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

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Presentation on theme: "Gravimetric Analysis. 1. Precipitation Methods Ag + (aq) + Cl - (aq) AgCl(s) 2. Volatilization Methods CuSO 4. 5H 2 0 (s) CuSO 4 (s) + 5H 2 O(g)"— Presentation transcript:

1 Gravimetric Analysis

2 1. Precipitation Methods Ag + (aq) + Cl - (aq) AgCl(s) 2. Volatilization Methods CuSO 4. 5H 2 0 (s) CuSO 4 (s) + 5H 2 O(g)

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

4 Some Common Gravimetric Analysis

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

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

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

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

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

10 1. Inorganic precipitating Reagents Cl -1 (AgCl), Br -1 (AgBr), I -1 (AgI) Element Precipitated AgNO 3 Precipitating Agent

11 1. Inorganic precipitating Reagents

12 2. Generation of Homogeneous precipitating agents dimethyl sulfate (CH 3 O) 2 SO 2 + 4H 2 O  2CH 3 OH + SO H 3 O + Ba +2, Ca +2, Sr +2, Pb +2

13 3. Organic precipitating agents Dimethylglyoxime (CH 3 C=NOH) 2 M HR  MR 2 + 2H + Ni(II) in NH 3 Pd(II) in HCl

14 4. Reducing agents Reducing Agent Analyte SO 2 Se, Au H 2 C 2 O 4 Au NaNO 2 Au Electrolytic Ag, Cu, Reduction Co, Ni

15 5. Organic functional groups methoxyl and ethoxyl groups ROCH 3 + HI  ROH + CH 3 I ROC 2 H 5 + HI  ROH + C 2 H 5 I CH 3 I + Ag+ + H 2 O  AgI (s) + CH 3 OH

16 Methods for the Generation of Homogeneous precipitating agents

17 Organic precipitating agents

18 Reducing agents used in gravimetric analysis

19 Organic functional groups

20 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

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

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

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

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

25 Colloidal Particles Primary Adsorption layer Counter-ion layer

26 Colloidal Particles (AgCl) n Ag + NO 3 - H+H+ Ag + NO 3 - H+H+ H+H+ H+H+ Ag + NO 3 - Ag + Colloidal AgCl particles in AgNO 3(aq) Counter-ion layr Primary Adsorption layer

27 Colloidal Particles (AgCl) n Cl - K+K+ K+K+ NO 3 - Cl - NO 3 - K+K+ K+K+ K+K+ Cl - K+K+ K+K+ K+K+ K+K+ K+K+ NO 3 - Cl - K+K+ NO 3 - K+K+ K+K+ K+K+ Ag + K+K+ Colloidal AgCl particles in KCl (aq) Counter-ion layer Primary Adsorption layer

28 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

29 Flocculation Coagulation Agglomeration Repulsion = Peptization

30 (AgCl) n Ag + NO 3 - H+H+ Ag + Ag+Ag+ NO 3 - H+H+ H+H+ H+H+ Ag + NO 3 - Ag + (AgCl) n Ag + NO 3 - H+H+ Ag + Ag+Ag+ NO 3 - H+H+ H+H+ H+H+ Ag + NO 3 - Ag + Coagulation Agglomeration Repulsion = Peptization Flocculation

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

32 Breakdown of the Barriers Addition of extra precipitating agent Addition of electrolyte Heating of solution Low Electrolyte Conc ChargeCharge EffectiveEffective Distance from surface Small excess AgNO 3 Large excess of AgNO 3 High Electrolyte Conc ChargeCharge EffectiveEffective Distance from surface Large excess of AgNO 3

33 Breakdown of the Barriers Addition of extra precipitating agent Addition of electrolyte Heating of solution Low Electrolyte Conc ChargeCharge EffectiveEffective Distance from surface Small excess AgNO 3 Large excess of AgNO 3

34 Breakdown of the Barriers Addition of extra precipitating agent Addition of electrolyte Heating of solution High Electrolyte Conc ChargeCharge EffectiveEffective Distance from surface Large excess of AgNO 3

35 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 –MgNH 4 PO 4 / MgKPO 4

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

37 VI. Drying &/or igniting of precipitate Heat to constant mass –removal of solvent Ignition –conversion to another substance –MgNH 4 PO 4  MgP 2 O 7 (900 o C)

38 VII. Calculations The Gravimetric Factor SO 3 BaSO 4 FM SO 3 / FM BaSO 4 Fe 3 O 4 Fe 2 O 3 FM 2Fe 3 O 4 / FM 3Fe 2 O 3 Fe Fe 2 O 3 FM 2Fe / FM Fe 2 O 3 MgO Mg 2 P 2 O 7 FM 2MgO / FM Mg 2 P 2 O 7 P 2 O 5 Mg 2 P 2 O 7 FM P 2 O 5 / FM Mg 2 P 2 O 7 Sought Measured Gravimetric Factor


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