Chapter 13

EDTA Titrations

EthyleneDiamineTetraAcetic acid

Box 13-1 Chelation Therapy & Thalassemia

• A successful drug for iron excretion

13-1 Metal-Chelate Complexes

EDTA forms strong 1:1 complexes with most metal ionsAs a metal-binding agent: page 265 for examples

Metal-ATP complex

Useful chelating agents

13-2 EDTA (ethylenediaminetetraacetic acid, a hexadentate)

(1) The most widely used chelating agent in titration (2) Forms strong 1:1 complexes regardless of the charge on

the cation

23

23f

233

4

3

232

2333

3

3

31

33

][NH ][Ag

])[Ag(NHK

)Ag(NH2NHAg

101][NH ][Ag

])[Ag(NHK

)Ag(NHNH)Ag(NH

102.5][NH ][Ag

])[Ag(NHK

)Ag(NHNHAg

Complexes: Formation Constant (Kf)stepwise formation constants (Ki)

8

23

23

fd

i

323

721f

104.0])[Ag(NH

][NH ][Ag

K

1

)(Kconstant on Dissociati

)(Kconstant y Instabilit

2NHAg)Ag(NH

constant.stability or constant formatoin

102.5KKK

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

(2) Neutral EDTA is a tetrabasic acid(3) Metal-EDTA complex is unstable at both low pH & hi

gh pH.• At low pH

– H+ & M n+

• At high pH– OH- & EDTA

For EDTA

][Y ][M

][MYK MYYM

4

4

f44

n

nnn

CHAPTER 13: TABLE 13.1

– Pb2+ as example:• At pH 10, tartrate is present

to prevent Pb(OH)2

• Pb-tartrate complex must be less stable than Pb-EDTA

(4) Auxiliary complexing agents: prevent metal ions from precipitating.

13-3 Metal Ion Indicators

• Metal ion indicator: a compound whose color changes when it binds to a metal ion.

• For an useful indicator, it must bind metal less strongly than EDTA does. the indicator must release its metal to EDTA

• Example: MgIn + EDTA MgEDTA + In• Indicator is pH dependent.• If metal block the indicator, use back titration.

NH3

NH3

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

pale bule deep bule

::

Demonstration 13-1 Metal Ion Indicator Color Changes

P.294

COLOR PLATE 8Titration of Mg2+ by EDTA, Using Eriochrome Black T Indicator(a) Before (left), near (center), and after (right) equivalence point. (b) Same titration with methyl red added as inert dye to alter colors. 

Demonstration 13-1 Metal Ion Indicator Color Changes

13-4 EDTA Titration Techniquesare useful for the determination of [metal]

• Direct titration– Titrate with EDTA– Buffered to an appropriate pH– Color distinct indicator– Auxiliary complexing agent

• Back titration– Excess EDTA, & titrate with metal ion– For analyte

• ppt in the absence of EDTA : – Ex: (Al3+-EDTA) at pH 7, indicator Calmagite) back titration wit

h Zn2+

• react slowly with EDTA • block the indicator

• Displacement titration– No satisfactory indicator– Ex1: Hg2+ + MgY2- HgY2- + Mg2+ Kf HgY2- > MgY2-

– Ex2: 2Ag+ + Ni(CN)42- 2Ag(CN)2 + Ni2+ , Ni2+ is titrated with EDTA

• Indirect titration– Determine [Anion] that precipitate metal ions: CO3

2-, CrO42- S2- SO4

2-

– Ex: SO42- + Ba2+ BaSO4(s) at pH 1

filter BaSO4(s) and boil with excess EDTA at pH 10 Ba(EDTA)2- and excess EDTA is back titration with Mg2+

• Masking– Masking prevents one element from interfering in the analysis of ano

ther element. Ex: Al3+ + Mg2+ + F- AlF63+ + Mg2+ then only Mg2+ can

be react with EDTA masking Al3+ with F- – Masking agent: CN- , F- (using with pH control to avoid HCN & HF)

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

Expt: The formation constants of the EDTA complexes of Ca2+ and Mg2+ are too close to differentiate between them in an EDTA titration, so they will titrate together. Ca2+ can actually be titrated in the presence of Mg2+ by raising the pH to 12 with strong alkali; Mg(OH)2 precipitates and does not titrate.

13-5 The pH-dependent Metal-EDTA Equilibrium

• Since the anion Y4- is the ligand species in complex formation, the complexation equilibria are affected markedly by the pH

• Fraction Composition of EDTA Solutions

.....KKK

][H

KK

][H

K

][H1

α

1

][Y

[EDTA]

KKKKKK][HKKKKK][HKKKK][HKKK][HKK][HK][H

KKKKKKα

][EDTAαY [EDTA]

][Yα

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

][Yα

654

3

65

2

6Y

4

654321543212

43213

3214

215

16

654321Y

Y

44

Y

4322345

26

4

Y

4

4

44

4

Species EDTA as a function of pH

Conditional Formation ConstantConditional Formation Constant

• most of the EDTA is not Y4- below pH=pK6=10.37. The species HY3-, H2Y2-, and so on, predominate at lower pH.

• It is convenient to express the fraction of free EDTA in the form Y4-

 

P.300

The number Ktf =αγ

4-Kf is called the conditional formation constant or the effective formation constant.

P.300

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

(2) Kf : HgY-2＞ PbY-2＞ CaY-2； 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

• Example at page 277• pH affects the titration of Ca2+ with EDTA

– Kf’ is smaller at lower pH.

13-6 EDTA Titration Curves

The end point break depends upon

1) [Mn+]

2) [L1]

3) [pH] selectivity

4) Kf

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

The titration rxn:Mn++ EDTA MYn-4

K’f = 4Kf

Three regions:(1) Before equivalence point : ex

cess Mn+

(2) At equivalence point [Mn+]= [EDTA]

(3) After equivalence point : exce

ss EDTA

Example at page 302