Chapter 13 EDTA Titrations EthyleneDiamineTetraAcetic acid

Gramicidin A antibiotic ion channel

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 ; Fe 2 O 3 / CrO 3 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 ) ∵ Z eff ↑  r ↓ ; IE ↑

Coordination Compounds └→colored & paramagnetic (often) consists of a complex ion (1)Coordination compounds are neutral species in which a small number of molecules or ions surround a central metal atom or ion. ex. [Co(NH 3 ) 5 Cl]Cl 2 complex ion : [Co(NH 3 ) 5 Cl] 2+

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(NH 3 ) 5 Cl ]Cl 2 H 2 O, NH 3, :Cl －.. ionic force counter ions central metal ligands complex ion

20.3 Coordination Compounds (2) Coordination number :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 : H 2 O, NH 3 bidentate : en, ox polydentate : EDTAEDTA  Chelating agents

Bonding in complex ions : The Localized Electron Model depend on coordination number Isomerism Coordinati on # Structure 2Linear (sp) 4tetrahedral (sp 3 ) or square planar ( dsp 2 ) 6Octahedral ( d 2 sp 3 )

[Cr(NH 3 ) 5 SO 4 ]Br [Cr(NH 3 ) 5 Br]SO 4 Fig20Fig20.10Fig20Fig20-11&12Fig20-16Fig20-16&17

Fig 20-10

Fig 20-11

Fig 20-12

Fig 20-16

Fig 20-17

The Crystal Field Model (1)Explains the bonding in complex ions soleoy in terms of electrostatic forces. (2)Two types of electrostatic forces : attraction : ( M ＋ ) & ( ligand ion － or ligand ： ) repulsion : ( ligand ： ) & ( metal e in d orbitals ) (3)Consider : octahedral complexes ● ●● ● ●● ● ● ● ●● ●● ● ●● ● ● ● ● 

The Crystal Field Model

Co 3 ＋, Fe 2 ＋, Fe 3 ＋ e g ─ ─ ELarge  e g ─ ─ E Small  t ２ g — ─ ─ t ２ g — ─ ─ Strong field (Low spin)Weak field (High spin) (a)(b) CN － > NO 2 － > en> NH 3 > H 2 O> OH － > F － > Cl － > Br － > I － Spectrochemical series ─→ weak field ligands     

The Crystal Field Model Spectrochemical series : a list of ligands arranged in order of their abilities to split the d orbital energies CO > CN － > en > NH 3 > H 2 O > F － > OH － > Cl － > Br － > I － strong-field ligandsweak-field ligands Magnetic properties paramagnetic dimagnetic High spin  more paramagnetic

Color : arise when complexes absorb light in some portion of the visible spectrum. ex.[Cu(H 2 O) 6 ] 2+ → blue  = E = h ex. [Ti(H 2 O) 6 ] 3+ max absorption at 498 nm[Ti(H 2 O) 6 ] 3 The Crystal Field Model

The complex ion Ti(H 2 O) 6 3+

The Crystal Field Model

13-1 Metal-Chelate Complexes EDTA forms strong 1:1 complexes with most metal ions

Lewis acid: Lewis base:

Metal-ATP complex

Figure 13-3 Synthetic chelate covalently attached to an antibody carries a metal isotope (M) to deliver lethal doses of radiation to tumor cells.

Figure 13-4 Iron(III)-enterobactin complex.

Chlorophyll is a porphyrin complex

Representation of the myoglobin molecule

Representation of the hemoglobin structure

Useful chelating agents

Box 13-1 Chelation Therapy & Thalassemia A successful drug for iron excretion

13-2 EDTA ( ethylenediaminetetraacetic acid, a hexadentate) (1)The most widely used chelating agent in titration (2)Forms strong 1:1 complexes regardless of the charge on the cation

Complexes: Formation Constant (Kf) stepwise formation constants (Ki)

(1)Multidentate chelating agents form stronger complexes ( K f ) with metal ions than bidentate or monodentate ligands. (2)Neutral EDTA is a tetrabasic acid (3)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 For EDTA

–Pb 2+ as example: At pH 10, tartrate is present to prevent Pb(OH) 2 Pb-tartrate complex must be less stable than Pb-EDTA (4) Auxiliary complexing agents: prevent metal ions from precipitating.

