CHEA 1201 L
Gravimetric Analysis of Sulfur
Pre-laboratory Activity
1. What is gravimetry? Enumerate the 5-6 steps in gravimetric analysis. . (3 pts)
Gravimetry is a macro analytical technique, and is considered as one of the most accurate
analytical techniques. In this technique, the analyte is converted to an insoluble form that is
washed, dried and weighed to determine the concentration of an analyte in a solution. This is
applies to samples where a precipitating agent is good and available. The precipitate is required to
be quantitative, easily washed and filtered and of an appropriate quantity in order to achieve an
accurate weighing of the analyte.
1.) Preparation of the Solution
The researcher must adjust the pH level of the solution in order for the precipitate to occur
quantitatively and get a precipitate that has the desired properties. The inferences are removed
and the volume of the sample must be adjusted in order to suit the amount of the precipitating
agent to be added.
2.) Precipitation
The precipitating agent solution is added to the sample solution. When the first drops of the the
precipitating agent is added, supersaturation of the solution occurs and nucleation starts to occur
where every few molecules of the precipitate aggregate together forming a nucleus. After that, an
addition of extra precipitating agent will cause either a formation of new nuclei or will build up
on existing nuclei to give a precipitate. This can be predicted through the Von Weimarn ratio.
According to this relation, the particle size is inversely proportional to a quantity called the
relative supersaturation where:
Relative Supersaturation = (Q-S) / S
Where q is the concentration of reactant before precipitation and S is the solubility of the
precipitate in the medium from which it is precipitated.
3.) Digestion of the Precipitate
The precipitate is left hot below boiling temperature for 30 minutes to 1 hour. This is
done in order for the particles to be digested. Digestion occurs when there is a dissolution of
small particles and reprecipitation on larger ones resulting in a particle growth and better
precipitate characteristics. This process is called the Ostwald ripening. An advantage of the
digestion is observed for colloidal precipitates where large amounts of absorbed ions cover the
huge area of the precipitate. Digestion then forces the small colloidal particles to agglomerate
which decreases their surface area and adsorption.
4.) Filtering and washing of the precipitate
It is critical to wash the precipitate very well. This must be done in order to remove all the
absorbed species which may add weight to the precipitate. The researcher must avoid using too
much water because it may cause some of the precipitate to be lost. In case of colloidal
precipitates, one must not use water as a washing solution because this may cause peptization.
During these situations, dilute nitric acid, ammonium nitrate, or diluted acetic acid may be used.
Coagulated particles will return to the colloidal state when washed with water, this process is
called peptization.
5.) Drying and Ignition
The precipitate is heated at about 120-150 oC in an oven or ignition in a muffle surface at
temperatures from 600-1200 oC. This is to get a material with exactly known chemical structure
so that the amount of analyte in the substance can be accurately determined.
6.) Calculations
The last step of gravimetric analysis is the performance of the analytical calculations.
2. What are the three types of gravimetric analysis? Provide a brief description for each. . (3 pts)
1.) Volatilization Gravimetry - An approach to gravimetry is to thermally or chemically
decompose the sample and measure the resulting change in its mass. Alternatively, we can trap
and weigh a volatile decomposition product. Because the release of a volatile species is an
essential part of these methods, we classify them collectively as volatilization gravimetric
methods of analysis.
2.) Precipitation Gravimetry - In precipitation gravimetry an insoluble compound forms when we
add a precipitating reagent, or precipitant, to a solution containing our analyte. In most methods
the precipitate is the product of a simple metathesis reaction between the analyte and the
precipitant; however, any reaction generating a precipitate can potentially serve as a gravimetric
method.
3.) Particulate Gravimetry - Precipitation and volatilization gravimetric methods require that the
analyte, or some other species in the sample, participate in a chemical reaction. In a direct
precipitation gravimetric analysis, for example, we convert a soluble analyte into an insoluble
form that precipitates from solution. In some situations, however, the analyte is already present as
in a particulate form that is easy to separate from its liquid, gas, or solid matrix. When such a
separation is possible, we can determine the analyte’s mass without relying on a chemical
reaction.
3. Provide the balanced chemical equations for the following: . (2 pts)
a. Reaction of sodium sulfate and barium chloride
BaCl2 (aq) + Na2SO4 (aq) → BaSO4 (s) + 2NaCl (aq)
b. Reaction of sodium chloride and silver nitrate
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq).
4. Calculate the number of moles of each reactant in the Procedure (see steps 2 and 4). Identify the
limiting reactant in the reaction and determine the theoretical amount of barium sulfate that
should be produced. (3 pts)
0.6g Sodium Sulfate
600mL beaker
Na2SO4
Molar Mass: 142.04 g/mol
100 mL of 0.05 M BaCl2
BaCl2 (aq) + Na2SO4 (aq) → BaSO4 (s) + 2NaCl (aq)
5. Suppose that 0.323 g of an unknown sulfate salt is dissolved in 50 mL of water. The
solution is acidified with 6 M HCl, heated, and an excess of aqueous BaCl2 is slowly
added to the mixture resulting in the formation of a white precipitate.
a. Assuming that 0.433 g of precipitate is recovered calculate the percent by mass
of SO42− in the unknown salt. (2 pts)
b. If it is assumed that the salt is an alkali sulfate determine the identity of the alkali
cation. (2 pts)