EDTA Titrations

1
EDTA Titrations

• Introduction
• 1.) Metal Chelate Complexes
• Any reagent which reacts with an analyte in a
  known ratio and with a large equilibrium constant
  can potentially be used in a titration.
• Complexation Titrations are based on the reaction
  of a metal ion with a chemical agent to form a
  metal-ligand complex.

Metal
Metal-Ligand Complex
Ligand
Note multiple atoms from EDTA are binding Mn2
Metal Lewis Acid or Electron-pair
acceptor Ligand Lewis Base or Electron-pair
donor
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EDTA Titrations

• Introduction
• 1.) Metal Chelate Complexes
• Complexation Titrations are essentially a Lewis
  acid-base reaction, in which an electron pair is
  donated from one chemical to another
• The ligands used in complexometric titrations are
  also known as chelating agents.
• Ligand that attaches to a metal ion through more
  than one ligand atom
• Most chelating agents contain N or O
• Elements that contain free electron pairs that
  may be donated to a metal

Fe-DTPA Complex
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EDTA Titrations

• Metal Chelation in Nature
• 1.) Potassium Ion Channels in Cell Membranes
• Electrical signals are essential for life
• Electrical signals are highly controlled by the
  selective passage of ions across cellular
  membranes
• Ion channels control this function
• Potassium ion channels are the largest and most
  diverse group
• Used in brain, heart and nervous system

K is chelated by O in channel
channel contains pore that only allows K to pass
K channel spans membrane
Opening of potassium channel allows K to exit
cell and change the electrical potential across
membrane
Current Opinion in Structural Biology 2001,
11408414
http//www.bimcore.emory.edu/home/molmod/Wthiel/Kc
hannel.html
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EDTA Titrations

• Metal Chelate Complexes
• 1.) Formation Constant (Kf)
• The equilibrium constant for the reaction between
  a metal ion (Mn) and a chelating agent (L-P) is
  known as a formation constant or stability
  constant.
• Applying different and specific names to the
  general equilibrium constant is a common
  occurrence
• Solubility (Ksp), acid-base (Ka, Kb), water
  dissociation (Kw), etc
• Chelate effect ability of multidentate ligands
  to form stronger metal complexes compared to
  monodentate ligands.

Kf 8x109
Kf 4x109
2 ethylenediamine molecules binds tighter than 4
methylamine molecules
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EDTA Titrations

• Metal Chelate Complexes
• 2.) Chelate Effect
• Usually chelating agents with more than one
  electron pair to donate will form stronger
  complexes with metal ions than chelating agents
  with only one electron pair.
• Typically more than one O or N
• Larger Kf values
• Multidentate ligand a chelating agent with more
  than one free electron pair
• Stoichiometry is 11 regardless of the ion charge
• Monodentate ligand a chelating agent with only
  one pair of free electrons

Multidentate ligand that binds radioactive metal
attached to monoclonal antibody (mAb). mAb is a
protein that binds to a specific feature on a
tumor cell delivering toxic dose of radiation.
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EDTA Titrations

• EDTA
• 1.) EDTA (Ethylenediaminetetraacetic acid)
• One of the most common chelating agents used for
  complexometric titrations in analytical
  chemistry.
• EDTA has 6 nitrogens oxygens in its structure
  giving it 6 free electron pairs that it can
  donate to metal ions.
• High Kf values
• 6 acid-base sites in its structure

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EDTA Titrations

• EDTA
• 2.) Acid-Base Forms
• EDTA exists in up to 7 different acid-base forms
  depending on the solution pH.
• The most basic form (Y4-) is the one which
  primarily reacts with metal ions.

EDTA-Mn Complex
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EDTA Titrations

• EDTA
• 2.) Acid-Base Forms
• Fraction (a) of the most basic form of EDTA (Y4-)
  is defined by the H concentration and acid-base
  equilibrium constants

Fraction (a) of EDTA in the form Y4-
where EDTA is the total concentration of all
free EDTA species in solution
aY4- is depended on the pH of the solution
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EDTA Titrations

• EDTA
• 3.) EDTA Complexes
• The basic form of EDTA (Y4-) reacts with most
  metal ions to form a 11 complex.
• Other forms of EDTA will also chelate metal ions
• Recall the concentration of Y4- and the total
  concentration of EDTA is solution EDTA are
  related as follows

Note This reaction only involves Y4-, but not
the other forms of EDTA
where aY4-is dependent on pH
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EDTA Titrations

• EDTA
• 3.) EDTA Complexes
• The basic form of EDTA (Y4-) reacts with most
  metal ions to form a 11 complex.