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.

Most indicators can be used only in certain pH ranges.

Demonstration 13-1 Metal Ion Indicator Color Changes P.294

COLOR PLATE 8 Titration of Mg 2+ 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. Demonstration 13-1 Metal Ion Indicator Color Changes

13-4 EDTA Titration Techniques are 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: (Al 3+ -EDTA) at pH 7, indicator Calmagite) back titration with Zn 2+ react slowly with EDTA block the indicator

Displacement titration –No satisfactory indicator –Ex1: Hg 2+ + MgY 2-  HgY 2- + Mg 2+ K f HgY 2- > MgY 2- –Ex2: 2Ag + + Ni(CN) 4 2-  2Ag(CN) 2 + Ni 2+, Ni 2+ is titrated with EDTA Indirect titration –Determine [Anion] that precipitate metal ions: CO 3 2-, CrO 4 2- S 2- SO 4 2- –Ex: SO Ba 2+  BaSO 4 (s) at pH 1 filter BaSO 4 (s) and boil with excess EDTA at pH 10  Ba(EDTA) 2- and excess EDTA is back titration with Mg 2+ Masking –Masking prevents one element from interfering in the analysis of another element. Ex: Al 3+ + Mg 2+ + F -  AlF Mg 2+ then only Mg 2+ can be react with EDTA  masking Al 3+ with F - –Masking agent: CN -, F - (using with pH control to avoid HCN & HF)

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 Ca 2+ and Mg 2+ are too close to differentiate between them in an EDTA titration, so they will titrate together. Ca 2+ can actually be titrated in the presence of Mg 2+ by raising the pH to 12 with strong alkali; Mg(OH) 2 precipitates and does not titrate.

13-5 The pH-dependent Metal-EDTA Equilibrium Since the anion Y 4- is the ligand species in complex formation, the complexation equilibria are affected markedly by the pH Fraction Composition of EDTA Solutions

Species EDTA as a function of pH

Conditional formation constant (K f ’) (1)We can use K’ f to calculate the equilibrium concentrations of the different species at a given pH. (2)K f : HgY -2 ＞ PbY -2 ＞ CaY -2 ； K f 不受 pH 值之影響， K f ’ 則受 pH 值之影響，上 述三者在 pH 值≦ 9.0 時， K f ’ 開始變小，也就是 EDTA 的滴定需在 (pH ＞ 9.0) 之鹼性溶液中進行 most of the EDTA is not Y 4- below pH=pK 6 = The species HY 3-, H2Y 2-, and so on, predominate at lower pH. It is convenient to express the fraction of free EDTA in the form Y 4-

Conditional Formation Constant most of the EDTA is not Y 4- below pH=pK 6 = The species HY 3-, H2Y 2-, and so on, predominate at lower pH. It is convenient to express the fraction of free EDTA in the form Y 4- P.300

The number K t f =α γ 4- K f is called the conditional formation constant or the effective formation constant. P.300 (1)We can use K’ f to calculate the equilibrium concentrations of the different species at a given pH. (2)K f : HgY -2 ＞ PbY -2 ＞ CaY -2 ； K f 不受 pH 值之影響， K f ’ 則受 pH 值之影響，上 述三者在 pH 值≦ 9.0 時， K f ’ 開始變小，也就是 EDTA 的滴定需在 (pH ＞ 9.0) 之鹼性溶液中進行

Example at page 300 pH affects the titration of Ca 2+ with EDTA –K f ’ is smaller at lower pH.

K f : cation with larger formation const provide good end point even in acidic media.

13-6 EDTA Titration Curves The end point break depends upon 1)[M n+ ] 2)[L 1 ] 3)[pH]  selectivity 4)K f The smaller K f, the more alkaline the solution must be to obtain a k’ f of 10 6.

The titration rxn: M n+ + EDTA  MY n-4 K’ f =  4 K f Three regions : (1)Before equivalence point : excess M n+ (2)At equivalence point [M n+ ]= [EDTA] (3)After equivalence point : excess EDTA Example at page 302