Gravimetric Analysis

1
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.

2
Gravimetric Analysis

• Accurate and precise.
• Possible sources of errors can be checked.
• It is an ABSOLUTE method.
• Relatively inexpensive

3
Gravimetric Analysis

• Precipitation methods.
• Volatilisation methods.
• Electrogravimetry.
• Thermogravimetry.

4
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.

5
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.

6
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 - 150C and weighed as AgCl.

7
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.

8
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.

9
Gravimetric Analysis

• Accuracy
• Solubility Products.
• Solubility.
• Particle size.
• Coprecipitates.
• Drying and ignition.

10
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

11
Gravimetric Analysis

• Equilibrium between AgCl precipitate and the
  saturated solution.

12
Gravimetric Analysis

• The corresponding thermodynamic equilibrium
  constant KT is given by

13
Gravimetric Analysis

• AgCl is in a solid phase therefore
• AgCl 1

KTSP AgCl-
14
Gravimetric Analysis

• Solubility.
• Common ion effect
• Ionic strength
• Fractional precipitation
• Complex ions
• Temperature
• Solvent

15
Gravimetric Analysis

• Particle size.
• Colloidal suspension (10-6 - 10-4 mm diameter) to
  crystalline precipitate.
• Depends on nucleation and particle growth.

16
Gravimetric Analysis

• Coprecipitates.
• Removal during precipitation of compounds which
  are otherwise soluble.
• Sources
• Surface adsorption
• Mixed crystal formation
• Occlusion
• Mechanical entrapment

17
Gravimetric Analysis

• Drying and ignition.
• Removes solvents and volatiles
• Decomposition to known form

18
Gravimetric Analysis

• Inorganic
• H2S, AgNO3, HCl, BaCl2
• Organic
• 2,4-DNP

19
Gravimetric Analysis

• 8-Hydroxyquinoline

• Selectivity through pH control

20
Gravimetric Analysis

• 8-Hydroxyquinoline Examples

21
Gravimetric Analysis

• Dimethylglyoxine

• Weakly alkaline conditions
• Nickel salt bright red

22
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.

23
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

24
Summary

• Disadvantages
• Careful and time consuming.
• Scrupulously clean glassware.
• Very accurate weighing.
• Coprecipitation.