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EDTA Titrations

• EDTA
• 3.) EDTA Complexes
• Substitute Y4- into Kf equation
• If pH is fixed by a buffer, then aY4- is a
  constant that can be combined with Kf

where EDTA is the total concentration of EDTA
added to the solution not bound to metal ions
Conditional or effective formation constant (at
a given pH)
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EDTA Titrations

• EDTA
• 3.) EDTA Complexes
• Assumes the uncomplexed EDTA were all in one form

at any pH, we can find aY4- and evaluate Kf
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EDTA Titrations

• EDTA
• 4.) Example
• What is the concentration of free Fe3 in a
  solution of 0.10 M Fe(EDTA)- at pH 8.00?

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EDTA Titrations

• EDTA
• 5.) pH Limitation
• Note that the metal EDTA complex becomes less
  stable as pH decreases
• Kf decreases
• Fe3 5.4x10-7 at pH 2.0 -gt Fe3 1.4x10-12
  at pH 8.0
• In order to get a complete titration (Kf 106),
  EDTA requires a certain minimum pH for the
  titration of each metal ion

End Point becomes less distinct as pH is lowered,
limiting the utility of EDTA as a titrant
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EDTA Titrations
Minimum pH for Effective Titration of Metal Ions

• EDTA
• 5.) pH Limitation
• By adjusting the pH of an EDTA titration
• one type of metal ion (e.g. Fe3) can be titrated
  without interference from others (e.g. Ca2)

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EDTA Titrations

• EDTA Titration Curves
• 1.) Titration Curve
• The titration of a metal ion with EDTA is similar
  to the titration of a strong acid (M) with a
  weak base (EDTA)
• The Titration Curve has three distinct regions
• Before the equivalence point (excess Mn)
• At the equivalence point (EDTAMn
• After the equivalence point (excess EDTA)

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EDTA Titrations

• EDTA Titration Curves
• 2.) Example
• What is the value of Mn and pM for 50.0 ml of
  a 0.0500 M Mg2 solution buffered at pH 10.00 and
  titrated with 0.0500 m EDTA when (a) 5.0 mL, (b)
  50.0 mL and (c) 51.0 mL EDTA is added?

Kf 108.79 6.2x108
aY4- at pH 10.0 0.30
mL EDTA at equivalence point
mmol of Mg2
mmol of EDTA
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EDTA Titrations

• EDTA Titration Curves
• 2.) Example
• (a) Before Equivalence Point ( 5.0 mL of EDTA)

Before the equivalence point, the Mn is equal
to the concentration of excess unreacted Mn.
Dissociation of MYn-4 is negligible.
moles of Mg2 originally present
moles of EDTA added
Original volume solution
Volume titrant added
Dilution effect
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EDTA Titrations

• EDTA Titration Curves
• 2.) Example
• (b) At Equivalence Point ( 50.0 mL of EDTA)

Virtually all of the metal ion is now in the form
MgY2-
Original volume of Mn solution
Moles Mg moles MgY2-
Original Mn
Original volume solution
Volume titrant added
Dilution effect
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EDTA Titrations

• EDTA Titration Curves
• 2.) Example
• (b) At Equivalence Point ( 50.0 mL of EDTA)

The concentration of free Mg2 is then calculated
as follows
Solve for x using the quadratic equation
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EDTA Titrations

• EDTA Titration Curves
• 2.) Example
• (c) After the Equivalence Point ( 51.0 mL of EDTA)

Virtually all of the metal ion is now in the form
MgY2- and there is excess, unreacted EDTA. A
small amount of free Mn exists in equilibrium
with MgY4- and EDTA.
Calculate excess EDTA
Volume excess titrant
Excess moles EDTA
Original EDTA
Original volume solution
Volume titrant added
Dilution effect
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EDTA Titrations

• EDTA Titration Curves
• 2.) Example
• (c) After the Equivalence Point ( 51.0 mL of EDTA)

Calculate MgY2-
Original volume of Mn solution
Moles Mg moles MgY2-
Original Mn
Only Difference
Original volume solution
Volume titrant added
Dilution effect
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EDTA Titrations

• EDTA Titration Curves
• 2.) Example
• (c) After the Equivalence Point ( 51.0 mL of EDTA)

Mg2- is given by the equilibrium expression
using EDTA and MgY2-
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EDTA Titrations

• EDTA Titration Curves
• 2.) Example
• Final titration curve for 50.0 ml of 0.0500 M
  Mg2 with 0.0500 m EDTA at pH 10.00.
• Also shown is the titration of 50.0 mL of 0.0500
  M Zn2

Note the equivalence point is sharper for Zn2
vs. Mg2. This is due to Zn2 having a larger
formation constant.
The completeness of these reactions is dependent
on aY4- and correspondingly pH.
pH is an important factor in setting the
completeness and selectivity of an EDTA titration
25
EDTA Titrations

