Chelating ligands can form complex ions with metals through multiple ligands. This is important in many areas, especially biochemistry.

Metals are Lewis acids that accept electron pairs from donating ligands that act as Lewis bases CN- is a common monodentate ligand, binding

to a metal ion through one atom (C) Metals can bind to multiple ligands (usually 6)

multidentate or a chelating ligand can attach to a metal by more than one atom

Chelating agents can be used for titration of metals to form complex ions (complexometric titration)

EDTA forms 1:1 complexes with metal ions with 6 ligands:

4 O & 2N; used in determination of water hardness.

EDTA is a hexaprotic system (H6Y2+) with 4 carboxylic acids and 2 ammoniums:

equilibrium for the formation of complex ion usually expressed in terms of the Y4- form (all six protons dissociated). However, this does not mean that only the Y4- form reacts

NH+

NH+

O OH

O

OH

O

OH

O OH 24.10K

16.6K

66.2K

0.2K

5.1K

0.0K

6

5

4

3

2

1

p

p

p

p

p

p

Similar to acids and bases, fractional compositions, α, can be defined as the fraction of “free” EDTA in a particular form. “Free” means uncomplexed EDTA thus, for Y4-:

EDTA

Y

YHYYHYHYHYHYH

Y

-4

-4-3-22

-345

26

-4

Y4

formation constant, Kf, or the stability constant:

Kf could have been defined for any form of EDTA, it should not be understood that only the Y4- reacts to form complex ion.

4

4

f44

Y M

MY MYYM

n

nnn K

The rings formed in the M-EDTA complex can become strained. If the oxygen atoms pull back toward the nitrogen atoms, the strain is relieved. This opens up the metal to other ligands. Water molecules frequently occupy these sites.

We saw from the fraction plot that most of the EDTA is not in the form of Y4- below a pH ~10.

We can derive a more useful equilibrium equation by rearranging the fraction relationship:

If we fix the pH of the titration with a buffer, then αY4- is a constant that can be combined with Kf

EDTAYEDTA

YY

-4-4

Y 44

EDTA M

MYY M

MY

-4Y

4

4

4

f

n

n

n

n

K

EDTA M

MY 4

fYf -4

n

n' KK

fYf4

4 MYEDTAM KK 'nn

Calculate the concentration of free Ca2+ in a solution of 0.10 M CaY2- at pH 10 and pH 6. Kf for CaY2- is 4.9x1010

At low pH, the metal-complex is less stable

22

2

f

1.0

EDTA Ca

CaY

x

xK '

6105fYf

1010fYf

101.1)109.4)(103.2( 6.00,pHat

108.1)109.4)(36.0( 10.00,pHat

4

4

KK

KK'

'

fYf

224 CaYEDTACa KK '

6@pH M 103.0

10@pH M 104.2Ca4-

62

x

For calcium, the end point becomes hard to detect below ~pH=8. The formation constant is too small below this point. This can be used to separate metals. At pH=4, Ca does not perform significant complexaion with EDTA. However, Fe can still form the complex, so it can be titrated without interference from Ca.

Like a strong acid/strong base titration, there are three points on the titration curve of a metal with EDTA: before, at, and after the equivalence point.

We’ll consider a titration where we have 50.0 mL of 0.040 M Ca2+ (buffered at pH=10) with 0.080 M EDTA.

Ve=25.0 mL

1010f 108.1)109.4)(36.0( 'K

What’s pCa2+ when we have added 5.0 mL of EDTA?

M 0291.055.0

50.0(0.040)

0.25

5.0-25.0Ca 2

FractionRemaining

InitialConcentration

DilutionFactor

1.54)0291.0log(pCa 2

What’s pCa2+ when we have added 25.0 mL of EDTA? At the equivalence point almost all the

metal is in the form CaY2-

Free Calcium is small and can be found w/ algebra

M 0267.075.0

50.0(0.040) CaY -2

InitialConcentration

DilutionFactor

91.5)102.1log(pCa 62

xxx -0.0267 Conc

0.0267 0 0 Conc

CaYEDTACa

f

i

22

1022

2

f 108.10267.0

EDTA Ca

CaY

x

xK '

M 102.1 6x

What’s pCa2+ when we have added 26.0 mL of EDTA? We have 1.0 mL excess EDTA

M 1005.176.0

1.0 (0.080) EDTA 3

InitialConcentration

DilutionFactor

86.8pCa 2

M 1063.276.0

50.0 (0.040) CaY 2-2

InitialConcentration

DilutionFactor

1032

2

2

2

f 108.1)1005.1(Ca

1063.2

EDTA Ca

CaY

'K M 104.1Ca 92

In aqueous solution, metal-hydroxide complexes or precipitates can form, especially at alkaline pH

We often have to use an auxiliary complexing agent This is a ligand that binds strongly enough to

the metal to prevent hydroxide precipitation, but weak enough to be displaced by EDTA

Ammonia is a common auxiliary complex for transition metals like zinc

To detect the end point of EDTA titrations, we usually use a metal ion indicator or an ion-selective electrode (Ch. 15)

Metal ion indicators change color when the metal ion is bound to EDTA:

Eriochrome black T is an organic ion

The indicator must bind less strongly than EDTA

(Blue) (Clear) (Clear) (Red)

EbT MgEDTA EDTA MgEbT

Direct titration: analyte is titrated with standard EDTA with solution buffered at a pH where Kf

’ is large

Back titration: known excess of EDTA is added to analyte. Excess EDTA is titrated with 2nd metal ion.

Displacement titration: For metals without a good indicator ion, the analyte can be treated with excess Mg(EDTA)2-. The analyte displaces Mg, and than Mg can be titrated with standard EDTA

Indirect titration: Anions can be analyzed by precipitation with excess metal ion and then titration of the metal in the dissolved precipitate with EDTA.

25.0 mL of an unknown Ni2+ solution was treated with 25.00 mL of 0.05283 M Na2EDTA. The pH of the solution was buffered to 5.5 and than back-titrated with 17.61 mL of 0.02299 M Zn2+. What was the unknown Ni2+ M?

EDTA mmol 1.32M) 3mL)(0.0528 (25.00 EDTA mol 22 Znmmol 4049.0M) 9mL)(0.0229 (17.61 Znmol

mmol 916.0 Znmmol 0.4049-EDTA mmol 321.1 Ni mol 22

M 0.0366mL) 00mmol)/(25. 916.0( Ni M 2

-2-42 ZnYY Zn

-2-42 NiY Y Ni