Types of chemistry

1. Types of chemistry • Although any type of chemical reaction may be used for titrimetric analysis, the most often used fall under the categories of: • Bronsted acid - base: HA + B ↔ HB+ + A- • Complex formation: M(aq) + nL(aq) ↔ MLn(aq) • Oxidation – reduction: Ox + Red ↔ Red’ + Ox’ • Precipitation: M(aq) + nL(aq) ↔ MLn(s) Lewis acid-base chemistry is often involved in precipitation and complex formation chemistry.

2. Complexation reactions Metal Complexes • Lewis acids are electron pair acceptors. • Coordination complexes: metal compounds formed by Lewis acid-base interactions. • Complexes: Have a metal ion (can be zero oxidation state) bonded to a number of ligands. Complex ions can be charged. Example, [Ag(NH3)2]+. • Ligands are Lewis bases. • Square brackets enclose the metal ion and ligands.

3. Chelate or chelon effect: More stable complexes are formed with chelating agents than the equivalent number of monodentate ligands. This is due to entropy (randomness) of the reaction – the more molecules, the lower the entropy and vice-versa. The interaction from all the different sites together is quite strong.

5. Ligands with More than One Donor Atom • [Ni(H2O)6]2+(aq) + 6NH3 [Ni(NH3)6]2+(aq) + 6H2O(l) Kf = 4  108 • [Ni(H2O)6]2+(aq) + 3en [Ni(en)3]2+(aq) + 6H2O(l) Kf = 2  1018 • Sequestering agents are chelating agents that are used to remove unwanted metal ions. • In medicine sequestering agents are used to selectively remove toxic metal ions (e.g. Hg2+ and Pb2+) while leaving biologically important metals.

6. One very important chelating agent is ethylenediaminetetraacetate (EDTA). • EDTA occupies 6 coordination sites, for example [CoEDTA]- is an octahedral Co3+ complex. • Both N atoms (blue) and O atoms (red) coordinate to the metal. • EDTA is used in consumer products to complex the metal ions which catalyze decomposition reactions.

8. Widely used chelator: (1) Direct titration (2) Indirect determination through a sequence of reactions

9. EDTA • * It forms 1:1 complexes with most metals. • (Not with Group 1A metals – Na, K, Li) * Forms stable water soluble complexes. * High formation constants. • A primary standard material – a highly purified • compound that serves as a reference material.

11. Highlighted, acidic protons • lost uponmetal complexation. pK1 = 0.0 pK2 = 1.5 pK3 = 2.0 pK4 = 2.66 Hydroxyl protons pK5 = 6.6 pK6 = 10.24 Ammonium protons

13. Fraction of EDTA in the form Y 4- [EDTA] : Total concentration of all “free” uncomplexed EDTA species in solution. Note that only the fully ionised , -4 – charged anion binds to metal ions

14. Fractional Composition Diagram for EDTA

15. At this range Y4- predominates, thus titrations are routinely done in buffered solutions near or above pH 10.

16. Formation Constant or Stability Constant: Equilibrium constant for the reaction of metal with a ligand. and Therefore,

17. Conditional Formation Constant Then is constant. Fixing the pH by buffering: Thus, conditional formation constant: Consider EDTA complex formation as if the uncomplexed EDTA is in one form. At any fixed pH, find and evaluate Kf’

18. Effective titration: *Reaction must go to completion. *Large Kf *Analyte and titrant essentially completely reacted at the equivalence point and: n(Metal) = n(Titrant) *Metals with higher Kf values can be titrated at lower pH *pH and thus Kf’ dependent

19. Effect of pH on EDTA Titration of Ca 2+ Less distinct end point

20. EDTA Titration Curves Titration reaction: For large Kf’: Reaction complete at each point in the titration. Titration curve: Plot pM (= -log[M]) vs. volume EDTA

21. EDTA Titration Curve Region 1 Excess Mn+ left after each addition of EDTA. Conc. of free metal equal to conc. of unreacted Mn+. Region 2 Equivalence point:[Mn+] = [EDTA] Some free Mn+ generated by MYn-4  Mn+ + EDTA Region 3 Excess EDTA. Virtually all metal in MYn-4 form.

