Experiment 1- Page 1
Analytical Chemistry: Gravimetric Analysis
Analytical chemistry is perhaps the most fundamental branch of chemistry as it involves either identifying the
components that make up a compound or determining the specific amount of a compound itself.
, the measurement of masses or densities, is one of the most fundamental types of chemical analyses since it
involves the direct comparison of masses of substances, the basis of all stoichiometry.
Unlike volumetric analyses (see
), solutions of standardized concentrations are not necessary in the gravimetric technique, so it is
quite useful when only a few samples are to be analyzed.
Repetitive routine analyses of many samples are often better
done by other techniques, however, due to the time involved for each gravimetric determination.
Also, for gravimetric
analyses to be viable, the element of concern must be present in the sample in a large enough quantity to give enough
precipitate to be measured with analytical precision.
The object of gravimetry is to quantitatively convert the species you are analyzing (and only that species) into a
solid, which can then be collected, dried, and its mass determined.
The mass of the solid and knowledge of its
composition permit the calculation of the moles of the species of interest in the initial sample.
: Aluminum 8-hydroxyquinolinate
In this experiment, you will precipitate the Al
ion by reacting it with an organic anion,
8-hydroxyquinolinate, forming a solid complex of aluminum known as aluminum 8-hydroxyquinolinate (also called
has several properties that make it desirable for gravimetric analysis (e.g., a large molar mass).
compound also has 'real world' applications.
Significant research is being performed on Alq
because it emits light
when a current is run through it, which makes it very desirable for display applications (e.g., computer monitors, etc.).
The 8-hydroxyquinolinate anion that reacts with Al
ion to form the Alq
compound is formed by the
deprotonation of the 8-hydroxyquinoline molecule via the following dissociation reaction:
M.M. = 145.16 g/mol
M.M. = 144.15 g/mol
Note that it is very important that this deprotonation reaction takes place.
The 8-hydroxyquinolinate anion must be
formed for the precipitation to occur; the 8-hydroxyquinoline molecule itself will not react with Al
As a result, careful attention must be paid to the
H of the Al
H is often a
major consideration in gravimetric analyses.
For the formation of a simple precipitate such as AgCl, this is a minor
is the anion of a strong acid, there is essentially no chance of the Cl
anion being tied up in the
protonated acid form at low
H (i.e., acidic) values (which would make it unavailable for precipitation).