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Gravimetric Analysis. A Gravimetric analysis is based upon the measurement of the weight of a substance that has a KNOWN composition AND IS chemically related to the analyte. Gravimetric Analysis. Accurate and precise. Possible sources of errors can be checked.

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gravimetric analysis
Gravimetric Analysis
  • A Gravimetric analysis is based upon the measurement of the weight of a substance that has a KNOWN composition AND IS chemically related to the analyte.
gravimetric analysis2
Gravimetric Analysis
  • Accurate and precise.
  • Possible sources of errors can be checked.
  • It is an ABSOLUTE method.
  • Relatively inexpensive
gravimetric analysis3
Gravimetric Analysis
  • Precipitation methods.
  • Volatilisation methods.
  • Electrogravimetry.
  • Thermogravimetry.
gravimetric analysis4
Gravimetric Analysis
  • In precipitation methods, the species to be determined is precipitated by a reagent that yields a sparingly soluble product that has a known composition or can be converted to such a substance.
gravimetric analysis5
Gravimetric Analysis
  • Precipitation methods.
    • Analyte (or chemically related species) isolated as a sparingly soluble precipitate of known composition.
    • Analyte (or chemically related species) isolated as a sparingly soluble precipitate that can be converted by heat to species of known composition.
gravimetric analysis6
Gravimetric Analysis
  • Determination of silver.
    • A solution of Ag+ is treated with an excess of NaCl or KCl solution, the precipitate is filtered off, washed well with water to remove soluble salts, dried at 130 - 150°C and weighed as AgCl.
gravimetric analysis7
Gravimetric Analysis
  • Frequently the constituent being estimated is weighed in a form other than that it was precipitated in.
  • Mg2+: precipitated as Mg(NH4)PO4.6H20 but is weighed as magnesium pyrophosphate Mg2P2O7 after ignition.
gravimetric analysis8
Gravimetric Analysis
  • Conditions:

1. Must be a stoichiometric reaction.

2. A stable product; no oxidation, dehydration or gelatinous precipitates.

3. Must avoid side reactions which result in coprecipitates.

gravimetric analysis9
Gravimetric Analysis
  • Accuracy
    • Solubility Products.
    • Solubility.
    • Particle size.
    • Coprecipitates.
    • Drying and ignition.
gravimetric analysis10
Gravimetric Analysis
  • Solubility Products
    • Even the most insoluble products have at least a certain solubility. It is therefore more correct to call these compounds sparingly soluble substances, eg: AgCl
gravimetric analysis11

AgCl(s) Ag+(aq) + Cl-(aq)

Gravimetric Analysis
  • Equilibrium between AgCl precipitate and the saturated solution.
gravimetric analysis12

[Ag+][Cl-]

[AgCl]

KT =

Gravimetric Analysis
  • The corresponding thermodynamic equilibrium constant KT is given by:
gravimetric analysis13
Gravimetric Analysis
  • AgCl is in a solid phase therefore

[AgCl] = 1

KTSP = [Ag+][Cl-]

gravimetric analysis14
Gravimetric Analysis
  • Solubility.
    • Common ion effect
    • Ionic strength
    • Fractional precipitation
    • Complex ions
    • Temperature
    • Solvent
gravimetric analysis15
Gravimetric Analysis
  • Particle size.
    • Colloidal suspension (10-6 - 10-4 mm diameter) to crystalline precipitate.
    • Depends on nucleation and particle growth.
gravimetric analysis16
Gravimetric Analysis
  • Coprecipitates.
    • Removal during precipitation of compounds which are otherwise soluble.
    • Sources:
      • Surface adsorption
      • Mixed crystal formation
      • Occlusion
      • Mechanical entrapment
gravimetric analysis17
Gravimetric Analysis
  • Drying and ignition.
    • Removes solvents and volatiles
    • Decomposition to known form
gravimetric analysis18
Gravimetric Analysis
  • Inorganic:
    • H2S, AgNO3, HCl, BaCl2
  • Organic
    • 2,4-DNP
gravimetric analysis19

O

Mg

+ 2

N

M

g

N

+ 2H

2+

+

N

O

O

H

Gravimetric Analysis
  • 8-Hydroxyquinoline
  • Selectivity through pH control
gravimetric analysis20
Gravimetric Analysis
  • 8-Hydroxyquinoline Examples
gravimetric analysis21

C

H

C

C

C

H

3

3

O

N

N

O

Ni

+ 2

2H+

C

H

C

C

C

H

2+

+

H

H

N

i

3

3

O

N

N

O

N

N

H

O

O

H

C

H

C

C

C

H

3

3

Gravimetric Analysis
  • Dimethylglyoxine
  • Weakly alkaline conditions
  • Nickel salt bright red
summary
Summary
  • Principles
    • Solution reaction between analytes and reagents to give sparingly soluble products.
    • Drying or ignition of precipitates.
    • Weighing
  • Apparatus
    • Flasks, beakers, pipettes, crucibles and filter papers.
    • Oven or furnace and a dessicator.
    • Analytical quality balance.
summary23
Summary
  • Applications
    • Extensive numbers of inorganic ions are determined with excellent precision and accuracy.
    • Routine assays of metallurgical samples.
    • Relative precision 0.1 to 1%.
    • Good accuracy
summary24
Summary
  • Disadvantages
    • Careful and time consuming.
    • Scrupulously clean glassware.
    • Very accurate weighing.
    • Coprecipitation.