1. Chapter 13
EDTA Titrations
EthyleneDiamineTetraAcetic acid

2. Box 13-1 Chelation Therapy & Thalassemia
A successful drug for iron excretion

3. 13-1 Metal-Chelate Complexes
EDTA forms strong 1:1 complexes with most metal ions
As a metal-binding agent: page 265 for examples

4. Metal-ATP complex

5. Useful chelating agents

7. (ethylenediaminetetraacetic acid, a hexadentate)
13-2 EDTA
(ethylenediaminetetraacetic acid, a hexadentate)
The most widely used chelating agent in titration
Forms strong 1:1 complexes regardless of the charge on the cation

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

10. For EDTA
Multidentate chelating agents form stronger complexes (Kf ) with metal ions than bidentate or monodentate ligands.
Neutral EDTA is a tetrabasic acid
Metal-EDTA complex is unstable at both low pH & high pH.
At low pH
H+ & M n+
At high pH
OH- & EDTA

11. CHAPTER 13: TABLE 13.1

12. (4) Auxiliary complexing agents: prevent metal ions from precipitating.
Pb2+ as example:
At pH 10, tartrate is present to prevent Pb(OH)2
Pb-tartrate complex must be less stable than Pb-EDTA

13. 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.

15. Demonstration 13-1 Metal Ion Indicator Color Changes
P.294

16. Demonstration 13-1 Metal Ion Indicator Color Changes
COLOR PLATE 8
Titration 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. 

17. 13-4 EDTA Titration Techniques are 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 with Zn2+
react slowly with EDTA
block the indicator

18. 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: CO32-, CrO42- S2- SO42-
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 another 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)

19. 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.

20. 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

21. Species EDTA as a function of pH

22. Conditional Formation Constant
most of the EDTA is not Y4- below pH=pK6= 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

23. The number Ktf =αγ4-Kf is called the conditional formation constant or the effective formation constant.
We can use K’f to calculate the equilibrium concentrations of the different species at a given pH.
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
P.300

24. pH affects the titration of Ca2+ with EDTA
Example at page 277
pH affects the titration of Ca2+ with EDTA
Kf’ is smaller at lower pH.

25. 13-6 EDTA Titration Curves
The end point break depends upon
[Mn+]
[L1]
[pH]  selectivity Kf
The smaller Kf, the more alkaline the solution must be to obtain a k’f of 106.

26. The titration rxn: Mn++ EDTA  MYn-4 K’f = a4Kf Three regions:
Before equivalence point : excess Mn+
At equivalence point [Mn+]= [EDTA]
After equivalence point : excess EDTA
Example at page 302