Complexation Reactions and Titrations

Chapter 17

Complexation Reactions and Titrations

1

Complexation Titration

? Also knowns as complexometric titration, complexometry, or chelatometry

? One of the classical titrimetric methods developed for the rapid and quantitative chemical analysis of metal ions.

? Based on complex formation between metal ion (cation) and complexing agent (ligand).

2

Bronsted-Lowery vs Lewis Acid-Base Concept

? Lewis acid: electron pair acceptor (metal cations, Mn+)

? Lewis base: electron pair donor (ligand, can be molecules or ions)

? Coordinate covalent bond: a bond formed when both electrons of the bond are donated by one atom.

A + B: A B

3

1

Ag+ + 2(:NH3 ) [H3N:Ag:NH3 ]+ Electron cofiguration Ag [Kr]4d10 5s15p0 Ag+ [Kr]4d105s05p0, sp hybrid orbitals accept 2 pairs of electrons, linear geometry

? Complex ion: A charged compound (+ or -) consisting of coordinate covalent bond. ? Complex (Coordinate compound): a compound of neutral complex species.

[Ag(NH3)2 ]+ vs [Ag(NH3)2 ](OH)

4

Complex ion

Co 32++

+ 6NH3

Co(NH

3

) 32++ 6

Acid

Base

Central atom Ligand

Coordination number (CN)

Cu2+ (blue) + 4:NH3 Cu(NH3 )42+ (dark blue)

5

? When a ligand has a single complexing or donor group in its structure, it is said to be unidentate (single-toothed),

:NH3, Cl- Co(NH3)62+, CuCl42-

? when there are two groups, it is bidentate,

6

2

? When there are three (four) groups, it is called tridentate (tetradentate) ligand, etc.

? When a bidentate (or higher number of donor groups present in the ligand) forms a complex with a metal cation, we call the resulting compound a metal chelate ("kee'late"-claw).

? As titrants, multidentate ligands, particularly those with 4 to 6 donor groups have the advantage that they usually react in a single step process, and their reactions with the metal cation are more complete than their unidentate counterparts.

7

Chelate Effect

? The ability of multidentate ligands to form more stable metal complexes than those formed by similar monodentate ligands

? Often results from the formation of 5membered "ring" with metal and two atoms on the ligand

8

Complexation Equilibria

Cu2 NH3 K1 Cu (NH3)2 Cu(NH3)2 NH3 K2 Cu (NH3)22 Cu(NH3)22 NH3 K3 Cu (NH3)23 Cu(NH3)23 NH3 K4 Cu (NH3)24

Cu2 4NH3 Kf Cu (NH3)24 K f K1 K2 K3K 4 4

Kf (4) ?formation constant 9

3

? Complexation reactions occur in a stepwise fasion

M + L ML

[ML] K1 [M][L]

ML + L ML2

K2

[ML2 ] [ML][L]

ML2 + L ML3

K3

[ML3 ] [ML2 ][L]

...

MLn-1 + L MLn

Kn

[MLn ] [MLn-1 ][L]

10

Formation Constants (i)

M + L ML

1

[ML] [M][L]

K1

M + 2L ML2

2

[ML2 ] [M][L]2

K1K2

M + 3L ML3

3

[ML3 ] [M][L]3

K1K2 K3

...

M + nL

ML n

n

[MLn ] [M][L]n

K1K 2 K3 ...K n

i : cumulated or collective formation constant with i L.

11

Alpha () Values Fraction of the Total Metal Concentration

M

=

[M] cM

ML

=

[ML] cM

ML2

=

[ML2 cM

]

MLn

=

[MLn cM

]

cM [M] [ML] [ML2 ] ... [MLn ]

[M] 1[M][L] 2[M][L]2 ... n[M][L]n

[M]{1 1[L] 2[L]2 ... n[L]n}

12

4

M

=

1

1[L]

1 2[L]2

...

n [L]n

ML

=

1

1[L]

1[L] 2[L]2

...

n

[L]n

ML2

=

1

1[

L]

2[L]2 2[L]2

...

n

[

L]n

MLn

=

1

1[L]

n [L]n 2[L]2

...

n

[ L]n

i = f (i ,[L]) [vs. i f (Ka ,[H+ ])]

13

Titration Curves of MLn

(A) Tetradentate ligand, 1:1 (B) Bidentate ligand 2:1 (C) Unidentate ligand, 4:1

Tetradentate or hexadentate ligands are more satisfactory as titrants than ligands with a lesser number of donor groups because their reactions with cations are more complete and they tend to form 1:1 complexes.

14

Ethylenediaminetetraacetic Acid (EDTA)

Most widely used complexometric titrant, Hexadentate ligand (4 ?COOH+2 amino groups) 15

5

EDTA

? It forms 1:1 complexes with most metals. (Not with Group 1A metals ? Na, K, Li)

? Forms stable water soluble complexes. ? High formation constants. ? A primary standard material ? a highly

purified compound that serves as a reference material. 16

Octahedron Structure of EDTA-M

5 ? five membered rings

17

18

6

Acid-Base Properties (H6Y2+)

The first four values apply to carboxyl protons, and the last two are for the ammonium protons. The neutral acid is tetraprotic, with the formula H4Y. A commonly used reagent is the disodium salt, Na2H2Y2H2O.

19

EDTA (H4Y) Disassociation

H6Y2+ K1 H5Y+ K2

H4Y K3 H3Y- K4 H2Y2- K5 HY3- K6 Y4-

0

1

2

3

4 (or Y4 )

Mn+ + Y4- MY(n-4)+

K MY

=

[MY(n-4)+ ] [Mn+ ][Y4- ]

4

[Y4- ] cT

[Y4- ] [EDTA]

[Y4- ]

[H6Y2+ ] [H5Y+ ] [H4Y] [H3Y- ] [H2Y2- ] [HY3- ] [Y4- ]

20

K MY

=

[MY(n-4)+ ] [MY(n-4)+ ] [Mn+ ][Y4- ] [Mn+ ]4cT

Conditional formation constant:

K

' MY

=

[MY(n-4)+ ] [Mn+ ]cT

4 KMY

K

' MY

KMY

as

4

1.0

K'MY is pH dependent!

21

7

4

[Y4- ] cT

[Y4- ] [H6Y2+ ] [H5Y+ ] [H4Y] [H3Y- ] [H2Y2- ] [HY3- ] [Y4- ]

=K1K2 K3K4 K5K6

/

[KH1K+ ]26K+K3K1[4H[H+ ]+5]2KK1K1K2[2HK+3]K4 +4 KK51[KH2+K]+3[KH1+K]32

K3

K4

K5

K6

4

Composition of EDTA solution as a function of pH.

22

Excel Computing of Y4- as a function of pH

1.20E+00 1.00E+00 8.00E-01 6.00E-01 4.00E-01 2.00E-01 0.00E+00

0.00 -2.00E-01

Alpha4

5.00

10.00

15.00

Alpha4

Distribution of unprotonated form of Y4- as a function of pH

23

Example 17-4 Use speadsheet to construct the titration curve of pCa versus volume of EDTA for 50.0 mL of 0.00500 M Ca2+ being titrated with 0.0100 M EDTA in a solution buffered to a constant pH of 10.0

(1) pH = 10.0, 4 = 0.35, KCaY = 5.0 e10, K'CaY = 4 KCaY = 1.75e10 (2) Equivalence point:

vEDTA = 50x0.00500/0.0100 = 25.0 mL (3) Initial pCa: [Ca2+] = 0.00500 M, pCa = 2.30

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