• Auxiliary Complexing Agents
• 1.) Metal Hydroxide
• In general, as pH increases a titration of a
  metal ion with EDTA will have a higher Kf.
• Larger change at the equivalence point.
• Exception If Mn reacts with OH- to form an
  insoluble metal hydroxide
• Auxiliary Complexing Agents a ligand can be
  added that complexes with Mn strong enough to
  prevent hydroxide formation.
• Ammonia, tartrate, citrate or triethanolamine
• Binds metal weaker than EDTA

Fraction of free metal ion (aM) depends on the
equilibrium constants (b) or cumulative formation
constants
Use a new conditional formation constant that
incorporates the fraction of free metal
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EDTA Titrations

• Auxiliary Complexing Agents
• 2.) Illustration
• Titration of Cu2 (CuSO4) with EDTA
• Addition of Ammonia Buffer results in a dark blue
  solution
• Cu(II)-ammonia complex is formed
• Addition of EDTA displaces ammonia with
  corresponding color change

Cu-ammonia
CuSO4
Cu-EDTA
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EDTA Titrations

• Metal Ion Indicators
• 1.) Determination of EDTA Titration End Point
• Four Methods
• Metal ion indicator
• Mercury electrode
• pH electrode
• Ion-selective electrode
• Metal Ion Indicator a compound that changes
  color when it binds to a metal ion
• Similar to pH indicator, which changes color with
  pH or as the compound binds H
• For an EDTA titration, the indicator must bind
  the metal ion less strongly than EDTA
• Similar in concept to Auxiliary Complexing Agents
• Needs to release metal ion to EDTA

Potential Measurements
End Point indicated by a color change from red to
blue
(red)
(colorless)
(colorless)
(blue)
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EDTA Titrations

• Metal Ion Indicators
• 2.) Illustration
• Titration of Mg2 by EDTA
• Eriochrome Black T Indicator

Addition of EDTA
Before Near
After Equivalence point
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EDTA Titrations

• Metal Ion Indicators
• 3.) Common Metal Ion Indicators
• Most are pH indicators and can only be used over
  a given pH range

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EDTA Titrations

• Metal Ion Indicators
• 3.) Common Metal Ion Indicators
• Useful pH ranges

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EDTA Titrations

• EDTA Titration Techniques
• 1.) Almost all elements can be determined by EDTA
  titration
• Needs to be present at sufficient concentrations
• Extensive Literature where techniques are listed
  in
• G. Schwarzenbach and H. Flaschka, Complexometric
  Titrations, MethuenLondon, 1969.
• H.A. Flaschka, EDTA Titrations, Pergamon
  PressNew York, 1959
• C.N. Reilley, A.J. Bernard, Jr., and R. Puschel,
  In L. Meites (ed.) Handbook of Analytical
  Chemistry, McGraw-HillNew York, 1963 pp. 3-76
  to 3-234.
• Some Common Techniques used in these titrations
  include
• Direct Titrations
• Back Titrations
• Displacement Titrations
• Indirect Titrations
• Masking Agents

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EDTA Titrations

• EDTA Titration Techniques
• 2.) Direct Titrations
• Analyte is buffered to appropriate pH and is
  titrated directly with EDTA
• An auxiliary complexing agent may be required to
  prevent precipitation of metal hydroxide.
• 3.) Back Titrations
• A known excess of EDTA is added to analyte
• Free EDTA left over after all metal ion is bound
  with EDTA
• The remaining excess of EDTA is then titrated
  with a standard solution of a second metal ion
• Approach necessary if analyte
• precipitates in the presence of EDTA
• Reacts slowly with EDTA
• Blocks the indicator
• Second metal ion must not displace analyte from
  EDTA

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EDTA Titrations

• EDTA Titration Techniques
• 4.) Displacement Titration
• Used for some analytes that dont have
  satisfactory metal ion indicators
• Analyte (Mn) is treated with excess Mg(EDTA)2-,
  causes release of Mg2.
• Amount of Mg2 released is then determined by
  titration with a standard EDTA solution
• Concentration of released Mg2 equals Mn

Requires
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EDTA Titrations

• EDTA Titration Techniques
• 5.) Indirect Titration
• Used to determine anions that precipitate with
  metal ions
• Anion is precipitated from solution by addition
  of excess metal ion
• ex. SO42- excess Ba2
• Precipitate is filtered washed
• Precipitate is then reacted with excess EDTA to
  bring the metal ion back into solution
• The excess EDTA is titrated with Mg2 solution

Total EDTA MYn-4 Y4-
complex
free
determine
Known
Titrate
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EDTA Titrations

• EDTA Titration Techniques
• 6.) Masking Agents
• A reagent added to prevent reaction of some metal
  ion with EDTA
• Demasking refers to the release of a metal ion
  from a masking agent

Al3 is not available to bind EDTA because of the
complex with F-
Requires