Chapter 12

EDTA Titrations

Antibiotic chelate captures its prey

Mn+ + nL MLnn+

Mn+: central metal ion (Lewis acid)L: ligand (Lewis base)

Complexometric titrations are useful for the determination of metal.

15A Forming Metal-Chelate Complexes

Complexes: Formation Constants

ligand: have a pair of unshared electronsEx: N, O, S

NH3

NH3

[H3N:Cu:NH3 ] 2+Cu2+ + 4:NH3

pale bule deep bule

::

23

23f

233

7

3

23f2

2333

3

3

3f1

33

][NH ][Ag

])[Ag(NHK

)Ag(NH2NHAg

102.5][NH ][Ag

])[Ag(NHK

)Ag(NHNH)Ag(NH

102.5][NH ][Ag

])[Ag(NHK

)Ag(NHNHAg

8

23

23

fd

i

323

7f2f1f

104.0])[Ag(NH

][NH ][Ag

K1

)(K constant onDissociati

)(K constant yInstabilit

2NHAg)Ag(NH

constant. stabilityor constant formatoin

102.5KKK

The most widely used chelating agent in titration is ethylenediaminetetraacetic acid (EDTA)

HO2CH2C

NH+CH2CH2H+N-O2CH2C

CH2CO2H

CH2CO2-

H4Y hexadentate

15D EDTA: (1) A hexadentate

(2) The chelon effect or chelate effect:

Multidentate chelating agents form stronger complexes (Kf ) with metal ions than bidentate or monodentate ligands.

指 multidentate 之 Kf 比 monodentate 之 Kf 為大， 即較穩定。

15D EDTA

(3) Neutral EDTA is a tetrabasic acid

Y][H][Y ][H

105.5K YHHY

Y][H][HY ][H

106.9K HYHYH

Y][H]Y[H ][H

102.2K YHHYH

Y][H]Y[H ][H

101.0K YHHYH

4

411-

a443

4

37-

a332

2

4

223-

a22

23

4

32-a134

(5) Five species EDTA as a function of pH

Effect of pH on EDTA equilibria

4HCaYYCa

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

][CaC

YHYH

YHHYYCaCaY

242

432234YH

2YH

4H

3H

22

H3H422

4

4

15D-2 EDTA Complex with Metal Ions

(1) Forms strong 1:1 complexes regardless of the charge on the cation

(2) Chelate with all cations

(3) Since the anion Y4- is the ligand species in complex formation, the complexation equilibria are affected markedly by the pH.

(4) The formation constant (Table 15-5)

[Y4-] ][Ca][CaY

K

CaYYCa

2

2

f

242

15D-3 Equilibrium: pH dependent M-Y

YH44

a4a3a2a1a3a2a12

a2a13

a14

a4a3a2a14

a4a3a2a1

4

a4a3a2

3

a4a3

2

a444

YH

4

4

Cα Y

KKKK][HKKK][HKK][HK][H

KKKKα

KKKK][H

KKK][H

KK][H

K][H

1α1

][Y

C

Ex

Calculate the fraction of EDTA that exists as Y-4 at pH 10, and from this calculate pCa in 100mL of solution of 0.100 M Ca+

2 at pH 10 after adding 100 mL of 0.100 M EDTA.

Calculate the fraction of EDTA that exists as Y-4 at pH 10, and from this calculate pCa in 100mL of solution of 0.100 M Ca+2 at pH 10 after adding 100 mL of 0.100 M EDTA.

∵ pH = 10 ∴ [H+] = 1.0 × 10-10

1/α4 = 1 + [H+]/Ka4 + [H+]2/Ka3Ka4 + ……

= 1 + [1.0 × 10-10 /5.5 × 10-11]+[(1.0 × 10-10)2/(6.9 × 10-7)(5.5 × 10-11)] +……

= 1 + 1.82 + 2.6 × 10-4 + …... = 2.82

 ∴ α4 = 0.35 Ca+2 + EDTA [or Y-4] = CaY-2

X 4[EDTA} [(0.1 × 100)/200] – X ≒ 0.05

Kf = [CaY-2]/[Ca+2] α4 [CH4Y] = 0.05/(X)(0.35)(X) = 5.0 × 1010

  X = 1.7 × 10-6 M

pCa = -log (1.7 × 10-6) = 5.77

The conditional formation constant (ex:15.5)

(1) We can use K’f to calculate the equilibrium concentrations of the different species at a given pH.

