1. Gravimetric Analysis

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

3. 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

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
AgNO3
Precipitating Agent

11. 1. Inorganic precipitating Reagents

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

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

14. 4. Reducing agents
Reducing Agent Analyte
SO Se, Au
H2C2O Au
NaNO Au
Electrolytic Ag, Cu,
Reduction Co, Ni

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

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

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

23. 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

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

27. 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)

28. 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

29. Flocculation
Coagulation
Agglomeration
Repulsion = Peptization

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

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
Charge
Effective
Distance from surface
Small excess AgNO3
Large excess of AgNO3
High Electrolyte Conc
Charge
Effective
Distance from surface
Large excess of AgNO3

33. 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

34. 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

35. 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

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
MgNH4PO4  MgP2O7 (900oC)

38. VII. Calculations The Gravimetric Factor
Sought Measured Gravimetric Factor
SO BaSO4 FM SO3 / FM BaSO4
Fe3O4 Fe2O3 FM 2Fe3O4 / FM 3Fe2O3
Fe Fe2O FM 2Fe / FM Fe2O3
MgO Mg2P2O FM 2MgO / FM Mg2P2O7
P2O Mg2P2O FM P2O5 / FM Mg2P2O7