1. Experiment 7

2. Objective Apply the concept of complexometric titration in the determination of total hardness in drinking water 3. Simple Water and Complexes 4. Water Drinking Hard water is better fordrinking because itcontains minerals 5. Water Cleaning Soft water is better forcleaning because itdoesnt form scum withsoap 6. Water Hardness of water Depends on source Caused by Ca2+ andMg2+ formingprecipitates with soap 7. Water Hardness of water Most Ca2+ in watercome from CaCO3exoskeletons of aquaticmicroorganisms likediatoms 8. Complexes Coordination betweenmetal and ligand An atom in the ligand (called the donor) donates electrons to the metal ion, forming a bond. New properties Complexes are usually more stable than the components due to the rearrangement of the energy levels of the d- orbital (crystal field theory) 9. Complexes - Ligands Monodentate Polydentate Chelating agents Gr.chelos, meaning teeth Polyanionic Complex-forming Reacts in a 1:1 ratio 10. Complexes - Usage Catalysts Polymerization, hydrogenation, hydroboration, etc. Medicine cis-diamminedichloroplatinum(II)ClNH3 Chelators Heavy Metal Poisoning Treatment Pt Cleaning agent Food enhancementClNH3 Anti-b acterial Titrants 11. Step by step discussion*simple dilutions are not discussed 12. Solution Preparation 13. Solution Preparation 250 mL 0.050 M std CaCO3soln + 20 mL dH2O, +few Cover with watch drops 6M HCl glass (slower 1.2511 g CaCO3 (Dissolveevaporation rate to standard precipitate, basic keep beaker from CO32-) getting dry)Rinse watch glass toEvaporate to 10 mL.beakerCool. Quantitatively transfer to 250 mL vol flask. Dilute to mark. 14. Solution Preparation pH 10 bufferTransfer to 250 mL142 mL conc NH3 +Adjust soln to pH 10vol flask. Dilute to17.5 g NH4Clmark. pOH pKb log([NH 4 ] /[ NH 3 ]) pOH log(1.8 x10 5 ) log(2.1016m ol/ 0.327m ol) pOH 4.74 0.808pH 8.45Add ammonia to increase pH! 15. Solution Preparation pH 10 buffer Transfer to 250 mL142 mL conc NH3 +Adjust soln to pH 10 vol flask. Dilute to17.5 g NH4Cl mark. Different minimum pH values for different cations Mn2+, Fe2+, 5-6 pH Fe3+, Th4+ 1-1.5 pH Ca2+ 8 Mg2+ 10 16. Solution Preparation pH 10 buffer Transfer to 250 mL142 mL conc NH3 +Adjust soln to pH 10 vol flask. Dilute to17.5 g NH4Cl mark. As a rule of thumb, The higher the pH (more basic solution), the sharper the endpoint The higher the formation of constant the lower the minimum pH 17. Solution Preparation 500 mL 0.050 M EDTA+1.0 g9.31 g Na2H2EDTA2H2O +200 mL dH2OMgCl26H2O Dissolve in 500 mL vol flask. Heat if necessary. 18. Solution Preparation 500 mL 0.050 M EDTA Why add 1.0 g MgCl26H2O? Diverse ion effect: increase solubility (easierpreparation) Sharper endpoint in titration Ensures presence of Mg2+ in sample Supposedly does not affect titration procedure What exactly happens when we add MgCl2? Good question. 