22. Example Consider the titration of 25.0 mL of 0.020 M MnSO4 with 0.010 M EDTA in a solution buffered at pH 8.00. Calculate pMn2+ at the following volumes of added EDTA and sketch the titration curve: 0 mL 50.0 mL 20.0 mL 50.1 mL 40.0 mL 55.0 mL 49.0 mL 60.0 mL 49.9 mL

23. Mn2+ + EDTA  MnY2- End point volume = 50.0 mL Region 1 • 0.0 mL EDTA: 0.020 M Mn2+: p Mn2+ = -log[Mn2+] = -log(0.020) = 1.70

24. 2. 20.0 mL EDTA: Initial Mn2+ volume Total volume of solution Original Mn2+ conc. Fraction Remaining Dilution Factor  [Mn2+] = 0.00671 M  pMn2+ = -log[Mn2+] = 2.18 Use same method to calculate pMn2+ for any EDTA volume before equivalence point (= 50.0 mL EDTA)

25. Region 2 50.0 mL EDTA At the Equivalence Point: virtually all metal is in MnY2- form. [Mn2+] = [EDTA] Assume negligible dissociation, then: Initial Mn2+ volume Total volume of solution Initial Mn2+ conc. Dilution Factor [MnY2-] = 6.67 x 10 –3 M

26. Region 2 (continued) At the Equivalence Point: Mn2+ + EDTA  MnY2- Initial conc. - - 0.00667 Final conc. xx 0.00667 - x and  x = 3.98 x 10–8 M  pMn2+ = -log[Mn2+] = 7.40

27. Region 3 After the equivalence point: All Mn2+ in the MnY2- form & there is excess EDTA. 55.0 mL EDTA: Excess EDTA volume Original EDTA Conc. Total volume of solution Dilution Factor  [EDTA] = 6.25 x 10–4 M

28. Initial Mn2+ volume Initial Mn2+ conc. Dilution Factor Total volume of solution  [MnY2-] = 6.25 x 10–3 M and  [Mn2+] = 2.31 x 10–14 M  pMn2+ = -log[Mn2+] = 13.62

30. EDTA Titration Curves for Ca 2+ and Sr 2+ (Buffered at pH 10) *Ca2+ end point more distinct. *Lower pH, Kf’ decreases, & End point less distinct. *We cannot raise pH arbitrarily: Metal hydroxides might precipitate.

31. Auxiliary Complexing Agents *Ligand strongly binds to metal & prevents hydroxide precipitation at high pH. *Auxiliary ligand binds less than EDTA binding to metal. *NH3 normally used:NH3 fixes pH and complexes metal species *Tartrate, citrate, or triethanolamine may be used.

32. Auxiliary Complexing Agents Metal – Ligand Equilibria M + L ⇌ ML M + 2L ⇌ ML2 i = overall or cumulative formation constant *Fraction of uncomplexed metal ion, M: CM is total concentration of all forms of metal M = M, ML, and ML2.

33. Auxiliary Complexing Agents CM = [M] + [ML] + [ML2] Mass balance expression and Therefore, 

34. Example Consider the titration of 50.0 mL of 0.00100 M Zn2+ with 0.00100 M EDTA at pH10 in the presence of 0.10 M NH3. (This is the concentration of NH3. There is Also NH4+ in the solution.) Find pZn2+ after addition of 20.0, 50.0, and 60,0 mL of EDTA. Note: We always assume that EDTA is a much stronger complexing agent than NH3. Kf for EDTA > Kf for NH3.

