Chapter 12 EDTA Titrations. Coordination Number. Ligands. Geometries. Complex Formation. Acids and Bases. K f

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1 Chapter 1 EDTA Titrations Acids and Bases Deinitions: Lewis Electrons (acid: electron pair acceptor); BrØnsted-Lowry (acid: proton donor) Lewis acid-base concept in etal-chelate Complexes etal ions (electron pair acceptor) Lewis acid Ligand (electron pair donor) Lewis base Coordination Number The atom o the ligand that supplies the nonbonding electrons or the metal-ligand bond is the donor atom. The number o these atoms is the coordination number. Complex Formation Formation o coordinate bonds between Lewis Acids/Bases Ag 1 Ag ( ( NH ) ( aq ) NH (aq) Ag ( NH ) NH ) ] [ Ag ][ NH ] 1 aq ) NH (aq) Ag ( NH ) ( aq ) ( aq ) Formation constants () are the equilibrium constants or complex ion ormation. The overall, or cumulative, ormation constants are denoted i Ag ( aq) NH (aq) Ag( NH ) ( aq) NH ) ] β 1 [ Ag ][ NH ] NH ) ] NH ) ][ NH ] Geometries Ligands There are two common geometries or metals with a coordination number o our: Tetrahedral & Square planar By ar the mostencountered geometry, when the coordination number is six, is octahedral. onodentate ligand: binds to a metal ion through only one atom, e.g., CN - ultidentate ligand or chelating ligand: has more than one ligand donor atoms. In ethylenediamine, NH CH CH NH (i.e., en), each N is a donor atom. en is bidentate. 1

2 Chelating Eect A multidentate ligand to orm more stable metal complexes than those ormed by similar monodentate ligand Chelating Agents Porphyrins (tetradentate ligands, in heme and chlorophyll) Adenosine triphosphate (ATP) EDTA Ethylenediaminetetraacetic acid (H EDTA or H ) Ethylenediaminetetraacetate anion (EDTA - or - ) Ethylenediaminetetraacetate, merciully abbreviated EDTA, has six donor atoms. EDTA is a primary standard material. Some etals Form 7 or 8 Coordinate Complexes EDTA Complexes EDTA orms 1:1 complexes with most metals (Not with Group 1A metals) EDTA complexes are usually stable water soluble complexes with high ormation constants Formation constant,, (or stability constant): [ ] n [ ][ ] n could have been deined or any orm o EDTA

3 ph aects EDTA titration Acid-Base Properties o EDTA: p HO CH CO H CH C HNCH CH NH HO CH CO H CH C H 6 p p p p. p ph: EDTA is a hexaprotic system (H 6 ) with carboxylic acids and ammoniums.66 n H n n Fraction o EDTA in the orm - [ ] [ H ] [ H ] [ H ] [ H ] [ H ] [ H ] [ ] α 6 ph Dependence o - α [ ] [ ] [ EDTA] C EDTA αα {[ H ] [ H ] 1 [ H ] 1 [ H ] 1 [ H ] 1 [ H ] } Fractional Composition Diagram or EDTA Conditional Formation Constant [ ] n n [ ][ ] - [ ] - [ ] [ ] α α [ EDTA] n [ EDTA] [ ][ ] Conditional ormation constant: n α - n [ ] [ ] α - [ EDTA] n [ ] [ ][ EDTA] Fixing the ph by buering, then α is a constant. At any ixed ph, ind α and evaluate

4 Example: lculate the concentration o ree in a solution o. - at ph and ph 6. or - is.6 (Table 1-) at ph., α [ ].1 x [ ][ EDTA] x EDTA at ph 6., α i α (.)( (1.8 EDTA.1 x x.1- x x [ ].6 )( ) ) 8. 6 at ph -. at ph 6 1. Excess n let ater each addition o EDTA.. o ree metal equal to conc. o unreacted n.. Equivalence point: [ n ] [EDTA] Some ree n generated by n- n EDTA. Excess EDTA. Virtually all metal in n- orm. 1. Titration reaction: n EDTA α. Reaction completes at each point in the titration i is large.. Plot p ( -log[ n ]) vs. volume o EDTA added EXAPLE: Derive a titration curve or the titration o. ml o. (buered at ph) with.,., and 6. ml o.8 EDTA. α EDTA.6.* 1. At equivalence point, Vol. o EDTA. ml. ml, Beore the equivalence point. ml, At the equivalence point 6. ml, Ater the equivalence point EXAPLE: Derive a titration curve or the titration o. ml o. (buered at ph) with.,., and 6. ml o.8 EDTA. At equivalence point, Vol. o EDTA. ml ml beore the equivalence point.-.. [ ] (.).91. Fraction Remaining.. p log(.91) 1. EXAPLE: Derive a titration curve or the titration o. ml o. (buered at ph) with.,., and 6. ml o.8 EDTA. At equivalence point, Vol. o EDTA. ml almost all the metal is in the orm, - i. [ ] - (.).67 EDTA.67 x x.67- x.. [ ] [ ][ EDTA].67 x 1.8 x p x log(1. ).91

5 EXAPLE: Derive a titration curve or the titration o. ml o. (buered at ph) with.,., and 6. ml o.8 EDTA. At equivalence point, Vol. o EDTA. ml 6. ml 1. ml excess EDTA, ater the equivalence point [ EDTA] (.8) [ ] (.).6 [ ] [ ][ EDTA] [ ] (1. ) 1. 9 p 8.86 ph aects the titration o with EDTA Auxiliary Complexing Agents A ligand that binds strongly enough to the metal to prevent hydroxide precipitation, but weak enough to be displaced by EDTA (e.g., ammonia, tartrate, citrate, or trithanolamine) Ammonia is a common auxiliary complex or transition metals like zinc (p. 9) α α Zn is the eective ormation constant at a ixed concentration o auxiliary complexing agent.

6 etal Ion Indicators etal Ion Indicators To detect the end point o EDTA titrations, we usually use a metal ion indicator or an ion-selective electrode (Ch. 1) etal ion indicators change color when the metal ion is bound to EDTA: gebt EDTA gedta EbT (Red) (Colorless) Eriochrome black T is an organic ion (Blue) The indicator must bind less strongly than EDTA EDTA Titration Techniques Direct titration: analyte is titrated with standard EDTA with solution buered at a ph where is large Back titration: known excess o EDTA is added to analyte. Excess EDTA is titrated with nd metal ion. Example:. ml o an unknown Ni solution was treated with. ml o.8 Na EDTA. The ph o the solution was buered to. and than back-titrated with ml o.99 Zn. What was the unknown Ni in? mol EDTA (. ml)(.8 ) 1. mmol EDTA Zn - - Zn mol Zn mol Ni (17.61mL)(.99 ).9 mmol Zn Ni - Ni 1.1mmol EDTA -.9 mmol Zn.916 mmol Ni (.916 mmol)/(. ml).66 - EDTA Titration Techniques Direct titration: analyte is titrated with standard EDTA with solution buered at a ph where is large Back titration: known excess o EDTA is added to analyte. Excess EDTA is titrated with nd metal ion. Displacement titration: For metals without a good indicator ion, the analyte can be treated with excess g(edta) -. The analyte displaces g, and than g can be titrated with standard EDTA Indirect titration: Anions can be analyzed by precipitation with excess metal ion and then titration o the metal in the dissolved precipitate with EDTA. asking agent: protects some componet o the analyte rom reaction with EDTA (render metal ions inactive without actually removing them rom solution). Demasking: releasing metal ion rom a masking agent. 6