Ch' 13 EDTA Titrations

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Ch. 13 EDTA Titrations
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Chelation in Biochemistry
Chelating ligands can form complex ions with
metals through multiple ligands. This is
important in many areas, especially biochemistry.
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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)

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Chelating Agents in Analytical Chemistry
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Ethylenediamenetetraacetic acid (EDTA)
EDTA forms 11 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.
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Acid/Base Properties of EDTA

• EDTA is a hexaprotic system (H6Y2) with 4
  carboxylic acids and 2 ammoniums
• 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

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Fraction of EDTA in Y4- Form

• Similar to acids and bases, we can define
  fractional compositions, a, defined as the
  fraction of free EDTA in a particular form.
• Free means uncomplexed EDTA
• So, for Y4-

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EDTA Complexes

• The equilibrium constant for a reaction of metal
  with EDTA is called the formation constant, Kf,
  or the stability constant
• Again, Kf could have been defined for any form of
  EDTA, it should not be understood that only the
  Y4- reacts to form complex ion.

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pH Dependence of aY4-
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Formation Constants for M-EDTA Complexes
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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.
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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
• If we fix the pH of the titration with a buffer,
  then aY4- is a constant that can be combined with
  Kf

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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)
• At low pH, the metal-complex is less stable

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Calcium/EDTA Titration Curve
For calcium, the end point becomes hard to detect
below pH8. The formation constant is too small
below this point. This can be used to separate
metals. At pH4, Ca does not perform significant
complexaion with EDTA. However, Fe can still
form the complex, so it can be titrated without
interference from Ca.
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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. Well consider a titration
where we have 50.0 mL of 0.040 M Ca2 (buffered
at pH10) with 0.080 M EDTA. Ve25.0 mL
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Before the Equivalence Point

• Whats pCa2 when we have added 5.0 mL of EDTA?

Dilution Factor
Fraction Remaining
Initial Concentration
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At the Equivalence Point

• Whats pCa2 when we have added 25.0 mL of EDTA?
• At the equivalence point almost all the metal is
  in the form CaY2-
• Free Calcium is small and can be found w/ algebra

Dilution Factor
Initial Concentration
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After the Equivalence Point

• Whats pCa2 when we have added 26.0 mL of EDTA?
• We have 1.0 mL excess EDTA

Initial Concentration
Dilution Factor
Initial Concentration
Dilution Factor
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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

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Metal Ion Indicators

• To detect the end point of EDTA titrations, we
  usually use a metal ion indicator or an
  ion-selective electrode (Ch. 15)
• Metal ion indicators change color when the metal
  ion is bound to EDTA
• Eriochrome black T is an organic ion
• The indicator must bind less strongly than EDTA

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Metal Ion Indicator Compounds
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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.

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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.

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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?