Gravimetric Analysis

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

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

Some Common Gravimetric Analysis

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

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

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

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

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

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

1. Inorganic precipitating Reagents

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

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

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

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

Methods for the Generation of Homogeneous precipitating agents

Organic precipitating agents

Reducing agents used in gravimetric analysis

Organic functional groups

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

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

Particle Size Collodial systems vs Crystalline suspensions Precipitate solubility temperature reactant concentration rate of mixing

Precipitate Formation nucleation & particle growth supersaturated solutions initiated by suspended solid contaminants Particle growth low relative supersaturation hot solutions slow addition of precipitating agent w/ stirring

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

Colloidal Particles Primary Adsorption layer Counter-ion layer

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

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

Ostwald Ripening LARGE CRYSTALS grow at the 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

Flocculation Coagulation Agglomeration Repulsion = Peptization

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

Breakdown of the Barriers Addition of extra precipitating agent Addition of electrolyte Heating of solution

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

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

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

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

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

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

VII. Calculations The Gravimetric Factor Sought Measured Gravimetric 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