1

Chapter 17

Complexation

Reactions and Titrations

Lewis acid-base concept

Lewis acid electron pair acceptor

[metal cations, Mn+]

Lewis base electron pair donor

[ligand, molecules or ions]

Coordinate covalent bond a bond formedwhen both electrons of the bondare donated by one atom.

Complex ion A metal ion with Lewis base attached to it through coordinate covalent bond.

Complex (Coordinate compound) a compound consisting either of complex ions and other ions of opposite charge or of neutral complex species.

2

Complex-Formation Reactions

• some elements (mostly metal ions) can form complexes with molecules/ions which have a “spare pair” of electrons (ligand).

• The central atom, (usually) a metal cation, accepts the pair of electrons from the ligand to form a coordinate covalent bond.

• The empty orbitals of the cation and the orbital occupied by the bonding pair of electrons on the donor ion or molecule (ligand) form a new molecular orbital.

• The number of coordinate covalent bonds that a cation tends to form with individual ligands (or functional groups on ligands) is known as its coordination number.

• Typical values for coordination numbers are 2, 4 and 6.

• The compounds formed can be neutral, positively or negatively charged depending on the charge of the reacting species.

3+ 3+3 3 6Co + 6NH Co(NH )

Central atom Ligand Coordination number (CN)

2+ 2+3 3 4Cu (blue) + 4:NH Cu(NH ) (dark blue)

2+ 2+

3

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

:NH3, I- Co(NH3)62+, CuI2-

• when there are two groups, it is bidentate,

• 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.

Chelate Effect

• The ability of multidentate ligands to formmore stable metal complexes than thoseformed by similar monodentate ligands

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

4

Complexation Equilibria

1

2

3

4

f

K 23

K 23 2

K 23 3

K 23 4

K 23 4

f 1

2

23

23 2

23

2 3 4

3

3

3

3

3

4

3

2

Cu(NH )Cu(NH )Cu

CuCu(NH )Cu(NH (NH )

Cu(NH )

4 Cu(N

NHNHNH)

Cu(NH )

Cu

NH

NH H )K K K K K

Kf (4) –formation constant

• Complexation reactions occur in a stepwise fasion

1

22 2

32 3 3

2

nn-1 n n

n-1

[ML]M + L ML

[M][L]

[ML ]ML + L ML

[ML][L]

[ML ]ML + L ML

[ML ][L]

...

[ML ]ML + L ML

[ML ][L]

K

K

K

K

Formation Constants (i)

1 1

22 2 1 22

33 3 1 2 33

n1 2 3

[ML]M + L ML

[M][L]

[ML ]M + 2L ML

[M][L]

[ML ]M + 3L ML

[M][L]

...

[ML ]M + L ML ...

[M][L]n n nn

K

K K

K K K

n K K K K

5

2 n

M MLM M

2 nML ML

M M

M 2 n

21 2

21 2

[M] [ML]= =

[ML ] [ML ]= =

[M] [ML] [ML ] ... [ML ]

[M] [M][L] [M][L] ... [M][L]

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

nn

nn

c c

c c

c

Alpha () Values Fraction of the Total Metal Concentration

2

n

M 21 2

1ML 2

1 2

22

ML 21 2

ML 21 2

1=

1 [L] [L] ... [L]

[L]=

1 [L] [L] ... [L]

[L]=

1 [L] [L] ... [L]

[L]=

1 [L] [L] ... [L]

nn

nn

nn

nn

nn

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.

6

EDTA Titrations

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

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.

Octahedron Structure of EDTA-M

5 –five membered rings

7

Acid-Base Properties (H6Y2+)

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

EDTA (H4Y) Disassociation

1 2

3 5 64

4Y

2+ +6 5

- 2- 3- 4-4 3 2

0 1 2 3 4

n+

H Y H Y

H Y H Y H Y HY Y

(or )

M + Y

K K

K K KK

(n-4)+4- (n-4)+

MY n+ 4-

[MY ] MY =

[M ][Y ]K

4- 4-

4T

4-

2+ + - 2- 3- 4-6 5 4 3 2

[Y ] [Y ]

[EDTA]

[Y ]

[H Y ] [H Y ] [H Y] [H Y ] [H Y ] [HY ] [Y ]

c

8

(n-4)+ (n-4)+

MY n+ 4- n+

(n-4)+'MY MYn+

'MY MY

4 T

4T

4

[MY ] [MY ] =

[M ][Y ] [M ]

Conditional formation constant:

[MY ] =

[M

1.0

]

c

c

K

K K

K K as

K’MY is pH dependent!

