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 Experiments
#2 & #4B), 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 Alq
properties that make it desirable for gravimetric analysis (e.g., a large molar mass).
This compound also has 'real
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 obtained by the
deprotonation of the 8-hydroxyquinoline molecule via the following dissociation reaction:
8-hydroxyquinoline (aka. OxH),
8-hydroxyquinolinate (aka. Ox
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 problem.