19. Solution Preparation 500 mL 0.050 M EDTA What happens when we DONT add MgCl2? Endpoint not as sharp if Mg2+ is not present in solution Only total hardness can be computed (Ca2+ and Mg2+)HO O SO O OOMgNN N O Figure 1. Structure of eriochrome black T magnesium complex 20. Standardization http://xkcd.com/927/ 21. Standardization 10.00m L 0.0050 M wrk+ 3 mL buffer Titrate to blue std CaCO3+ 6 drps EBTendpoint Reactions: (Consider w/o MgCl2) Start: Ca2+ + HIn2- CaHIn (Kf = 2.5x105) Before equivalence point: Ca2+ + H2Y2- CaH2Y(Kf = 5.0x1010) At the end point: CaHIn + Y2- CaH2Y + HIn2- 22. Standardization w/o MgCl2Ca2+ + HIn2- CaHIn Ca2+ + H2Y2- CaH2YCaHIn + Y2- CaH2Y + HIn2- 23. Standardization 10.00 mL 0.0050 M wrk+ 3 mL bufferTitrate to blue std CaCO3+ 6 drps EBT endpoint Reactions: (Consider w/ MgCl2) Start: Ca2+ + HIn2- CaHIn (Kf = 2.5x105)Calcium-EBT complexation Before equivalence point: Ca2+ + H2Y2- CaH2Y(Kf = 5.0x1010)Calcium consumption Ca2+ + MgH2Y CaH2Y + Mg2+ Magnesium Displacement Mg2+ + CaHIn MgHIn + Ca2+ Magnesium-EBT complexation Near the endpoint: Assume all Ca2+ consumed Mg2+ + H2Y2- MgH2Y Released Magnesium consumption At the end point: MgHIn + H2Y2- MgH2Y + HIn2- Magnesium Displacement 24. Standardization w/ MgCl2Ca2+ + HIn2- Mg2+ + H2Y2- MgH2YCaHIn Ca2+ + H2Y2- CaH2Y Ca2+ + MgH2Y CaH2Y + Mg2+ MgHIn + H2Y2- MgH2Y + HIn2- Mg2+ + CaHIn MgHIn + Ca2+ 25. Standardization w/ MgCl2Amount of EDTA corresponding toFree EDTA to titrate released Mg2+ Amount of EDTA to titrate Ca2+Amount of EDTA to titrate Mg2+ in indicator (negligible) 26. Standardization The amount of MgCl2 that was supposed to be added was not in significant (0.0049 mol, compared to EDTA that has 0.025 mol) The addition of MgCl2 should be stoichiometric (Y=X) or negligible (Y0) to EDTA in order for the titration to be unaffected 27. Sample Analysis 28. Sample Analysis 50 mL + 3 mL buffer Titrate to blue Viva+ 6 drps EBTendpoint Reactions: (Consider w/o MgCl2) Start: Mg2+ + HIn2- MgHIn (Kf = 1.0x107) Magnesium-EBT complexation Before equivalence point: Ca2+ + H2Y2- CaH2Y(Kf = 5.0x1010) Calcium consumption Near the endpoint: Assume all Ca2+ consumed Mg2++H2Y2-MgH2Y(Kf=4.9x108) Present Magnesium consumption At the end point: MgHIn + H2Y2- MgH2Y + HIn2- Magnesium Displacement 29. Sample Analysis w/o MgCl2 Mg2+ + H2Y2- MgH2YMg2+ + HIn2- MgHInCa2+ + H2Y2- CaH2YCaHIn + Y2- CaH2Y + HIn2- 30. Sample Analysis 50 mL+ 3 mL bufferTitrate to blue Viva + 6 drps EBT endpoint Reactions: (Consider w/ MgCl2) Start: Mg2+ + HIn2- MgHIn (Kf = 1.0x107) Magnesium-EBT complexation Before equivalence point: Ca2+ + H2Y2- CaH2Y(Kf = 5.0x1010)Calcium consumptionCa2+ + MgH2Y CaH2Y + Mg2+Magnesium release Near the endpoint: Assume all Ca2+ consumed Mg2++H2Y2-MgH2Y(Kf=4.9x108) Present Magnesium consumptionReleased Magnesium consumption At the end point: MgHIn + H2Y2- MgH2Y + HIn2-Magnesium Displacement 31. Sample Analysis w/ MgCl2Mg2+ + HIn2- MgHInMg2+ + H2Y2- MgH2YCa2+ + H2Y2- CaH2YCa2+ + MgH2Y CaH2Y + Mg2+ MgHIn + H2Y2- MgH2Y + HIn2- 32. Sample Analysis w/ MgCl2 Amount of EDTA corresponding to Free EDTA to titrate released and present Mg2+ Amount of EDTA to titrate