4- 4-

4 2+ + - 2- 3- 4-T 6 5 4 3 2

+ 6 + 5 + 4 + 31 1 2 1 2 3

1 2 3 4 5 6 + 2 +1 2 3 4 1 2 3 4 5 1 2 3 4 5 6

[Y ] [Y ]

[H Y ] [H Y ] [H Y] [H Y ] [H Y ] [HY ] [Y ]

[H ] + [H ] [H ] + [H ] = /

[H ] [H ]+

c

K K K K K KK K K K K K

K K K K K K K K K K K K K K K

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Composition of EDTA solution as a function of pH.

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

-2.00E-01

0.00E+00

2.00E-01

4.00E-01

6.00E-01

8.00E-01

1.00E+00

1.20E+00

0.00 5.00 10.00 15.00

Alpha4

Alpha4

Excel Computing of Y4- as a function of pH

9

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

(4) Pre-equivalence point: Ca (excess) + Y CaY (disassociation negligible)

[Ca] = [50.00x0.00500-vEDTAx0.0100]/(50.00+vEDTA)

vEDTA (mL) [Ca] pCa5.00 3.64e-3 2.4410.00 2.46e-3 2.6120.00 7.14e-4 3.1524.00 1.35e-4 3.87

(5) At equivalence point:[CaY]/[Ca]^2 = K’MY, [Ca] = {[CaY]/K’MY}^(1/2)[Ca] = {[50.00x0.00500/(50.00+25.00)]/1.75e10}^0.5

=4.36e-7M , pCa = 6.36

(6) Post-equivalence point:Ca + Y CaYx excess ~equivalence concentration

(50.00x0.00500)/(50.00+vEDTA)/{[Ca][(0.0100xvEDTA-50.00x0.00500)/(50.00+vEDTA)]}=K’MY

[Ca]=0.25/{[0.0100xvEDTA-0.25]xK’MY}=1.43e-11/(0.0100xvEDTA-0.25)

VEDTA(mL) [Ca] pCa26.00 1.42e-9 8.8530.00 2.86-10 9.5440.00 9.53e-11 10.0250.00 5.71e-11 10.24

10

0

2

4

6

8

10

12

0 10 20 30 40 50

V (EDTA, mL)

pC

a

Ca-EDTA Titration Curve at pH 10

EDTA Titration Curves forCa (K’2.75e10) and Mg (K’1.72e8) at pH 10

Larger the K’MY

Larger the pM change

Eriochrome Black T

Typical Metal Ion Indicator--Eriochrome Black T(EBT)

A Weak Acid and azo dye

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Indicators for EDTA Titrations

At the end point:MgIn + EDTA MgEDTA + In(red) (colorless) (colorless) (Blue)

Before Titration:Mg2+ + In MgIn

(colorless) (blue) (red)

During Titration: Before the end pointMg2+ + EDTA MgEDTA

(free Mg2+ ions) (Solution red due to MgIn complex)

Compounds changing colour when binding to metal ion.Kf for Metal-In < Kf for Metal-EDTA.

EDTA Titration Solution pH must be Controlled for EBT Indicator

2 1

32- -

2--

Factors Influencing Chemical Specification of Eriochrome Black T (EBT):

1. pH, as EBT is a weak acid,

HIn (blue)

rcolo varies on pH:

= 5e-

HIn (blue)

rH In ( ed)

In (orange)

7K 2

f [M-In]

2+f [M-In]

f [M-In] f [M-Y]

2-

= 2.8e-12

so pH must be controlled ~7-12 [dominated by ]

2. of M-In complex,

Mg

+ =1.0e

HIn (blue)

In (

n

MgI 7

must

blue

be smaller tha

n) (red)

K

K

K

K K

Influence of pH on Titration Curves of Ca-EDTA

The higher pHThe larger pCa changepH should > 8 forCa-EDTA titrations

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Lower pHs OKfor Large KMY Complexes

pH = 6.0

Minimum pH needed for satisfactory titration of

various cations with EDTA.

