) a ligand that has a single
  donor group Ex NH3
• Bidentate (???) a ligand that has two groups
  available for covalent bonding Ex glycine
• Tridentate, tetradentate,

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The Formation of Complexes

• Macrocycles metal ions and cyclic organic
  compounds
• the organic compounds contain nine or more
  atoms in the cycle and include at least three
  heteroatoms, usually O, N, S.

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The ions of alkali metals cam form complexes
with crown ether and cryptand D. J. Cram, C. J.
Pedersen and J.-M. Lehn Nobel prize in Chemistry
in 1987
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Complexation Equilibria
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ß the overall formation constant
451
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a the fraction of the total metal or metal
complex concentration existing
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Calculation of a for metal complexes
CT CM M ML ML2 MLn
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ML ß1 M L ML2 ß2 M L2 ML3 ß3
M L3 MLn ßn M Ln
CT CM M ML ML2 MLn
M ß1M Lß2M L2 ...ßnM
Ln M 1 ß1Lß2L2
...ßnLn
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The Formation of Insoluble Species
Ligands That Can Protonate

• Side reaction involving the metal or the ligand
• For ligand if the ligand is weak acid, then
  ligand can
• be protonated.

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Complexation with protonating ligands
Le Châteliers principle
M L L the conjugate base of polyprotic
acid Adding acid reduces the concentration
of free L available to complex with M,
decrease the effectiveness of L as a complexing
agent.
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??
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Conditional Formation Constant

• Conditional Formation Constant
• (Effective Formation Constant)
• the effect of pH on the free ligand
  concentration in a complexation reaction.

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At a particular pH value , ?2 is constant
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Titrations with Inorganic Complexing Agents
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Figure 17-1 Titration curves for complexometric
titrations. Titration of 60.0 mL of a solution
that is 0.020 M in metal M with (A) a 0.020 M
solution of the tetradentate ligand D to give MD
as the product (B) a 0.040 M solution of the
bidentate ligand B to give MB2 and (C) a 0.080 M
solution of the unidentate ligand A to give MA4.
The overall formation constant for each product
is 1020.
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Aminocarboxylic Acid Titrations

• Ethylenediaminetetraacetic acid EDTA

The EDTA molecule has six potential sites for
bonding a metal ion.
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Four carboxyl group H4Y ??EDTA
Acidic Properties of EDTA
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Figure 17-2 Composition of EDTA solutions as a
function of pH.
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Figure 17F-1 Structure of H4Y and its
dissociation products. Note that the fully
protonated species H4Y exists as the double
zwitterion with the amine nitrogens and two of
the carboxylic acid groups protonated. The first
two protons dissociate from the carboxyl groups,
while the last two come from the amine groups.
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Figure 17-3 Structure of a metal/EDTA complex.
Note that EDTA behaves here as a hexadentate
ligand in that six donor atoms are involved in
bonding the divalent metal cation.
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Complexes of EDTA and Metal Ions

• The reagent combines with metal ions in a 11
  ratio regardless of the charge on the cation.

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Equilibrium Calculations Involving EDTA

• A titration curve for the reaction of Mn and
  EDTA
• a polt of pM versus reagent volume

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From 15H

• Tetraacetic acid
• H4Y H3Y- H2Y2- HY3- Y4-

CT Y4- HY3- H2Y2- H3Y- H4Y
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Conditional Formation Constants
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Example 17-1 Calculate the molar Y4-
concentration in a 0.0200M EDTA solution buffered
to a pH of 10.00
???????Y4-, 0.0200 M EDTA ??,pH??10.0
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Calculation of the cation concentration in EDTA
solutions
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Example 17-2 Calculate the equilibrium
concentration of Ni2 in a solution with an
analytical NiY2- concentration of 0.0150M at pH
(a) 3.0 and (b) 8.0
?????pH??,0.0150M?NiY2-??????Ni2
??conditional formation constant ??
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Example 17-3 Calculate the concentration of Ni2
in a solution that was prepared by mixing 50.0mL
of 0.0300M Ni2 with 50.00mL of 0.05M EDTA. The
mixture was buffered to a pH of 3.0
?50.0mL,0.0300M Ni2?50.00mL,0.05M
EDTA??? ?,??????pH????3.0????Ni2???
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EDTA Titration Curves

