Chapter 9

Complexation and Precipitation Titrations

Forming Complexes

Most metals ions react with electron-pair donors to form coordination compounds or complexes.

The donor species (ligand) must have at least one pair of unshared electrons available for bond formation.

Cu(H2O)4+2, Cu(NH3)4

+2, Cu(NH2CH2COO)2

CuCl4-2

Ligands

A ligands is a neutral molecule or ion having a lone pair that can be used to form a bond to a metal ion.

Chelating agents: unidentate, bidentate, tridentate, tetradentate, pentadentate, hexadentate

Titration of a Single Anion

Calculate the pAg of the solution during the titration of 50ml of 0.05M NaCl with 0.1M AgNO3 after the addition of the following volumes of reagent: (a) 0ml (b) 24.5 ml (c) 25 ml (d) 25.5ml Ksp=1.82×10-10

A: 50 ml of 0.05M NaCl with 0.1 M AgNO3

B: 50 ml of 0.005M NaCl with 0.01 M AgNO3

Signaling the End Point

Mohr methodFormation of a colored precipitate

The Mohr method must be carried out at a pH of 7-10 because chromate ion is the conjugate base of the weak chromic acid.

2Ag+(aq)+X-(aq)

2Ag+(aq)+CrO4-2(aq) Ag2CrO4(s)

AgX(s) white

red

Ethylenediaminetetraacetic Acid

C CN N

CH2-COOH

CH2-COOH

HOOC-H2C

HOOC-H2C

H H

HH

The EDTA molecule has six potential sites for bonding a metal ion: the four carboxyl groups and two amino groups.

Acidic Properties of EDTA

C CN N

CH2-COOH

CH2-COOH

HOOC-H2C

HOOC-H2C

H H

HH

C CN N

CH2-COOH

CH2-COO-

-OOC-H2C

HOOC-H2C

H H

HH

H++H

H4Y

C CN N

CH2-COOH

CH2-COO-

-OOC-H2C

-OOC-H2C

H H

HH

H++H

C CN N

CH2-COO-

CH2-COO-

-OOC-H2C

-OOC-H2C

H H

HH

H++H

H3Y-

K1=1.02×10-2

H2Y-2

K2=2.14×10-3

C CN N

CH2-COO-

CH2-COO-

-OOC-H2C

-OOC-H2C

H H

HH

H+

C CN N

CH2-COO-

CH2-COO-

-OOC-H2C

-OOC-H2C

H H

HH

HY-3

K3=6.92×10-7

Y-4

K4=5.5×10-11

The Nature of EDTA Complexes with Metal Ions

The reagent combines with metal ions in a 1:1 ratio regardless of the charge on the cation.

M+n+Y-4 MY+(n-4)

]][[

][4

)4(

YM

MYK

n

n

MY

Equilibrium Calculations Involving EDTA

EDTA titrations are always performed in solutions that are buffered to a known pH to avoid interferences by other cations or to ensure satisfactory indicator behavior.

Conditional Formation Constant

.applicable is αfor which pH at theonly constant a is K

][

][

][

][

]][[

][

][][][][][

][][][][

4'MY

)4(

4'

4

)4(

4

)4(

432

234

43213212

213

14

43214

Tn

n

MYMY

Tn

n

MYn

n

MY

T

CM

MYKK

CM

MYK

YM

MYK

YHYHYHHYYC

KKKKHKKKHKKHKH

KKKK

D

HKKK

D

HKK

D

HK

D

H

KKKKHKKKHKKHKHD

][

][

][

][

][][][][

3213

221

2

31

1

4

0

43213212

213

14

Calculation of the Cation Concentration in EDTA Solutions

Calculate 4 Calculate conditional formation constants K’

Ni+2+Y-4 NiY-2

184

)4(

102.4]][[

][

YNi

NiYK

n

n

MY

0.015M-xx x

Calculate equilibrium [Ni+2] at pH=3 and pH=8

pH 4 pH 4

2 3.7×10-14 8 5.4×10-3

3 2.5×10-11 9 5.2×10-2

4 3.6×10-9 10 3.5×10-1

5 3.5×10-7 11 8.5×10-1

6 2.2×10-5 12 9.8×10-1

7 4.8×10-4

Value for 4 for EDTA at selected pH value

)(101.8][Ni1027.2015.0

1027.2102.4104.5K 8pH

)(102.1][Ni1005.1015.0

1005.1102.4105.2K 3pH

KαK

CY][H]Y[H]Y[H][HY][Y][Ni

102162

16183'NiY

5282

81811'NiY

NiY4'NiY

T432

2342

Mxx

x

Mxx

x

Indicator for EDTA TitrationsErtichrome Black T

H2O+H2In-HIn-2+H3O+

H2O+HIn-2In-3+H3O+

O2N

SO3-

OH

N

N

OH

K1=5×10-7

K2=2.8×10-12

red blue

blue orange

MIn-+HY-3 HIn-2+MY-2

red blue

At the end point: MgIn + EDTA MgEDTA + In-

(red) (colourless) (colourless) (Blue)

Before Titration:• Mg2+ + In- MgIn (colourless) (blue) (red)

During Titration: Before the end point• Mg2+ + 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.

Requirements for Indicator

Metal-indicator complex must be less stable than the metal-EDTA complex.

Binding between metal and indicator must not be too weak. It has to avoid EDTA replacing at the beginning of the titration.

In general, the metal-indicator complex should be 10 to 100 times less stable than the metal-titrant complex.

Titration of Ca+2 and Mg+2

The K’ of EDTA of Ca+2 and Mg+2 are too close to differentiate between them in an EDTA titration.

Generally, they will titrate together. This titration is used to determine total

hardness of water.

Titration of Ca+2

EB-T cannot be used to indicate the direct titration of Ca+2 in the absence of Mg+2 with EDTA.

The indicator forms too weak a complex with Ca+2 to give a sharp end point.

Resolution

A small measured amount of Mg+2 is added to the Ca+2 solution.

Ca+2 gives more stable K’ than Mg+2. A correction is made for the amount of EDTA

used for titration of the Mg+2 .

At the end point: MgIn + EDTA MgEDTA + In-

(red) (colourless) (colourless) (Blue)

Before Titration:• Ca+2+EDTA+Mg2++In- MgIn+CaEDTA

During Titration: Before the end point• Ca2+ + EDTA CaEDTA

Kf for Mg+2-EDTA < Kf for Ca+2-EDTA.

EDTA Titration CurveEDTA Titration CurveRegion 1

Excess Mn+ left after each additionof EDTA. Conc. of free metal equal to conc. of unreacted Mn+.

Region 2

Equivalence point:[Mn+] = [EDTA]Some free Mn+ generated by MYn-4 Mn+ + EDTA

Region 3Excess EDTA. Virtually all metalin MYn-4 form.