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Complex Formation Titration

EDTA forms 1:1 complexes with most metal ions, binding through the four oxygen and the two nitrogen atoms.

Multidentate ligands are preferred over monodentate ligands.

ATP Metal-ATP Complex

Chelate covalently attached to an antibody carries a metal isotope (M) to deliver lethal doses of radiation to tumor cells.

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Analytically useful chelating agents

Chelation therapy and thalassemia

Orally taken iron chelator Fractional composition diagram of EDTA

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Complexes of EDTA and Metal Ions

Ag+ + Y4 - ßà AgY3-

Mg2+ + Y4- ßà MgY2-

Al3+ + Y4- ßà AlY-

Mn+ + Y4- ßà [MY(n-4)] KMY = [MY(n-4)] / [Mn+][Y4-]

α4 = [Y4-]/ cT Where: cT = [Y4-] + [HY3-] + [H2Y2-] + [H3Y-] + [H4Y]

α4 = K1K2K3K4 [H+]4+K1[H+]3+K1K2[H+]2+K1K2K3[H+]+K1K2K3K4

Conditional Formation Constant α4 = [Y4-]/ cT Where: cT = [Y4-] + [HY3-] + [H2Y2-] + [H3Y-] + [H4Y] = total molar concentration of uncomplexed EDTA

KMY = [MY(n-4)] / [Mn+][Y4-]

K’MY = α4KMY = [MY(n-4)] / [Mn+]cT

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Calculate the equilibrium concentration of Ni2+

in a solution with an analytical concentration of 0.0150 M at pH a)3.00 and b) 8.0 KfNiY2- = 4.2 x 1018

Calculate the concentration of Ni2+ in a soln that was prepared by mixing 50.0 mL of 0.0300 M Ni2+ with 50.0 mL of 0.0500 M EDTA. The mixture was Buffered to pH 3.0. KfNiY2- = 4.2 x 1018

Example. 50.0 mL of 0.0400 Ca2+ is titrated with 0.0800 M EDTA at pH 10. Calculate pCa at the ff. volumes of titrant added. a.  0.00 mL b.  10.0 mL c.  25.0 mL d.  50.0 mL log Kf = 10.65 α4 = 0.30 at pH 10

Titration curves for the reaction of 50.0 mL of 0.0400 M metal ion with 0.0800 M EDTA at pH 10.

Effect of reaction completeness on titration curve: As Kf increases, the reaction becomes more complete ⇒ ΔpM/ΔV increases (steeper slope) at the equivalence point region.

Effect of pH on titration curve: as pH increases, the slope at the equivalence point region becomes steeper (more distinct break at the equivalence point).

An auxiliary complexing agent like NH3, tartrate, citrate or triethanolamine, prevents precipitation of metal hydroxide.

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How to improve selectivity of EDTA 1.  Interferences can be controlled by pH

regulation Trivalent cations can be titrated without interference from divalent cations at pH 1-2 2. Buffer solution to pH 7 for Cd and Zn 3. Add masking reagent/s (ex: cyanide complexes with Cd, Co, Cu, Ni, Zn and Pd, so Mg and Ca can be titrated)

Metal ion indicators change color when they bind to a metal ion.

Hardness of water

A 100-mL sample of water containing Ca2+ and Mg2+ ions is titrated with 15.28 mL of 0.01016 M EDTA in an ammonia buffer at pH 10.00. Another sample of 100 mL is treated with NaOH to precipitate Mg(OH)2, and then titrated at pH 13.00 with 10.43 mL of the same EDTA solution. Calculate the ppm of CaCO3 (100.09) and of MgCO3 (84.314) in the sample.

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Liebig Titration of Cyanide with AgNO3 Basis: 2 CN- + Ag+ D Ag(CN)2

- K ~ 1021

End point: appearance of turbidity due to the precipitation of silver cyanide.

Ag+ + Ag(CN)2

- D 2 AgCN (s) or Ag+ + Ag(CN)2

- D Ag[Ag(CN)2] (s) Deniges modification: iodide ion is added since AgI is less soluble than silver cyanide; NH3 is added ⇒ Ag(NH3)2

+ forms which retards the precipitation of AgI until very near the equivalence point.

Example. A 0.7562-g sample containing NaCN is dissolved in H2O and then concentrated NH3 and some KI solution are added. The solution requires 23.58 mL of 0.0988 M AgNO3 for titration. Calculate the percentage of NaCN (49.007) in the sample.