Effect of Other Complexing Agentson EDTA Titration Curves

• A second complexing agent added to maintain theanalyte metal ion in solution (many metal ions forminsoluble hydroxides or oxides at slightly high pH).

• To "mask" or remove interfering ions present in thesample matrix.

• The second complexing agent (“masking agent“) usuallyhas a higher affinity for the interfering ion than the EDTAto prevent it from reacting with the EDTA.

• Most buffers will complex metal ions because they alsocontain functional groups (-OH, -COOH, -NH2) which canform coordinate covalent bonds and their effect on thefree metal ion concentration must be considered.

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Influence of [NH3] on the Zn-EDTA Titration at pH 9.00

2+ 2+3 3 4

3-

2-3 4

Before Titration Zn + 4NH Zn(NH )

HY EDTA (during Titration)

ZnY + 3NH NH

Ammonia decreases the change in pZn in the

equivalence-point region.

n

M

4 T 4 T

4

M

Y

MY

Y

M

Y M

M

M

M

M

M + Y MY (EDTA complexing)

M + nL ML (second complexing agent)

[MY] = ([M],[Y]--free concentration)

[M][Y]

[MY] [MY] =

( )(

Conditional formation constant:

[MY]"

() )

=

c

K

c

K K

c

c

Kc

21

M T

M2M

1Where

1 [L] [L] ... [L

[M]

]nn

c

c

EDTA Titration Techniques

1. Direct Titration

*Buffer analyte to pH where Kf’ for MY is large,*and M-In colour distinct from free In color.

*Auxiliary complexing agent may be used.

2. Back Titration

*Known excess std EDTA added.

*Excess EDTA then titrated with a std sol’n of a secondmetal ion.

*Note: Std metal ion for back titration must not displace analyte from MY complex.

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2. Back Titration: When to apply it

*Analyte precipitates in the absence of EDTA.

*Analyte reacts too slowly with EDTA.

*Analyte blocks indicator

3. Displacement Titration

*Analyte treated with excess Mg(EDTA)

M + MgY MY + Mg

* Kf’ for MY > Kf’ for MgY

*Metal ions with no satisfactory indicator.

4. Indirect Titration

*Anions analyzed: CO32-, CrO4

2-, S2-, and SO42-.

Precipitate SO42- with excess Ba2+ at pH 1.

*BaSO4(s) washed & boiled with excess EDTA at pH 10.

BaSO4(s) + EDTA(aq) BaY2-(aq) + SO42-(aq)

Excess EDTA back titrated:EDTA(aq) + Mg2+MgY2-(aq)

Alternatively: *Precipitate SO42- with excess

Ba2+ at pH 1.

*Filter & wash precipitate.

*Treat excess metal ion in filtrate with EDTA.

5. Masking

*Masking Agent: Protects some component of analytefrom reacting with EDTA.

*F- masks Hg2+, Fe3+, Ti4+, and Be2+.

*CN- masks Cd2+, Zn2+, Hg2+, Co2+, Cu+, Ag+, Ni2+, Pd2+, Pt2+, Hg2+, Fe2+, and Fe3+,

but not Mg2+, Ca2+, Mn2+, Pb2+.

*Triethanolamine: Al3+, Fe3+, and Mn2+.

*2,3-dimercapto-1-propanol: Bi3+, Cd2+, Cu2+, Hg2+, and Pb2+.

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*Demasking: Releasing masking agent from analyte.

mHCOmHCNM mnm 2

mH2C

CN

OH

Mn+

Metal-CyanideComplex

Formaldehyde

*Oxidation with H2O2 releases Cu2+ from Cu+-Thiourea complex.

*Thus, analyte selectivity:1. pH control2. Masking3. Demasking