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 with soap
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 aquatic microorganisms like diatoms
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)
Complexes - Ligands Monodentate Polydentate Chelating agents – Gr. “chelos”, meaning teeth Polyanionic Complex-forming Reacts in a 1:1 ratio
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.
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!
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
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
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.
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.
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
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)
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
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
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 Final vol working std: 50 mL
Molarity of Primary Standard The molarity of the working standard can be computed from the given
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 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 >.
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