(2) K’f value holds for only a specified pH.(3) HgY-2 之 Kf ＞ PbY-2 之 Kf ＞ CaY-2 之 Kf ； Kf 不受 pH 值之影響，

Kf’ 則受 pH 值之影響，上述三者在 pH 值≦ 9.0 時， Kf

’ 開始變小，也就是 EDTA 的滴定需在 (pH ＞ 9.0) 之鹼性溶液中進行

YH2

2'f4f

YH42

2

f

44]C[Ca

][CaYKαK

C]α[Ca][CaY

K

15D-4 EDTA Titration Curves The end point break depends upon

1) [Mn+]2) [L1]3) [pH] selectivity4) Kf

The smaller Kf, the more alkaline the solution must be to obtain a k’f of 106.

15D-4 The titration rxn:Mn++ EDTA MYn-4 K’f = 4Kf

Three regions:(1) Before(2) At(3) Afterequivalence point

The Kf for CaY-2 is 5.0 × 1010. At pH 10, α4 is c

alculated to be 0.35 to give a Kf` of 1.8 × 1010.

Calculate the pCa in 100 mL of a solution of 0.100 M Ca+2 at pH 10 after addition of (a) 0 mL (b) 50 mL (c) 100 mL (d) 150 mL of 0.100 M EDTA.

The Kf for CaY-2 is 5.0 × 1010. At pH 10, α4 is calculated to be 0.

35 to give a Kf` of 1.8 × 1010. Calculate the pCa in 100 mL of a

solution of 0.100 M Ca+2 at pH 10 after addition of (a) 0 mL (b) 50 mL (c) 100 mL (d) 150 mL of 0.100 M EDTA. 

(a).  pCa = -log [Ca+2] = -log 0.1 = 1.00 (b).  pCa = -log [0.1 × 100 – 0.1 × 50]/150 = 1.48 (c).  [0.05 - X]/[X][X] = 1.8 × 1010

X = 1.7 × 10-6 M = [Ca+2]

pCa = -log [1.7 × 10-6] = 5.77 (d).  [CaY-2] = 10 mmol/250 mL = 0.04 M

[CH4Y] = [0.1 × 150 – 0.1 × 100]/250 = 0.02 M

0.04/[Ca+2](0.02) = 1.8 × 1010 = Kf`

[Ca+2] = 1.1 × 10-10 M; pCa = -log 1.1 × 10-10 = 9.95

Various pH: K’ becomes smaller as the pH decreases

Kf: cation with larger formation const provide good end point even in acidic media.

Minimum permissible pH for a satisfactory end point

以 EDTA 滴定一系列金屬離子，當各自之 Kf`≒106 且能給出一個 sharp end p

oint 之最小 pH 值。 整條曲線依金屬之 Kf 可約略分為三區，

於 pH ＜ 3 可滴定者， 3 ＜ pH ＜ 7 可滴定者， pH ＞ 7 可滴定者。

當 pH ＜ 3 時只有第一區之 metals 能以 EDTA 滴定來分析， 3 ＜ pH ＜ 7時，則第一、二區之 metals 理論上都可以 EDTA 滴定來分析。但當 pH 為強鹼時，雖然理論上所有的金屬都可被 EDTA 來滴定，但卻無法區分不同之金屬，此外也有不少金屬在強鹼時會產生氫氧化物的沈澱，也無法以 EDTA 滴定來分析

15D-5 How do other complexing agents affect EDTA titration

At the highest pH range, all the metals will react, but not all can be titrated directly due to precipitation of hydroxides. Ex: Fe3+ or Th4+

[ammonia] influence

15D-6 Detection of the End Point:

IndicatorsO,O’-dihydroxy azo type (metallochromic indicators)EX: Eriochrome Black T: (EBT)

-O3S N N

OHOH

O2NH3In

H2In- HIn2- + H+ pKa=6.3 red blueHIn2- In3- pKa=11.6 orangepH > 7MIn + HY3- HIn2- + MY2-

red blue

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

The formation constants of the EDTA complexes of calcium and magnesium are too close to differentiate between them in an EDTA titration, so they will titrate together.

Eriochrome Black T cannot be used to indicate the direct titration of calculium alone with EDTA, because the indicator forms too weak a complex with calcium to give a sharp end point.

Calcium can actually be titrated in the presence of magnesium by raising the pH to 12 with strong alkali; Mg(OH)2 precipitates and does not titrate.

EBT(pH<6) H2In- red

(pH 6~12) HIn2- blue(pH>12) In3- orange