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 H3YH4Y 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 = =