《分析化学》课程教学课件（PPT讲稿，英文）Chapter 5 Complexometric Titration

G归东理王大军 Analytical Chemistry Chapter 5 Complexometric Titration Learning objectives >To master the fundamental principles and relevant calculations about complex equilibrium; >To master the calculation of titration curve,the end point and the titration and the titration break; >To understand the calculation of titration error; >To know about the application of complexometric titr

Analytical Chemistry 1 Chapter 5 Complexometric Titration Learning objectives ➢To master the fundamental principles and relevant calculations about complex equilibrium; ➢To master the calculation of titration curve, the end point and the titration and the titration break; ➢To understand the calculation of titration error; ➢To know about the application of complexometric titration

©山东理王大军 Analytical Chemistry 5.1 Metal-chelate complexes Metal ions are Lewis acids,accepting electron pairs from electron-donating ligands that are lewis bases. Most transition metal ions bind six ligand atoms,a ligand that attaches to a metal ion through more than one ligand atom is said to be multidentate. A multidentate ligand such as EDTA is called a chelating ligand. Ag+2CN→Ag(CN)2 2

Analytical Chemistry 2 5.1 Metal-chelate complexes Metal ions are Lewis acids, accepting electron pairs from electron-donating ligands that are lewis bases. Most transition metal ions bind six ligand atoms, a ligand that attaches to a metal ion through more than one ligand atom is said to be multidentate. A multidentate ligand such as EDTA is called a chelating ligand. - 2 - Ag + 2CN  Ag(CN) +

⑤归东理工大 Analytical Chemistry A titration based on complex formation is called a complexometric titration.The equilibrium constant for the reaction of a metal with a ligand is called the formation constant, also termed the stability constant,Ks. For example,the stability constant of the reaction above is Ks= [Ag(CN) [Ag+JICN] We could write the equilibrium in the opposite direction as dissociation. 3

Analytical Chemistry 3 A titration based on complex formation is called a complexometric titration. The equilibrium constant for the reaction of a metal with a ligand is called the formation constant, also termed the stability constant, Ks . For example, the stability constant of the reaction above is - 2 - 2 s [Ag ]lCN ] [Ag(CN) ] + K = We could write the equilibrium in the opposite direction , as dissociation

G山东我王大军 Analytical Chemistry And the equilibrium constant is simple the reciprocal of the formation constant,and it is called instability constant Ki or dissociation constant,Ka _1_[Ag*JICN K=[Ag(CN):]

Analytical Chemistry 4 And the equilibrium constant is simple the reciprocal of the formation constant, and it is called instability constant Ki or dissociation constant, Kd . [Ag(CN) ] 1 [Ag ]lCN ] - 2 - 2 d + = = Ks K

G归东理王大军 Analytical Chemistry 5.2 EDTA EDTA (ethylenediaminetetreacetic acid)is the most widely used chelator in analytical chemistry. >EDTA reacts with metal ion in a 1:1 molar ratio; >All EDTA chelators are soluble in water; >Most of them are colorless,with the exception of the chelators of a few transition metal ion. 1.Acid-base properties EDTA is a hexaprotic system,designated HoY2+.The neutral acid is tetraprotic,with the formula H4Y. 5

Analytical Chemistry 5 5.2 EDTA EDTA (ethylenediaminetetreacetic acid) is the most widely used chelator in analytical chemistry. ➢EDTA reacts with metal ion in a 1:1 molar ratio; ➢All EDTA chelators are soluble in water; ➢Most of them are colorless, with the exception of the chelators of a few transition metal ion. 1. Acid-base properties EDTA is a hexaprotic system, designated H6Y2+ . The neutral acid is tetraprotic, with the formula H4Y

G山东理王大 Analytical Chemistry HOOCH2C CH2CQO :NHt一C—CNH H2H2 OOCH2C CH2COOH A commonly used reagent is the disodium salt,Na2H2 Y.2H2O. the fraction of EDTA in each of its protooonated forms is plotted in the figure beow. 6

Analytical Chemistry 6 NH + C H2 C H2 HOOCH2 C - OOCH2 C NH + CH2 COOH CH2 COO - ： ： ： ： ·· ·· A commonly used reagent is the disodium salt, Na2H2Y·2H2O. the fraction of EDTA in each of its protooonated forms is plotted in the figure beow

G山东理工大军 Analytical Chemistry 6 H.Y HY, 0.8 0.6 10 12 14 PH We can define for each species as the fraction of EDTA in that form.For example

Analytical Chemistry 7 H6Y - HY Y H2Y H5Y H3Y￾H4Y We can define for each species as the fraction of EDTA in that form. For example

©山东龙子大军 Analytical Chemistry [Y4-] [H,Y2+]+[HY+]+H4Y]+H3Y-]+H2Y2-]+[HY3]+[Y4] if we let [Y']represent the total concentration of all free EDTA species in the solution.By free'we mean EDTA not complexed to metal ions,then, [Y] QYH) [Y4] where ayo is the reciptrocal of the fraction of Y4,and we call it coefficient of acid effect. values of ay can be calculated if the pH is given,or obtained from the table. 8

Analytical Chemistry 8 [H Y ] [H Y ] [H Y] [H Y ] [H Y ] [HY ] [Y ] [Y ] 2- 3 4 5 4 3 2 2 6 4- Y + + − − − + + + + + +  = if we let [Y´] represent the total concentration of all free EDTA species in the solution. By ‘free’ we mean EDTA not complexed to metal ions, then, [Y ] [Y'] Y(H) 4−  = where αY(H) is the reciptrocal of the fraction of Y4- , and we call it coefficient of acid effect. values of αY(H) can be calculated if the pH is given, or obtained from the table

⑤归东理工大 Analytical Chemistry 2.EDTA complexes the formation constant Kvy of a metal-EDTA complex is the equilibrium constant for the reaction M+Y→MY KMY [MY] [MJ[Y] Note that Kyy is defined for reaction of the species Y4-with the metal ion.This species is only one of the seven different forms of free EDTA present in the solution. 9

Analytical Chemistry 9 2. EDTA complexes the formation constant KMY of a metal-EDTA complex is the equilibrium constant for the reaction M+Y MY [M][Y] [MY] KMY = Note that KMY is defined for reaction of the species Y4- with the metal ion. This species is only one of the seven different forms of free EDTA present in the solution

G山东龙子大军 Analytical Chemistry the formation constants for most EDTA complexes are quite large and tend to be larger for more positively charged metal ions. 3.Conditional formation constant Most of the EDTA is not Y4-below pH 10.24,so [Y-]-IY] aY(H) the equilibrium constant for a metal ion can now be written as KMYMY KMY M[Y'] CY(H) 10

Analytical Chemistry 10 the formation constants for most EDTA complexes are quite large and tend to be larger for more positively charged metal ions. 3. Conditional formation constant Most of the EDTA is not Y4- below pH 10.24, so Y(H) 4 [Y'] [Y ]  = − the equilibrium constant for a metal ion can now be written as Y(H) MY MY [M][Y'] [MY] '  K K = =