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 ions As a metal-binding agent: page 265 for examples

Metal-ATP complex

Useful chelating agents

(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

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

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

CHAPTER 13: TABLE 13.1

(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

13-3 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.

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

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. 

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

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)

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

Species EDTA as a function of pH

Conditional 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 =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

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.

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.

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