35. Solution Zn2+ - NH3 complexes: Zn(NH3)2+, Zn(NH3)22+, Zn(NH3)32+, and Zn(NH3)42+ 1 = 1.51 x 102, 2 = 2.69 x 104, 3 = 5.50 x 106, and 4 = 5.01 x 108 [L] = [NH3] = 0.10 M  • Very little free Zn2+ in the presence of 0.10 M NH3. • Most Zn2+ complexed by NH3

36. At pH 10, = (1.8 x10-5) (0.36) (1016.50) = 2.05 x 1011 1. Addition of 20.0 mL EDTA sol’n: = 4.3 x 10-4 M  pZn2+ = -log[Zn2+] = 8.11

37. 2. Equivalence point: Addition of 50.0 mL EDTA: = 5.00 x 10-4 M  X = =4.9 x 10-8 M  pZn2+ = -log[Zn2+] = 12.05

38. 3. After the equivalence point: 60.0 mL EDTA = 9.1 x 10-5 M = 4.5 x 10-4 M  [Zn2+] = 4.3 x 10–16 M  pMn2+= 15.36 Note: Past equivalence point problem independent on presence of NH3. Both [EDTA] and [ZnY2-] known.

39. EDTA Titrations at Different Concentrations of Auxiliary Complexing Reagent (NH3). Small  pZn near equivalence point. Significant  pZn Near equiv. Point. (More distinct end point)

40. Metal Ion Indicators Compounds changing colour when binding to metal ion. Kf for Metal-In < Kf for Metal-EDTA. • Before Titration: • Mg2+ + In  MgIn • (colourless) (blue) (red) • During Titration: Before the end point • Mg2+ + EDTA  MgEDTA • (free Mg2+ ions) (Solution red due to MgIn complex) At the end point: 3. MgIn + EDTA  MgEDTA + In (red) (colourless) (colourless) (Blue)

43. EDTA Titration Techniques 1. Direct Titration *Buffer analyte to pH where Kf’ for MYn-2 is large, *and M-In colour distinct from free In colour. *Auxiliary complexing agent may be used. 2. Back Titration *Known excess std EDTA added. *Excess EDTA then titrated with a std sol’n of a second metal ion. *Note: Std metal ion for back titration must not displace analyte from MYn-2 complex.

44. 2. Back Titration: When to apply it *Analyte precipitates in the absence of EDTA. *Analyte reacts too slowly with EDTA. *Analyte blocks indicator 3. Displacement Titration *Metal ions with no satisfactory indicator. *Analyte treated with excess Mg(EDTA)2- Mn+ + MgYn-2 MYn-4 + Mg2+ * Kf’ for MYn-2 > Kf’ for MgYn-2

45. 4. Indirect Titration *Anions analysed: CO32-, CrO42-, S2-, and SO42-. Precipitate SO42- with excess Ba2+ at pH 1. *BaSO4(s) washed & boiled with excess EDTA at pH 10. BaSO4(s) + EDTA(aq)  BaY2-(aq) + SO42-(aq) Excess EDTA back titrated:EDTA(aq) + Mg2+MgY2-(aq) Alternatively: *Precipitate SO42- with excess Ba2+ at pH 1. *Filter & wash precipitate. *Treat excess metal ion in filtrate with EDTA.

46. 5. Masking *Masking Agent: Protects some component of analyte from reacting with EDTA. *F- masks Hg2+, Fe3+, Ti4+, and Be2+. *CN- masks Cd2+, Zn2+, Hg2+, Co2+, Cu+, Ag+, Ni2+, Pd2+, Pt2+, Hg2+, Fe2+, and Fe3+, but not Mg2+, Ca2+, Mn2+, Pb2+. *Triethanolamine:Al3+, Fe3+, and Mn2+. *2,3-dimercapto-1-propanol: Bi3+, Cd2+,Cu2+, Hg2+, and Pb2+.

47. *Demasking: Releasing masking agent from analyte. Metal-Cyanide Complex Formaldehyde *Oxidation with H2O2 releasesCu2+ from Cu+-Thiourea complex. • pH control • Masking • Demasking *Thus, analyte selectivity: