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Experiment 7
Objective Apply the concept of complexometric titration in the determination of total hardness in drinking water
Simple Water and Complexes
Water Drinking   Hard water is better for    drinking because it    contains minerals
Water         Cleaning           Soft water is better for            cleaning because it            doesn’t form scum wi...
Water Hardness of water   Depends on source   Caused by Ca2+ and    Mg2+ forming    precipitates with soap
Water         Hardness of water           Most Ca2+ in water            come from CaCO3            exoskeletons of aquat...
Complexes Coordination between  metal and ligand    An atom in the ligand     (called the donor)     donates electrons t...
Complexes - Ligands                Monodentate                Polydentate                   Chelating agents – Gr.     ...
Complexes - Usage Catalysts   Polymerization, hydrogenation, hydroboration, etc. Medicine   cis-diamminedichloroplatin...
Step by step discussion*simple dilutions are not discussed
Solution Preparation
Solution Preparation 250 mL 0.050 M std CaCO3soln                           + 20 mL dH2O, +few             Cover with wat...
Solution Preparation pH 10 buffer                                                Transfer to 250 mL  142 mL conc NH3 +   ...
Solution Preparation pH 10 buffer                                             Transfer to 250 mL  142 mL conc NH3 +      ...
Solution Preparation pH 10 buffer                                           Transfer to 250 mL142 mL conc NH3 +          ...
Solution Preparation 500 mL 0.050 M EDTA                                          +1.0 g9.31 g Na2H2EDTA•2H2O   +200 mL d...
Solution Preparation 500 mL 0.050 M EDTA Why add 1.0 g MgCl2•6H2O?   Diverse ion effect: increase solubility (easier   ...
Solution Preparation 500 mL 0.050 M EDTA What happens when we DON’T add MgCl2?   Endpoint not as sharp if Mg2+ is not p...
Standardization           http://xkcd.com/927/
Standardization 10.00m L 0.0050 M wrk    + 3 mL buffer       Titrate to blue std CaCO3                + 6 drps EBT        ...
Standardization     w/o MgCl2Ca2+ + HIn2- CaHIn           Ca2+ + H2Y2-  CaH2Y                                  CaHIn + ...
Standardization 10.00 mL 0.0050 M wrk        + 3 mL buffer        Titrate to blue std CaCO3                    + 6 drps EB...
Standardization     w/ MgCl2Ca2+ + HIn2-                            Mg2+ + H2Y2-  MgH2YCaHIn           Ca2+ + H2Y2-  C...
Standardization w/ MgCl2                                Amount of EDTA corresponding to                                Fr...
Standardization The amount of MgCl2 that was supposed to be added was not in significant (0.0049 mol, compared to EDTA th...
Sample Analysis
Sample Analysis             50 mL         + 3 mL buffer       Titrate to blue             Viva          + 6 drps EBT      ...
Sample Analysis     w/o MgCl2                                 Mg2+ + H2Y2-  MgH2YMg2+ + HIn2-  MgHIn          Ca2+ + H2...
Sample Analysis             50 mL        + 3 mL buffer        Titrate to blue             Viva         + 6 drps EBT       ...
Sample Analysis     w/ MgCl2Mg2+ + HIn2-  MgHIn                    Mg2+ + H2Y2-  MgH2Y          Ca2+ + H2Y2-  CaH2Y   ...
Sample Analysis w/ MgCl2                             Amount of EDTA corresponding to                             Free EDT...
Sample Analysis w/ MgCl2                  Y             X    Amount of EDTA to                  titrate Mg2+ in          ...
Sample Analysis w/ MgCl2    Let X =    Y=                 Y              X
Sample Analysis w/ MgCl2    Let X =    Y=                 Y              X
Sample Analysis The amount of MgCl2 that was supposed to be  added was significant (0.0049 mol, compared to  EDTA that ha...
Sample Analysis Back Titration with EDTA is possible    Add standardized amount of EDTA    Back titrate with Mg2+
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...
Molarity of Primary Standard The molarity of the working standard can be computed from the given
Volume of titrant used instandardization               Trial 1   Trial 2   Trial 3   Volumeworking std      10        10  ...
Molarity of EDTA       Volume    Net volume                              Molarity      wrking std   EDTAStd A   10 mL     ...
Titer of EDTA                 nEDTA =       Molarity            Titer                 nCaCO3Std A 0.003646 M    1 Titer:
Total Hardness of Viva         Volume of              Total                                          Total hardness in ppm...
Collective Data  Team      Group   Trial1   Trial2   Trial3   Average              1     142.58   142.58   139.02    141.4...
To Viva or not to Viva, that is the question
PPM Viva Total hardness of 192.49 ppm CaCO3
Conclusion 25.8% difference between the mean and the claimed  value Viva’s water is softer than they claim it to be
References   J. Roger Hart; J. Chem. Educ., 1984, 61 (12), p 1060.   Blitz, Jonathan P. COMPLEXOMETRIC DETERMINATION OF ...
“He who asks is a fool for five minutes, but he who            does not ask remains a fool forever.”                      ...
Quantitative Determination of Total Hardness in Drinking Water by Complexometric EDTA Titration
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Quantitative Determination of Total Hardness in Drinking Water by Complexometric EDTA Titration

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Chem 28.1 Experiment 7

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Quantitative Determination of Total Hardness in Drinking Water by Complexometric EDTA Titration

  1. 1. Experiment 7
  2. 2. Objective Apply the concept of complexometric titration in the determination of total hardness in drinking water
  3. 3. Simple Water and Complexes
  4. 4. Water Drinking  Hard water is better for drinking because it contains minerals
  5. 5. Water  Cleaning  Soft water is better for cleaning because it doesn’t form scum with soap
  6. 6. Water Hardness of water  Depends on source  Caused by Ca2+ and Mg2+ forming precipitates with soap
  7. 7. Water  Hardness of water  Most Ca2+ in water come from CaCO3 exoskeletons of aquatic microorganisms like diatoms
  8. 8. Complexes Coordination between metal 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. 9. Complexes - Ligands  Monodentate  Polydentate  Chelating agents – Gr. “chelos”, meaning teeth  Polyanionic  Complex-forming  Reacts in a 1:1 ratio
  10. 10. Complexes - Usage Catalysts  Polymerization, hydrogenation, hydroboration, etc. Medicine  cis-diamminedichloroplatinum(II) Cl NH3 Chelators  Heavy Metal Poisoning Treatment Pt  Cleaning agent  Food enhancement Cl NH3  Anti-b acterial  Titrants
  11. 11. Step by step discussion*simple dilutions are not discussed
  12. 12. Solution Preparation
  13. 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 (Dissolve evaporation rate to standard precipitate, basic keep beaker from CO32-) getting dry) Rinse watch glass to Evaporate to 10 mL. beaker Cool. Quantitatively transfer to 250 mL vol flask. Dilute to mark.
  14. 14. Solution Preparation pH 10 buffer Transfer to 250 mL 142 mL conc NH3 + Adjust soln to pH 10 vol flask. Dilute to 17.5 g NH4Cl mark. pOH pKb log([NH 4 ] /[ NH 3 ]) pOH log(1.8 x10 5 ) log(2.1016m ol/ 0.327m ol) pOH 4.74 0.808 pH 8.45Add ammonia to increase pH!
  15. 15. Solution Preparation pH 10 buffer Transfer to 250 mL 142 mL conc NH3 + Adjust soln to pH 10 vol flask. Dilute to 17.5 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. 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. 17. Solution Preparation 500 mL 0.050 M EDTA +1.0 g9.31 g Na2H2EDTA•2H2O +200 mL dH2O MgCl2•6H2O Dissolve in 500 mL vol flask. Heat if necessary.
  18. 18. Solution Preparation 500 mL 0.050 M EDTA Why add 1.0 g MgCl2•6H2O?  Diverse ion effect: increase solubility (easier preparation)  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. 19. Solution Preparation 500 mL 0.050 M EDTA What happens when we DON’T add MgCl2?  Endpoint not as sharp if Mg2+ is not present in solution  Only total hardness can be computed (Ca2+ and Mg2+) HO O S O O O O Mg N N N O Figure 1. Structure of eriochrome black T – magnesium complex
  20. 20. Standardization http://xkcd.com/927/
  21. 21. Standardization 10.00m L 0.0050 M wrk + 3 mL buffer Titrate to blue std CaCO3 + 6 drps EBT endpoint 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. 22. Standardization  w/o MgCl2Ca2+ + HIn2- CaHIn Ca2+ + H2Y2-  CaH2Y CaHIn + Y2-  CaH2Y + HIn2-
  23. 23. Standardization 10.00 mL 0.0050 M wrk + 3 mL buffer Titrate 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. 24. Standardization  w/ MgCl2Ca2+ + HIn2-  Mg2+ + H2Y2-  MgH2YCaHIn Ca2+ + H2Y2-  CaH2Y Ca2+ + MgH2Y  CaH2Y + Mg2+ MgHIn + H2Y2-  MgH2Y + HIn2- Mg2+ + CaHIn  MgHIn + Ca2+
  25. 25. Standardization w/ MgCl2 Amount of EDTA corresponding to Free EDTA to titrate released Mg2+ Amount of EDTA to titrate Ca2+ Amount of EDTA to titrate Mg2+ in indicator (negligible)
  26. 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 (Y≈0) to EDTA in order for the titration to be unaffected
  27. 27. Sample Analysis
  28. 28. Sample Analysis 50 mL + 3 mL buffer Titrate to blue Viva + 6 drps EBT endpoint 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. 29. Sample Analysis  w/o MgCl2 Mg2+ + H2Y2-  MgH2YMg2+ + HIn2-  MgHIn Ca2+ + H2Y2-  CaH2Y CaHIn + Y2-  CaH2Y + HIn2-
  30. 30. Sample Analysis 50 mL + 3 mL buffer Titrate 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 consumption Ca2+ + MgH2Y  CaH2Y + Mg2+ Magnesium release  Near the endpoint: Assume all Ca2+ consumed  Mg2++H2Y2-MgH2Y(Kf=4.9x108) Present Magnesium consumption Released Magnesium consumption  At the end point:  MgHIn + H2Y2-  MgH2Y + HIn2- Magnesium Displacement
  31. 31. Sample Analysis  w/ MgCl2Mg2+ + HIn2-  MgHIn Mg2+ + H2Y2-  MgH2Y Ca2+ + H2Y2-  CaH2Y Ca2+ + MgH2Y  CaH2Y + Mg2+ MgHIn + H2Y2-  MgH2Y + HIn2-
  32. 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. 33. Sample Analysis w/ MgCl2 Y X Amount of EDTA to titrate Mg2+ in indicator (negligible)
  34. 34. Sample Analysis w/ MgCl2 Let X = Y= Y X
  35. 35. Sample Analysis w/ MgCl2 Let X = Y= Y X
  36. 36. Sample Analysis The amount of MgCl2 that was supposed to be added was significant (0.0049 mol, compared to EDTA that has 0.025 mol) The addition of MgCl2 should be negligible (EDTA:Mg≈0) with respect to EDTA in order for the titration to be unaffected
  37. 37. Sample Analysis Back Titration with EDTA is possible  Add standardized amount of EDTA  Back titrate with Mg2+
  38. 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. 39. Molarity of Primary Standard The molarity of the working standard can be computed from the given
  40. 40. Volume of titrant used instandardization Trial 1 Trial 2 Trial 3 Volumeworking std 10 10 10 CaCO3Final volume 14.1 28 41.8 Initial 0.4 14.1 28 volume Net volume 13.7 13.9 13.8
  41. 41. Molarity of EDTA Volume Net volume Molarity wrking std EDTAStd A 10 mL 13.7 mL Molarity:
  42. 42. Titer of EDTA nEDTA = Molarity Titer nCaCO3Std A 0.003646 M 1 Titer:
  43. 43. Total Hardness of Viva Volume of Total Total hardness in ppm Water Vol EDTA Amount of CaCO3 Sample CalciumSample A 50 mL 19.5 mL Total Amount of Calcium: Total hardness in ppm CaCO3:
  44. 44. Collective Data Team Group Trial1 Trial2 Trial3 Average 1 142.58 142.58 139.02 141.40 1 2 3 140.61 139.89 140.61 140.37Team Mean 140.88 4 148.57 149.32 147.83 148.57 2 5 151.40 140.83 140.13 144.12 6 141.29 142.02 144.92 142.74Team Mean 145.14 7 148.38 138.63 136.54 141.19 8 3 9 10 139.15 142.66 138.45 140.09Team Mean 140.64 Mean 142.64 Stdev 4.73 RSD 33.2
  45. 45. To Viva or not to Viva, that is the question
  46. 46. PPM Viva Total hardness of 192.49 ppm CaCO3
  47. 47. Conclusion 25.8% difference between the mean and the claimed value Viva’s water is softer than they claim it to be
  48. 48. References J. Roger Hart; J. Chem. Educ., 1984, 61 (12), p 1060. Blitz, Jonathan P. COMPLEXOMETRIC DETERMINATION OF Mg2+ and Ca2+. 2010. 25 January 2012 <http://www.ux1.eiu.edu/~cfjpb/teaching/quant/labs/experiment8.pdf>. Garrett, Simon J. CEM 333 EDTA Formation Constants. 1998. 25 January 2012 <http://www.cem.msu.edu/~cem333/EDTATable.html>. Jackson School of Geosciences. COORDINATION CHEMISTRY. n.d. 25 January 2012 <http://www.geo.utexas.edu/courses/376m/coord_chem.htm>. Jon A. McCleverty, Thomas J. Meyer. "Applications of Coordination Complexes." 2003. Platinum Metals Review. 25 January 2012 <http://www.platinummetalsreview.com/pdf/101-104-pmr- jul04.pdf>. Mccord, Dr. Stephen P. Determination of Water Hardness using Complexometric titration. 2005. 25 January 2012 <http://mccord.cm.utexas.edu/courses/spring2005/ch455/Spr05455Wk4Lab.pdf>. Old Dominion University. Chapter 12: EDTA Titrations. n.d. 25 January 2012 <http://www.odu.edu/sci/xu/chem321/chem321chapter12.pdf>. Prince Georges Community College. ANALYSIS OF CALCIUM BY EDTA TITRATION TO ASSESS WATER . n.d. 25 January 2012 <http://academic.pgcc.edu/psc/chm103/EDTA_Ca.pdf>. Reckhow Research Group. CHAPTER XVI VOLUMETRIC METHODS. 17 June 2011. 25 January 2012 <http://www.ecs.umass.edu/cee/reckhow/courses/572/572bk16/572BK16.html>. Sinex, Scott A. EDTA - A Molecule with a Complex Story. 1 August 2007. 25 January 2012 <http://www.chm.bris.ac.uk/motm/edta/edtah.htm>. UC Davis Department of Chemistry. EDTA TITRATIONS. 31 March 2004. 25 January 2012 <http://www-chem.ucdavis.edu/2C/Old/06EDTA.pdf >.
  49. 49. “He who asks is a fool for five minutes, but he who does not ask remains a fool forever.” Chinese Proverb

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