EDTA Titrations

Chelation in Biochemistry

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

Metal-Chelate Complexes• 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)

• A ligand that can attach to a metal by more than one atom is multidentate or a chelating ligand

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

Chelating Agents in Analytical Chemistry

Ethylenediamenetetraacetic acid (EDTA)

EDTA forms 1:1 complexes with metal ions by with 6 ligands: 4 O & 2N. EDTA is the most used chelating agent in analytical chemistry, e.g. water hardness.

Acid/Base Properties of EDTA• EDTA is a hexaprotic system (H6Y2+) with 4 carboxylic acids

and 2 ammoniums:

• We usually express the equilibrium for the formation of complex ion in terms of the Y4- form (all six protons dissociated). You should not take this to mean that only the Y4- form reacts

24.10p

16.6p

66.2p

0.2p

5.1p

0.0p

6

5

4

3

2

1

K

K

K

K

K

K

NH+

NH+

O OH

O

OH

O

OH

O OH

Fraction of EDTA in Y4- Form• Similar to acids and bases, we can define

fractional compositions, α, defined as the fraction of “free” EDTA in a particular form.– “Free” means uncomplexed EDTA– So, for Y4-:

EDTA

Y

YHYYHYHYHYHYH

Y

-4

-4-3-22

-345

26

-4

Y4

EDTA Complexes• The equilibrium constant for a reaction of metal

with EDTA is called the formation constant, Kf, or the stability constant:

• Again, 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

pH Dependence of αY4-

Formation Constants for M-EDTA Complexes

Some Metals Form 7 or 8 Coordinate Complexes

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.

Conditional Formation Constant• 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

Y44 Y

-4-4

Y

EDTA M

MY

Y M

MY

-4Y

4

4

4

f

n

n

n

n

K

EDTA M

MY 4

fYf -4

n

n' KK fYf

44 MYEDTAM KK 'nn

Example• 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 (Table 13-2)

• 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 '

xxx -0.1 Conc

0.1 0 0 Conc

CaYEDTACa

f

i

22

6@pH M 103.0

10@pH M 104.2Ca4-

62

x

Calcium/EDTA Titration Curve

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.

Generic Titration Curve

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

Before the Equivalence Point• 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

At the Equivalence Point• 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

After the Equivalence Point• 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

Auxiliary Complexing Agents• 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

Metal Ion Indicators• 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

Metal Ion Indicator Compounds

EDTA Titration Techniques• 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.

EDTA Titration Techniques (2)• 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.

Example Titration• 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