Ch. 17 Complexation reactions and titrations A. The formation of complexes Most metal ion : react with electron-donor species to form coordination compounds or complexes. * Electron-donor : Ligand(water, ammonia, halide ions) Co-ordinate group : Ligand 중 donor 원자를 갖는 基 * Complexometric methods : Titrimetric methods based on complex formation * Coordination number : 2, 4, 6 Single donor group : unidentate Poly donor group : bidentate, tridentate, tetradentate, pentadentate, hexadentate * Chelate ring : 여러자리 배위자에 의한 배위화합물은 중심 금속 이온에 의하여 환을 이루게 되는 이를 chelate ring 이라 한다. chelate ring 형성반응 : chelation

Unidentate Bidentate Polydentate

Complexation equilibria

Calculation of Alpha value for Metal complexes

The formation of insoluble species BiI 3 Cl - 에 Ag + 을 첨가 초기 : AgCl 침전 생성 과량의 Cl - 첨가 : AgCl 2 -, AgCl 3 2-, AgCl 4 3- 생성

Ligands that can protonate 금속 M, ligand L(the conjugated base of a polyprotic acid : form HL, HL, HL ……) Ex. Ferric ion(Fe 3+ ) 은 oxalate ion 과 착이온 형성 (FeO X )+, (FeO X2 ) - 과 (FeO x3 ) 3- 착이온 생성 Oxalate 는 양성자를 내어 놓아 HO X - 와 H 2 O x 를 생성 Fe 3+ 와 반응하기 전 대부분의 Oxalate 는 O x 2- 로 존재하는 염기성 용액에서는 fe/oxalate 착이온은 대단히 안정. * 산을 가하면 Fe- 착화합물의 해리를 유발

Since we are interested in the free oxalate concentration

Conditional formation constants The Effect of pH on the ligand concentration in a complexation reaction : introduce conditional or effective formation constant. These constants : pH-dependent equilibrium constants. Ex.) For the reaction Fe 3+ with Oxalate The formation constant for first complex At a particular pH value, is constant Conditional constants : simplifies calculations(because C is known, free ligand concentration is not easily determined. * The overall formation constant(beta values) for the higher complexes, (FeO x2 ) -, (FeOx 3 ) 3-, can also be written as conditional constant)

Titrations with inorganic complexing agent

Organic complexing agents Most widely used organic complexing agent Masking agent(section D-8) : 금속과 결합하여 정량을 방해하는 안정한 착화합물 형성

Ethylenediaminetetraacetic acid(EDTA) Six potential sites for bonding a metal ion (four carboxyl group, two amino group) K 1 = 1.02 × 10 -2, K 2 = 2.14 × 10 -3, K 3 = 6.92 × 10 -7, K 4 = 5.50 × # Na 2 H 2 Y. 2H 2 O Aminocarboxylic acid titration

Composition of EDTA solutions as a function of pH.

Complexes of EDTA and Metal ions EDTA : Metal ion = 1 : 1

Equilibrium calculations involving EDTA Titration curve for the reaction of a cation M n+ with EDTA : pM vs reagent volume Conditional formation constants

Computing aplha 4 value for EDTA solutions

EDTA titration curves for 50 mL of 0.005M Ca 2+ (K CaY ’ = 1.75 * ) and Mg 2+ (K MgY ’ = 1.72 * 10 8 ) at pH 10.0.

Influence of pH on the titration of 0.01 M Ca 2+ with 0.01M EDTA

Titration curves for 50 mL 0f 0.01M solutions of various cations at pH 6.0.

Minimum pH needed for satisfactory titration of various cations with EDTA.

The effect of other complexing agents on EDTA titration curves Many cations : form hydroxide precipitates(at high pH) * need auxiliray complexing agent(to keep the cation in solution) For example : Zn(II) : very high concentration of NH 3 and NH 4 Cl Influence of ammonia on the end point for the titration of 50 mL of 0.005M Zn 2+. Solutions are buffered to pH 9.00.

Indicators for EDTA titration Structure and molecular model of Eriochrome Black T. The compound contains a sulfonic group that completely dissociates in water and two phenolic groups that only partially dissociate.

Structure and molecular model of Eriochrome Black T. The compound contains a sulfonic acid group that completely dissociates in water and two phenolic groups that only partially dissociate.

Titration methods employing EDTA  Direct titration - Methods based on indicators for analyte - Methods based on indicators for metal ion - Potentiometric methods - Spectrophotometric methods  Back-titration methods - In the case of slow reaction(Cr(III), Co(III))  Displacement methods - Add excess Mg-EDTA, Zn-EDTA solution React with other Metal ions and form stable complex, Analyze the Mg and Zn with EDTA -