EDTA Titrations

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Chapter 13

• EDTA Titrations

EthyleneDiamineTetraAcetic acid
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Gramicidin A antibiotic ion channel
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The Transition Metals

• Industry Fe , Cu , Ti , Ag , ??
• Biosystem transport , storage , catalyst
• (1) General Properties ( Sc ? Cu )
• a) Great similarities within a period as well as
  a group ? d subshells incomplerely filled.
• ? distinctive coloring
• ? formation of paramagnetic compounds
• ? catalytic behavior
• ? tendency to form complex ions.
• b) difference m.p W / Hg
• Hard / soft Fe , Ti / Cu , Au , Ag
• Reactivity oxides Cu / Fe Fe2O3 /
  CrO3

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The Transition Metals

• (2) Electron configurations 4s before 3d
• (3) Oxidation states
• most common 2 , 3 ( 2 7 )
• more than one oxidation states
• (4) Reduction Potentials
• -----? period
• reducing ability ? ( Zn , Cr )
• ? Zeff ? ? r ? IE ?

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Coordination Compounds

• ? colored paramagnetic (often)
• consists of a complex ion
• Coordination compounds are neutral species in
  which a small number of molecules or ions
  surround a central metal atom or ion.
• ex.
• Co(NH3)5ClCl2
• complex ion Co(NH3)5Cl2

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Coordination Compounds

• coordinate covalent bond
• Complex ion metal cation ligands
• e acceptor e donor
• center (one) surrounding ( ? 2 )
• transion metal
• Lewis acid Lewis base
• Co(NH3)5Cl Cl2

ionic force
counter ions
central metal ligands
complex ion
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20.3 Coordination Compounds

• (2) Coordination number
• The of donor atoms surrounding the central
  metal
• The most common 4 or 6
• (3) Ligands
• A neutral molecule or ion having a line pair that
  can be used to from a bond to a metal ion.
• monodentate H2O , NH3
• bidentate en , ox
• polydentate EDTA

? Chelating agents
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Bonding in complex ions
Isomerism

• The Localized Electron Model
• depend on coordination number

Coordination Structure
2 Linear (sp)
4 tetrahedral (sp3 ) or square planar ( dsp2 )
6 Octahedral ( d2sp3 )
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Cr(NH3)5SO4Br Cr(NH3)5BrSO4
Fig20.10
Fig20-1112
Fig20-1617
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Fig 20-10
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Fig 20-11
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Fig 20-12
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Fig 20-16
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Fig 20-17
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The Crystal Field Model

• Explains the bonding in complex ions soleoy in
  terms of electrostatic forces.
• Two types of electrostatic forces
• attraction ( M ) ( ligand ion - or ligand
  )
• repulsion ( ligand ) ( metal e in d
  orbitals )
• Consider octahedral complexes

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The Crystal Field Model
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The Crystal Field Model
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The Crystal Field Model

• Co3 , Fe2 , Fe3
• eg - -
• E Large D eg - -
• E Small D
• t2g - - t2g - -
• Strong field (Low spin) Weak field (High spin)
• (a) (b)
• CN-gt NO2-gt engt NH3gt H2Ogt OH-gt F-gt Cl-gt Br-gt I-
• Spectrochemical series -? weak field
  ligands

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The Crystal Field Model

• Spectrochemical series
• a list of ligands arranged in order of their
  abilities to split the d orbital energies
• CO gt CN-gt en gt NH3gt H2O gt F- gt OH-gt Cl-gt Br-gt
  I-
• strong-field ligands weak-field ligands
• Magnetic properties
• paramagnetic
• dimagnetic
• High spin ? more paramagnetic

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The Crystal Field Model
Color arise when complexes absorb light in some
portion of the visible spectrum.
ex. Cu(H2O)62 ? blue D E hn ex.
Ti(H2O)63 max absorption at 498 nm
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The complex ion Ti(H2O)63
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The Crystal Field Model
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The Crystal Field Model
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The Crystal Field Model
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13-1 Metal-Chelate Complexes
EDTA forms strong 11 complexes with most metal
ions
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• Lewis acid
• Lewis base

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Metal-ATP complex
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Figure 13-3 Synthetic chelate covalently attached
to an antibody carries a metal isotope (M) to
deliver lethal doses of radiation to tumor cells.
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Figure 13-4 Iron(III)-enterobactin complex.
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Chlorophyll is a porphyrin complex
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Representation of the myoglobin molecule
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Representation of the hemoglobin structure
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Useful chelating agents
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Box 13-1 Chelation Therapy Thalassemia

• A successful drug for iron excretion

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13-2 EDTA (ethylenediaminetetraacetic acid, a
hexadentate)

1. The most widely used chelating agent in titration
   
2. Forms strong 11 complexes regardless of the
   charge on the cation

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Complexes Formation Constant (Kf)stepwise
formation constants (Ki)
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For EDTA

• Multidentate chelating agents form stronger
  complexes (Kf ) with metal ions than bidentate or
  monodentate ligands.
• Neutral EDTA is a tetrabasic acid
• Metal-EDTA complex is unstable at both low high
  pH.
• At low pH
• H competes with M n
• At high pH
• OH- competes with EDTA

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(4) Auxiliary complexing agents prevent metal
ions from precipitating.

