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Gravimetric Analysis

Gravimetric Analysis. Ag + (aq) + Cl - (aq) AgCl(s). 2. Volatilization Methods. CuSO 4 . 5H 2 0 (s) CuSO 4 (s) + 5H 2 O(g). 1. Precipitation Methods. Substance Analyzed. Precipitate formed. Precipitate weighed. Cl -1. AgCl. AgCl. Interferences.

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Gravimetric Analysis

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  1. Gravimetric Analysis

  2. Ag+(aq) + Cl-(aq) AgCl(s) 2. Volatilization Methods CuSO4. 5H20 (s) CuSO4 (s) + 5H2O(g) 1. Precipitation Methods

  3. Substance Analyzed Precipitate formed Precipitate weighed Cl-1 AgCl AgCl Interferences Br-1, I-1, SCN-1, CN-1, S-2, S2O3-2 Some Common Gravimetric Analysis

  4. Gravimetric Procedures • Sample Preparation • Preparation of Solution • Precipitation • Digestion • Filtration & washing of precipitate • Drying &/or igniting of precipitate • Calculations

  5. I. Sample Preparation • Collect sample • Dry sample • Determine mass of sample • Dissolve Sample

  6. II. Preparation of Solution • Volume of solution • Good concentration range • Concentrations of interferences • Temperature • pH

  7. III. Precipitation • The precipitating agent should be specific (at least selective) in its reaction with the analyte

  8. Precipitating Agents • Inorganic precipitating Reagents • Homogeneous generation of precipitating agents • Organic precipitating agents • Reducing agents used in gravimetric analysis • Organic functional groups

  9. Precipitating Agent Element Precipitated AgNO3 Cl-1(AgCl), Br-1(AgBr), I-1(AgI) 1. Inorganic precipitating Reagents

  10. 1. Inorganic precipitating Reagents

  11. 2. Generation of Homogeneous precipitating agents dimethyl sulfate (CH3O)2SO2 + 4H2O  2CH3OH + SO4-2+2H3O+ Ba+2, Ca +2, Sr +2, Pb +2

  12. 3. Organic precipitating agents Dimethylglyoxime (CH3C=NOH)2 M+2 + 2HR  MR2 + 2H+ Ni(II)in NH3 Pd(II)in HCl

  13. 4.Reducing agents Reducing AgentAnalyte SO2 Se, Au H2C2O4 Au NaNO2 Au Electrolytic Ag, Cu, Reduction Co, Ni

  14. 5.Organic functional groups methoxyl and ethoxyl groups ROCH3 + HI  ROH + CH3I ROC2H5 + HI  ROH + C2H5I CH3I + Ag+ + H2O  AgI(s)+ CH3OH

  15. Ideal properties of a precipitate • Easily filtered & washed free of contaminants • Low solubility to reduce loss of mass during filtration and washing • Un-reactive with environment • Known composition after drying or ignition

  16. The Ideal Precipitate Should be easy to filter & wash free of impurities Large Particles

  17. Particle SizeCollodial systemsvsCrystalline suspensions • Precipitate solubility • temperature • reactant concentration • rate of mixing

  18. Precipitate Formationnucleation & particle growth • Nucleation • supersaturated solutions • initiated by suspended solid contaminants • Particle growth • low relative supersaturation • hot solutions • slow addition of precipitating agent w/ stirring

  19. IV. Digestion • Colloidal Particles • Primary Adsorption layer • Counter-ion layer • Ostwald Ripening • Flocculation • Purification • impurities

  20. Colloidal Particles Primary Adsorption layer Counter-ion layr Ag+ NO3- NO3- H+ NO3- Ag+ NO3- NO3- Ag+ Ag+ Ag+ NO3- Ag+ NO3- Ag+ (AgCl)n NO3- H+ NO3- Ag+ H+ Ag+ Ag+ Ag+ NO3- H+ NO3- Ag+ NO3- NO3- NO3- NO3- Colloidal AgCl particles in AgNO3(aq)

  21. Colloidal Particles Primary Adsorption layer Counter-ion layer K+ K+ K+ K+ K+ Cl- K+ NO3- Cl- Cl- Ag+ K+ NO3- Cl- Cl- (AgCl)n NO3- K+ K+ Cl- K+ Cl- Cl- Cl- Cl- NO3- K+ NO3- K+ K+ K+ K+ Colloidal AgCl particles in KCl(aq)

  22. Ostwald Ripening The precipitate (small crystals) is allowed to stand in the presence of the mother liquor ( solution from which it was precipitated) LARGE CRYSTALS grow at the expense of the small crystals

  23. Ag+ Ag+ NO3- NO3- NO3- NO3- H+ H+ NO3- NO3- Ag+ Ag+ NO3- NO3- NO3- NO3- Ag+ Ag+ Ag+ Ag+ Ag+ Ag+ NO3- NO3- Ag+ Ag+ NO3- NO3- Ag+ Ag+ (AgCl)n (AgCl)n NO3- NO3- H+ H+ NO3- NO3- Ag+ H+ Ag+ H+ Ag+ Ag+ Ag+ Ag+ NO3- Ag+ Ag+ NO3- H+ H+ NO3- NO3- Ag+ Ag+ NO3- NO3- NO3- NO3- NO3- NO3- NO3- NO3- Flocculation Coagulation Agglomeration Repulsion = Peptization

  24. Low Electrolyte Conc High Electrolyte Conc Effective Small excess AgNO3 Large excess of AgNO3 Charge Effective Charge Large excess of AgNO3 Distance from surface Distance from surface Breakdown of the Barriers • Addition of extra precipitating agent • Addition of electrolyte • Heating of solution

  25. Purification • Co-precipitation • AgBr with AgCl • Occlusions • adsorbed ion trapped inside • Mechanical entrapment • holes in crystals • Surface adsorption • after complete precipitation • Post-precipitation • Isomorphous Replacement • MgNH4PO4 / MgKPO4

  26. V. Filtration and Washing of precipitate • Wash with electrolyte • Avoids peptization • (reverse of coagulation)

  27. VI. Drying &/or igniting of precipitate • Heat to constant mass • removal of solvent • Ignition • conversion to another substance • MgNH4PO4  MgP2O7 (900oC)

  28. VII. Calculations • The Gravimetric Factor SoughtMeasuredGravimetric Factor SO3 BaSO4 FM SO3 / FM BaSO4 Fe3O4 Fe2O3 FM 2Fe3O4 / FM 3Fe2O3 Fe Fe2O3 FM 2Fe / FM Fe2O3 MgO Mg2P2O7 FM 2MgO / FM Mg2P2O7 P2O5 Mg2P2O7 FM P2O5 / FM Mg2P2O7

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