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

AgCl

Precipitate formed

AgCl

Precipitate weighed

Br-1, I-1, SCN-1, CN-1, S-2, S2O3-2

Interferences

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

3. Organic precipitating agents

Dimethylglyoxime (CH3C=NOH)2M+2 + 2HR MR2 + 2H+

Ni(II) in NH3

Pd(II) in HCl

4. Reducing agents

Reducing Agent AnalyteSO2 Se, AuH2C2O4 AuNaNO2 Au

Electrolytic Ag, Cu, Reduction Co, Ni

5. Organic functional groups

methoxyl and ethoxyl groups

ROCH3 + HI ROH + CH3IROC2H5 + HI ROH + C2H5I

CH3I + Ag+ + H2O AgI(s) + CH3OH

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

The Ideal Precipitate

Large Particles

Should be easy to filter &wash free of impurities

Particle Size

Collodial systems vs Crystalline suspensions

• Precipitate solubility• temperature• reactant concentration• rate of mixing

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

IV. Digestion• Colloidal Particles

– Primary Adsorption layer– Counter-ion layer

• Ostwald Ripening• Flocculation • Purification

– impurities

Colloidal Particles

(AgCl)n

Ag+ NO3-

H+

Ag+

Ag+

Ag+

Ag+

Ag+

Ag+

Ag+

Ag+

NO3-

NO3-

H+

H+

H+

Ag+

NO3-NO3

-

NO3-

NO3-

NO3-

NO3-

NO3-

NO3-

NO3-

NO3-NO3

-

NO3-

Ag+

Ag+

Colloidal AgCl particles in AgNO3(aq)

Counter-ion layrPrimary Adsorption layer

Colloidal Particles

(AgCl)n

Cl- K+

K+

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

NO3-

Cl-

NO3-

NO3-

K+

K+

K+

Cl-

K+K+

K+

K+

K+NO3

-

Cl-

K+

NO3-

K+K+

K+

Ag+

K+

Colloidal AgCl particles in KCl(aq)

Counter-ion layerPrimary Adsorption layer

Ostwald RipeningThe 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

(AgCl)n

Ag+ NO3-

H+

Ag+

Ag+

Ag+

Ag+

A

g+

Ag+

Ag+Ag+

NO3-

NO3-

H+

H+

H+

Ag+

NO3-NO3

-

NO3-

NO3-

NO3-

NO3-

NO3-

NO3-

NO3-

NO3-NO3

-

NO3-

Ag+

Ag+

(AgCl)n

Ag+ NO3-

H+

Ag+

Ag+

Ag+

Ag+

A

g+

Ag+

Ag+Ag+

NO3-

NO3-

H+

H+

H+

Ag+

NO3-NO3

-

NO3-

NO3-

NO3-

NO3-

NO3-

NO3-

NO3-

NO3-NO3

-

NO3-

Ag+

Ag+

Coagulation Agglomeration

Repulsion = Peptization

Flocculation

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

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

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

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

Sought Measured Gravimetric Factor