Ca2+Amount of EDTA to titrate Mg2+ in indicator (negligible) 33. Sample Analysis w/ MgCl2Y XAmount of EDTA totitrate Mg2+ inindicator (negligible) 34. Sample Analysis w/ MgCl2Let X =Y= YX 35. Sample Analysis w/ MgCl2Let X =Y= YX 36. Sample Analysis The amount of MgCl2 that was supposed to beadded was significant (0.0049 mol, compared toEDTA that has 0.025 mol) The addition of MgCl2 should be negligible(EDTA:Mg0) with respect to EDTA in order for thetitration to be unaffected 37. Sample Analysis Back Titration with EDTA is possible Add standardized amount of EDTA Back titrate with Mg2+ 38. Molarity of Primary Standard Weight 1o std: 1.2511 g %Purity 1o std: 99.9% Final volume std: 250 mL Vol std soln: 5 mL Final vol working std: 50 mL 39. Molarity of Primary Standard The molarity of the working standard can be computed from the given 40. Volume of titrant used instandardization Trial 1 Trial 2 Trial 3 Volumeworking std101010 CaCO3Final volume14.1 28 41.8Initial0.4 14.1 28 volume Net volume 13.713.913.8 41. Molarity of EDTA VolumeNet volumeMolaritywrking std EDTAStd A 10 mL 13.7 mL Molarity: 42. Titer of EDTA nEDTA = MolarityTiter nCaCO3Std A 0.003646 M1 Titer: 43. Total Hardness of Viva Volume ofTotalTotal hardness in ppmWaterVol EDTA Amount of CaCO3Sample CalciumSample A50 mL 19.5 mL Total Amount of Calcium: Total hardness in ppm CaCO3: 44. Collective DataTeamGroup Trial1 Trial2 Trial3 Average1 142.58 142.58 139.02141.401 23 140.61 139.89 140.61 140.37Team Mean140.884 148.57 149.32 147.83 148.572 5 151.40 140.83 140.13 144.126 141.29 142.02 144.92 142.74Team Mean145.14 7148.38 138.63 136.54 141.19 83 9 10 139.15 142.66 138.45 140.09Team Mean140.64Mean 142.64Stdev 4.73RSD 33.2 45. To Viva or not to Viva, that is the question 46. PPM Viva Total hardness of 192.49 ppm CaCO3 47. Conclusion 25.8% difference between the mean and the claimedvalue Vivas water is softer than they claim it to be 48. References J. Roger Hart; J. Chem. Educ., 1984, 61 (12), p 1060. Blitz, Jonathan P. COMPLEXOMETRIC DETERMINATION OF Mg2+ and Ca2+. 2010. 25 January2012 . Garrett, Simon J. CEM 333 EDTA Formation Constants. 1998. 25 January 2012. Jackson School of Geosciences. COORDINATION CHEMISTRY. n.d. 25 January 2012. Jon A. McCleverty, Thomas J. Meyer. "Applications of Coordination Complexes." 2003. PlatinumMetals Review. 25 January 2012 . Mccord, Dr. Stephen P. Determination of Water Hardness using Complexometric titration. 2005. 25January 2012 . Old Dominion University. Chapter 12: EDTA Titrations. n.d. 25 January 2012. Prince Georges Community College. ANALYSIS OF CALCIUM BY EDTA TITRATION TO ASSESSWATER . n.d. 25 January 2012 . Reckhow Research Group. CHAPTER XVI VOLUMETRIC METHODS. 17 June 2011. 25 January2012 . Sinex, Scott A. EDTA - A Molecule with a Complex Story. 1 August 2007. 25 January 2012. UC Davis Department of Chemistry. EDTA TITRATIONS. 31 March 2004. 25 January 2012. 49. He who asks is a fool for five minutes, but he whodoes not ask remains a fool forever. Chinese Proverb