• Pb2 as example
• At pH 10, tartrate is present to prevent Pb(OH)2
• Pb-tartrate complex must be less stable than
  Pb-EDTA

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13-3 Metal Ion Indicators

• Metal ion indicator a compound whose color
  changes when it binds to a metal ion.
• For an useful indicator, it must bind metal less
  strongly than EDTA does. ? the indicator must
  release its metal to EDTA
• Example MgIn EDTA ? MgEDTA In
• Indicator is pH dependent.
• If metal block the indicator, use back titration.

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• Most indicators can be used only in certain pH
  ranges.

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• Demonstration 13-1 Metal Ion Indicator Color
  Changes

P.294
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Demonstration 13-1 Metal Ion Indicator Color
Changes
COLOR PLATE 8 Titration of Mg2 by EDTA, Using
Eriochrome Black T Indicator (a) Before (left),
near (center), and after (right) equivalence
point. (b) Same titration with methyl red added
as inert dye to alter colors. 
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13-4 EDTA Titration Techniquesare useful for the
determination of metal

• Direct titration
• Titrate with EDTA
• Buffered to an appropriate pH
• Color distinct indicator
• Auxiliary complexing agent
• Back titration (example at p295)
• Excess EDTA, titrate with metal ion
• For analyte
• ppt in the absence of EDTA
• Ex (Al3-EDTA) at pH 7, indicator Calmagite)
  back titration with Zn2
• react slowly with EDTA
• block the indicator

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• Displacement titration
• No satisfactory indicator
• Ex1 Hg2 MgY2- ? HgY2- Mg2 Kf HgY2- gt
  MgY2-
• Ex2 2Ag Ni(CN)42- ? 2Ag(CN)2 Ni2 , Ni2
  is titrated with EDTA
• Indirect titration
• Determine Anion that precipitate metal ions
  CO32-, CrO42- S2- SO42-
• Ex SO42- Ba2 ? BaSO4(s) at pH 1
• filter BaSO4(s) and boil with excess
  EDTA at pH 10
• ? Ba(EDTA)2- and excess EDTA is back
  titration with Mg2
• Masking
• Masking prevents one element from interfering in
  the analysis of another element. Ex Al3 Mg2
  F- ? AlF63 Mg2 then only Mg2 can be react
  with EDTA ? masking Al3 with F-
• Masking agent CN- , F- (using with pH control
  to avoid HCN HF)

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• In general, the metal-indicator complex should be
  10 to 100 times less stable than the
  metal-titrant complex
• Expt The formation constants of the EDTA
  complexes of Ca2 and Mg2 are too close to
  differentiate between them in an EDTA titration,
  so they will titrate together. Ca2 can actually
  be titrated in the presence of Mg2 by raising
  the pH to 12 with strong alkali Mg(OH)2
  precipitates and does not titrate.

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13-5 The pH-dependent Metal-EDTA Equilibrium

• Since the anion Y4- is the ligand species in
  complex formation, the complexation equilibria
  are affected markedly by the pH
• Fraction Composition of EDTA Solutions

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Species EDTA as a function of pH
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Conditional formation constant (Kf)

• most of the EDTA is not Y4- below pHpK610.37.
  The species HY3-, H2Y2-, and so on, predominate
  at lower pH.
• It is convenient to express the fraction of free
  EDTA in the form Y4-

• We can use Kf to calculate the equilibrium
  concentrations of the different species at a
  given pH.
• Kf HgY-2gtPbY-2gtCaY-2 Kf??pH????,Kf??pH????,??
  ???pH??9.0?, Kf????,???EDTA?????(pHgt9.0)????????

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• Conditional Formation Constant
• most of the EDTA is not Y4- below pHpK610.37.
  The species HY3-, H2Y2-, and so on, predominate
  at lower pH.
• It is convenient to express the fraction of free
  EDTA in the form Y4-
•  
• 
  

P.300
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• The number Ktf a?4-Kf is called the conditional
  formation constant or the effective formation
  constant.

• We can use Kf to calculate the equilibrium
  concentrations of the different species at a
  given pH.
• Kf HgY-2gtPbY-2gtCaY-2 Kf??pH????,Kf??pH????,??
  ???pH??9.0?, Kf????,???EDTA?????(pHgt9.0)????????

P.300
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• Example at page 300
• pH affects the titration of Ca2 with EDTA
• Kf is smaller at lower pH.

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• Kf cation with larger formation const provide
  good end point even in acidic media.

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13-6 EDTA Titration Curves

• The end point break depends upon
• Mn
• L1
• pH ? selectivity
• Kf
• The smaller Kf, the more alkaline the solution
  must be to obtain a kf of 106.

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• The titration rxn
• Mn EDTA ? MYn-4
• Kf a4Kf
• Three regions
• Before equivalence point excess Mn
• At equivalence point Mn EDTA
• After equivalence point excess EDTA
• Example at page 302