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.

It is an ABSOLUTE method.

Relatively inexpensive
Gravimetric Analysis

Precipitation methods.

Volatilisation methods.

Electrogravimetry.

Thermogravimetry.
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 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 Analysis

Determination of silver.
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.
A
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 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 Analysis

Accuracy
 Solubility
Products.
 Solubility.
 Particle
size.
 Coprecipitates.
 Drying and ignition.
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 Analysis

Equilibrium between AgCl precipitate
and the saturated solution.
AgCl(s)
Ag+(aq) + Cl-(aq)
Gravimetric Analysis

The corresponding thermodynamic
equilibrium constant KT is given by:
+][Cl-]
[Ag
KT =
[AgCl]
Gravimetric Analysis

AgCl is in a solid phase therefore
[AgCl] = 1
KTSP = [Ag+][Cl-]
Gravimetric Analysis

Solubility.
 Common
ion effect
 Ionic strength
 Fractional precipitation
 Complex ions
 Temperature
 Solvent
Gravimetric Analysis

Particle size.
suspension (10-6 - 10-4 mm diameter)
to crystalline precipitate.
 Colloidal
 Depends
on nucleation and particle growth.
Gravimetric Analysis

Coprecipitates.
 Removal
during precipitation of compounds
which are otherwise soluble.
 Sources:
 Surface
adsorption
 Mixed crystal formation
 Occlusion
 Mechanical entrapment
Gravimetric Analysis

Drying and ignition.
 Removes
solvents and volatiles
 Decomposition to known form
Gravimetric Analysis

Inorganic:
 H2S, AgNO3,

Organic
 2,4-DNP
HCl, BaCl2
Gravimetric Analysis
8-Hydroxyquinoline

O
Mg2+ + 2
N
Mg
N
N
OH
Selectivity
O
through pH control
+ 2H+
Gravimetric Analysis

8-Hydroxyquinoline Examples
Metal
pH
pH
Initial Ppt.
Complete
Metal
pH
pH
Initial Ppt.
Complete
Ppt
Ppt
Aluminium
2.9
4.7 – 9.8
Manganese
4.3
5.9 – 9.5
Bismuth
3.7
5.2 – 9.4
Molybdenum
2.0
3.6 – 7.3
Cadmium
4.5
5.5 – 13.2
Nickel
3.5
4.6 – 10.0
Calcium
6.8
9.2 – 12.7
Thorium
3.9
4.4 – 8.8
Cobalt
3.6
4.9 – 11.6
Titanium
3.6
4.8 – 8.6
Copper
3.0
>3.3
Tungsten
3.5
5.0 – 5.7
Iron(III)
2.5
4.1 – 11.2
Uranium
3.7
4.9 – 9.3
Lead
4.8
8.4 – 12.3
Vanadium
1.4
2.7 – 6.1
Magnesium
7.0
>8.7
Zinc
3.3
>4.4
Gravimetric Analysis

Dimethylglyoxine
CH3
Ni2+ + 2 CH3
C
C
CH3
N N
HO

OH
C
C
O N N O
H
H + 2H+
Ni
O N N O
CH3
C
C
Weakly alkaline conditions

CH3
Nickel salt bright red
CH3
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.
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
Summary

Disadvantages
 Careful
and time consuming.
 Scrupulously clean glassware.
 Very accurate weighing.
 Coprecipitation.