# Chapter 12 EDTA Titrations.

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Chapter 12 EDTA Titrations

Antibiotic chelate captures its prey

15A Forming Metal-Chelate Complexes
Mn+ + nL  MLnn+ Mn+: central metal ion (Lewis acid) L: ligand (Lewis base) Complexometric titrations are useful for the determination of metal.

Complexes: Formation Constants
ligand: have a pair of unshared electrons Ex: N, O, S

The most widely used chelating agent in titration is ethylenediaminetetraacetic acid (EDTA)

15D EDTA (2) The chelon effect or chelate effect:
Multidentate chelating agents form stronger complexes (Kf ) with metal ions than bidentate or monodentate ligands. 指 multidentate 之 Kf 比 monodentate 之Kf為大， 即較穩定。

(3) Neutral EDTA is a tetrabasic acid

(5) Five species EDTA as a function of pH

Effect of pH on EDTA equilibria

15D-2 EDTA Complex with Metal Ions
(1) Forms strong 1:1 complexes regardless of the charge on the cation (2) Chelate with all cations

(3) Since the anion Y4- is the ligand species in complex formation, the complexation equilibria are affected markedly by the pH. (4) The formation constant (Table 15-5)

15D-3 Equilibrium: pH dependent M-Y

Ex Calculate the fraction of EDTA that exists as Y-4 at pH 10, and from this calculate pCa in 100mL of solution of M Ca+2 at pH 10 after adding 100 mL of M EDTA.

Calculate the fraction of EDTA that exists as Y-4 at pH 10, and from this calculate pCa in 100mL of solution of M Ca+2 at pH 10 after adding 100 mL of M EDTA. ∵ pH = ∴ [H+] = 1.0 × 10-10 1/α4 = 1 + [H+]/Ka4 + [H+]2/Ka3Ka4 + …… = 1 + [1.0 × /5.5 × 10-11]+[(1.0 × 10-10)2/(6.9 × 10-7)(5.5 × 10-11)] +…… = × …...  = 2.82  ∴ α4 = Ca+2 + EDTA [or Y-4] = CaY-2 X a4[EDTA} [(0.1 × 100)/200] – X ≒ 0.05 Kf = [CaY-2]/[Ca+2] α4 [CH4Y] = 0.05/(X)(0.35)(X) = 5.0 × 1010   X = 1.7 × 10-6 M pCa = -log (1.7 × 10-6) = 5.77

The conditional formation constant (ex:15.5)
We can use K’f to calculate the equilibrium concentrations of the different species at a given pH. K’f value holds for only a specified pH. HgY-2之Kf ＞PbY-2之Kf＞CaY-2之Kf； Kf不受pH值之影響，Kf’則受pH值之影響，上述三者在pH值≦9.0時， Kf’開始變小，也就是EDTA的滴定需在(pH＞9.0)之鹼性溶液中進行

15D-4 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 k’f of 106.

15D-4 The titration rxn: Mn++ EDTA  MYn-4 K’f = a4Kf Three regions:
Before At After equivalence point

The Kf for CaY-2 is 5. 0 × 1010. At pH 10, α4 is calculated to be 0
The Kf for CaY-2 is 5.0 × At pH 10, α4 is calculated to be 0.35 to give a Kf` of 1.8 × Calculate the pCa in 100 mL of a solution of M Ca+2 at pH 10 after addition of (a) 0 mL (b) 50 mL (c) 100 mL (d) 150 mL of M EDTA.

The Kf for CaY-2 is 5. 0 × 1010. At pH 10, α4 is calculated to be 0
The Kf for CaY-2 is 5.0 × At pH 10, α4 is calculated to be 0.35 to give a Kf` of 1.8 × Calculate the pCa in 100 mL of a solution of M Ca+2 at pH 10 after addition of (a) 0 mL (b) 50 mL (c) 100 mL (d) 150 mL of M EDTA. (a).  pCa = -log [Ca+2] = -log 0.1 = 1.00 (b).  pCa = -log [0.1 × 100 – 0.1 × 50]/150 = 1.48 (c).  [ X]/[X][X] = 1.8 × 1010 X = 1.7 × 10-6 M = [Ca+2] pCa = -log [1.7 × 10-6] = 5.77 (d).  [CaY-2] = 10 mmol/250 mL = 0.04 M [CH4Y] = [0.1 × 150 – 0.1 × 100]/250 = 0.02 M 0.04/[Ca+2](0.02) = 1.8 × 1010 = Kf` [Ca+2] = 1.1 × M; pCa = -log 1.1 × = 9.95

Various pH: K’ becomes smaller as the pH decreases

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

Minimum permissible pH for a satisfactory end point

15D-5 How do other complexing agents affect EDTA titration
At the highest pH range, all the metals will react, but not all can be titrated directly due to precipitation of hydroxides. Ex: Fe3+ or Th4+ [ammonia] influence

15D-6 Detection of the End Point: Indicators
O,O’-dihydroxy azo type (metallochromic indicators) EX: Eriochrome Black T: (EBT)

H2In-  HIn2- + H+ pKa=6.3 red blue HIn2-  In pKa=11.6 orange pH > 7 MIn + HY3-  HIn2- + MY2- red blue

In general, the metal-indicator complex should be 10 to 100 times less stable than the metal-titrant complex’ The formation constants of the EDTA complexes of calcium and magnesium are too close to differentiate between them in an EDTA titration, so they will titrate together.

Eriochrome Black T cannot be used to indicate the direct titration of calculium alone with EDTA, because the indicator forms too weak a complex with calcium to give a sharp end point.

Calcium can actually be titrated in the presence of magnesium by raising the pH to 12 with strong alkali; Mg(OH)2 precipitates and does not titrate. EBT (pH<6)  H2In-  red (pH 6~12)  HIn2-  blue (pH>12)  In3-  orange