Chapter 12 EDTA Titrations

1 1 Chapter 12 EDTA TitrationsAcids and Bases Definitions: Lewis Electrons ( acid : electron pair acceptor); Br nsted-Lowry ( acid : proton donor)Metal ions (electron pair acceptor) Lewis acidLigand (electron pair donor) Lewis baseLewis acid - base concept in Metal-Chelate ComplexesCoordination Number The atom of the ligand that supplies the nonbonding electrons for the metal-ligand bond is thedonor atom. The number of these atoms is thecoordination FormationFormation of coordinate bonds between Lewis Acids/BasesFormation constants (Kf) are the equilibrium constants for complex ion formation.

2 The overall, or cumulative, formation constants are denoted i2122323]][[])([KKNHAgNHAgKf ===++b]][[])([331 NHAgNHAgK++=]][)([])([33232 NHNHAgNHAgK++=)()( (aq))(33aqNHAgNHaqAg+++)()( (aq))()(2333aqNHAgNHaqNHAg+++K1K2)()( (aq)2)(233aqNHAgNHaqAg+++KfGeometriesThe re are two common geometries for metals with a coordination number of four:Tetrahedral & Square planarBy far the most-encountered geometry, when the coordination number is six, is Monodentate ligand: binds to a metal ion through only one atom, , CN- Multidentate ligand or chelating ligand:has more than one ligand donor atoms.

3 In ethylenediamine, NH2CH2CH2NH2( , en), each N is a donor atom. en is bidentate. 2 Chelating EffectA multidentate ligand to form more stable metal complexes than those formed by similar monodentate ligand Chelating Agents Porphyrins (tetradentate ligands, in heme and chlorophyll) Adenosine triphosphate (ATP)EDTAE thylenediaminetetraacetate, mercifully abbreviated EDTA, has six donor atoms. Ethylenediaminetetraacetic acid (H4 EDTA or H4Y)Ethylenediaminetetraacetate anion (EDTA-4or Y-4)EDTA is a primary standard Metals Form 7 or 8 Coordinate ComplexesEDTA Complexes EDTA forms 1:1 complexes with most metals (Not with Group 1A metals) EDTA complexes are usually stable water soluble complexes with high formation constants Formation constant, Kf, (or stability constant): Kfcould have been defined for any form of EDTA[][][]-+---+= +44f44Y MMY MYYM nnnnK3pH affects EDTA titrationHNCH2CH2 NHCH2CO2 HCH2CO2 HHO2CH2 CHO2CH2CH6Y2+pH.

4 acid - base Properties of EDTA: YH MYYM4n44---+ +nnnEDTA is a hexaproticsystem (H6Y2+) with 4 carboxylic acids and 2 [][] [][][] [] [] []----++-++++++=-43223452644 YHYYHYHYHYHYHYYaFraction of EDTA in the form Y4-EDTAYCYEDTAY][][][444--==-a[][][][][] []{}654321543214321232132141566543214 KKKKKKKKKKKHKKKKHKKKHKKHKHHKKKKKKY++++++ =++++++-a=-4 YapH Dependence of Y4-Fractional Composition Diagram for EDTAC onditional Formation ConstantFixing the pH by buffering, then-4 Yais a formation constant:At any fixed pH, find-4 Yaand evaluate Kf [][][][]EDTAYEDTAY44Y-4-4Y--= =aa[][][][][][]EDTA MMYY MMY-4Y444fa+--+-==nnnnK[][][]EDTA MMY4fYf-4+-==nn'KKa44--+ +nnMYYM]][[][44-+-=YMMYKnnf4 Example: Calculate the concentration of free Ca2+in a solution of M CaY2-at pH 10 and pH 6.

5 Kffor CaY2-is (Table 12-2)[][][] CaCaYxxK'-==+ )10)( ( ,pHat )10)( ( ,pHat 44 = === ====---KKKK''aafYf224 CaYEDTACaKK'-= +-+ 0 0 ConcCaYEDTACa fi22-+ +[]6pHat M 10pHat M == ==-+xEDTA Titration Curve1. Excess Mn+left after each addition of EDTA. Conc. of free metal equal to conc. of unreactedMn+. 2. Equivalence point: [Mn+] = [EDTA] Some free Mn+generated by MYn-4 Mn++ EDTA 3. Excess EDTA. Virtually all metal in Titration Curve2. Reaction completes at each point in the titration if Kf is Plot pM (= -log[Mn+]) of EDTA added4-+ +nnMYEDTAMfYfKK-=4'a1.

6 Titration ml, Before the equivalence ml, At the equivalence ml, After the equivalence pointEXAMPLE:Derive a titration curve for the titration of mL of M Ca2+(buffered at pH=10) with , , and mL of M EDTA. EDTA Titration CurveAt equivalence point, Vol. of EDTA = * === +--+KKCaYEDTACaY'a[]M ( ) + =+ ) (pCa2=-=+EXAMPLE:Derive a titration curve for the titration of mL of M Ca2+(buffered at pH=10) with , , and mL of M EDTA. EDTA Titration Curve5mL before the equivalence pointAt equivalence point, Vol. of EDTA = mLFractionRemainingEXAMPLE:Derive a titration curve for the titration of mL of M Ca2+(buffered at pH=10) with , , and mL of M EDTA.

7 EDTA Titration Curvealmost all the metal is in the form, CaY2-At equivalence point, Vol. of EDTA = mL[]M ( ) CaY-2= += ) (pCa62= -=-+ 0 0 ConcCaYEDTACa fi22-+ +[][][] CaCaY =-==+-xxK'M =x5 EXAMPLE:Derive a titration curve for the titration of mL of M Ca2+(buffered at pH=10) with , , and mL of M EDTA. EDTA Titration mL mL excess EDTA, after the equivalence pointAt equivalence point, Vol. of EDTA = mL[]M ( ) EDTA3- = +=[]M ( ) CaY2-2- = += +[][][][] ) ( CaCaY = ==-+-+-'KM + =pH affects the titration of Ca2+with EDTAA uxiliary Complexing Agents A ligand that binds strongly enough to the metal to prevent hydroxide precipitation, but weak enough to be displaced by EDTA ( , ammonia, tartrate, citrate, or trithanolamine)fYZnfKK-+=42''aaAmmonia is a common auxiliary complex for transition metals like zinc (p.)

8 239)Kf is the effective formation constant at a fixed concentration ofauxiliary complexing 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 mustbind less strongly than EDTA(Blue) )(Colorless (Red) EbT MgEDTA EDTA MgEbT+ +Metal Ion IndicatorsEDTA 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.

9 Excess EDTA is titrated with 2nd metal : mL of an unknown Ni2+solution was treated with mL of M Na2 EDTA. The pH of the solution was buffered to and than back-titrated with mL of M Zn2+. What was the unknown Ni2+in M?EDTA mmol ) 3mL)( ( EDTA mol==++==22 Znmmol ) 9mL)( ( Znmolmmol Znmmol mmol Ni mol22==++M ) 00mmol)/(25. ( Ni M2==+-2-42 ZnYY Zn ++-2-42 NiY Y Ni ++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.

10 Excess EDTA is titrated with 2nd metal ion. Indirect titration: Anions can be analyzed by precipitation with excess metal ion and then titration of the metal in the dissolved precipitate with EDTA. Displacement titration: For metals without a good indicator ion, the analyte can be treated with excess Mg(EDTA)2-. The analytedisplaces Mg, and than Mg can be titrated with standard EDTA Masking agent: protects some componet of the analyte from reaction with EDTA (render metal ions inactive without actually removing them from solution). Demasking:releasing metal ion from a masking agent.