• Example 17-4
• Use a spreadsheet to construct the titration
  curve of pCa versus volume of EDTA for 50.00mL of
  0.00500M Ca2 being titrated with 0.0100M EDTA in
  a solution buffered to a constant pH of 10.0

?pH??10.0?,??0.0100M EDTA????50.00mL 0.00500M
Ca2 ??,????????
pCa vs EDTA(??)
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Figure 17-5 Spreadsheet for the titration of
50.00 mL of 0.00500 M Ca2 with 0.0100 M EDTA in
a solution buffered at pH 10.0.
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Figure 17-6 EDTA titration curves for 50.0 mL of
0.00500 M Ca2 (KCaY1.751010) and Mg2
(KMgY1.72108) at pH 10.0. Note that because of
the larger formation constant, the reaction of
calcium ion with EDTA is more complete, and a
larger change occurs in the equivalence-point
region. The shaded areas show the transition
range for the indicator Eriochrome Black T.
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Figure 17-7 Influence of pH on the titration of
0.0100 M Ca2 with 0.0100 M EDTA. Note that the
end point becomes less sharp as the pH decreases
because the complex formation reaction is less
complete under these circumstances.
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Figure 17-8 Titration curves for 50.0 mL of
0.0100 M solutions of various cations at pH 6.0.
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Figure 17-9 Minimum pH needed for satisfactory
titration of various cations with EDTA. (From
C.N.Reilley and R.W.Schmid, Anal. Chem.,
1958,30,947.copyrigh 1958 American Chemical
Society. Reprinted with permission of the
American Chemical Society.)
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The Effect of Other Complexing Agents on EDTA
Titration Curves

• pH increase OH-

???M(OH)x????
An auxiliary complexing agent is needed to keep
the cation in solution, cause the end points to
be less sharp. Auxiliary ???
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Figure 17-10 Influence of ammonia concentration
on the end point for the titration of 50.0 mL of
0.00500 M Zn2. Solutions are buffered to pH
9.00. The shaded region shows the transition
range for Eriochrome Black T. Note that ammonia
decreases the change in pZn in the
equivalence-point region.
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Feature 17-5EDTA titration curves when a
complexing agent is present
????4??
?M ??NH3???,Zn(NH3)x???????
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aM the fraction of the total metal or metal
complex concentration existing
KZnY ???pH???NH3??????????
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Example calculate the pZn of solutions prepared
by adding 20.0, 25.0, and 30.0 mL of 0.0100M EDTA
to 50.0 mL of 0.00500M Zn2. Assume that both the
Zn2 and EDTA solutions are 0.100M in NH3 and
0.175M NH4Cl to provide a constant pH of 9.0
?pH??9.0??????(NH3 NH4Cl),??0.0100M
EDTA????50.00mL 0.00500M Zn2 ??,?pZn??
EDTA??(a) 20.0 mL (b)25.0 mL (c) 30.0 mL
KZnY ???pH???NH3??????????
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Figure 17-11 Structure and molecular model of
Eriochrome Black T. The compound contains a
sulfonic acid group that completely dissociates
in water and two phenolic groups that only
partially dissociate.
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Figure 17-12 Structural formula and molecular
model of Calmagite. Note the similarity to
Eriochrome Black T (see Figure 17-11).
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Example 17-5 Determine the transition ranges
for Eriochrome Black T in titrations of Mg2 and
Ca2 at pH 10.0, given that (a) the second acid
dissociation constant for the indicator is (b)
The formation constant for MgIn- is (c) Ca2
Kf 2.5x105
?pH??10.0????,??EBT????,??Ca2?Mg2?,EBT??????
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Figure 17F-2 Typical kit for testing for water
hardness in household water.
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