EXPERIMENT 2
Gravimetric Analysis of a Soluble Chloride
REFERENCES:
D. A. Skoog, D. M. West, F. J. Holler, and S. R. Crouch Analytical
Chemistry: An Introduction, 7th ed. Chapter 8, pp. 179-201.
BACKGROUND
Gravimetric analysis is a method based
on the isolation of the desired constituent in pure form or in
some combined form from a sample, and weighing the isolated constituent.
The desired constituent is usually isolated or separated by precipitation.
From the weights of sample and precipitate, the percentage of
constituent can be calculated.
Precipitation is effected by inorganic
or organic precipitating agents. Two common inorganic precipitating
agents are silver nitrate, which is used to precipitate halide
ions such as chloride, and barium chloride, which is used to precipitate
sulfate ion. Potassium, ammonium, rubidium, and cesium ions are
precipitated by sodium tetraphenylborate. In all of these precipitation
reactions, the product is a salt because it is formed by reactions
between cations and anions and the bonding is electrovalent.
Organic precipitating reagents contain
functional groups which combine with inorganic ions to form insoluble
salts. The organic reagent may contain groups such as carboxyl
or hydroxyl which ionize to form anions that combine with cations
to form insoluble salts. In this reaction, the bonding is also
electrovalent. Some organic reagents contain nitrogen or oxygen
that can combine with metal ions by forming covalent or coordinate
bonds. Other organic reagents may contain two or more functional
groups which can combine with a single cation to form a ring structure.
Such a reagent would be called a bidentate ligand if it contained
two functional groups. The product formed between a cation and
a multidentate ligand, a ring, is named a chelate.
Silver ions undergo the following reaction
with chloride:
Ag+ + Cl- ® AgCl (white solid)
Silver chloride is a relatively insoluble
compound with a solubility product (Ksp = [Ag+][Cl-])
of 1.8 x 10-10.
An excess of silver ions is added so that
the chloride concentration at equilibrium will be negligible.
If enough silver nitrate solution is not used, the precipitation
will be incomplete, resulting in a substantial error and a low
value for the % Cl in the sample.
PROCEDURE
1. Clean and dry three porcelain crucibles (see note below). Make sure crucibles are marked so they can be distinguished from one another.
2. Dry crucibles in the oven at 100-110 °C for one hour or overnight. The crucibles should be put in a labeled beaker and covered with a watch glass when in the oven.
3. Cool the crucibles in a desiccator for 20 minutes and weigh.
4. Repeat step 2 and 3, this time drying for only 20 minutes.
5. Repeat this procedure until the mass
of each crucible agrees to within 0.3 mg.
Note: Cleaning Procedure for Porcelain
Crucibles
If there is a gray or white residue inthe crucible, add a few drops of conc. NH3(aq) and apply vacuum to
pull the resulting solution through the fritted bottom.
Follow by rinsing with copious quantities of H2O.
Make sure you have about 100 mL of H2O in the filter flask to dilute
the acid when it comes through. If a dark
stain remains, empty the contents of the filter flask and add
a few mL of conc. HNO3 to the crucible. Apply vacuum
as before, and after the solution goes through, empty the flask
and rinse the crucible. If there is a reddish stain in the crucible, a few drops of
conc. HCl should remove it nicely. Proceed as with HNO3
above. If crucible does not filter rapidly after cleaning see
the TA.
Waste Note: The NH3 (a base) waste is separate from the HNO3
and HCl (acids) waste.
Preparation of Chloride Unknown 1. Dry the sample in an oven at 100-120
°C
for 1-2 hours. 2. Weigh out accurately three portions
of the dried sample of about 0.5 to 0.7 g each. 3. Dissolve each portion in a separate
400-mL beaker, using 150 mL of distilled water to which
about 1 mL of concentrated nitric acid has been added. Precipitation of Chloride with Silver
4. Heat the chloride solutions
to boiling and with constant stirring add the silver nitrate
in approximately 5 mL aliquots until
the precipitation of the silver chloride is complete. To check
for complete precipitation, silver nitrate must be added in small
quantities and vigorously stirred. Allow it then to settle a bit
and add some more silver nitrate solution (no stirring yet), if
the solution becomes cloudy, keep adding. If the solution remains
clear add about 10% more silver nitrate solution, then set the
beaker (covered) in your locker for at least one hour.
Filtration and final weighing (this
procedure should be done separately for each sample).
5. After the solution has cooled for 1 hour, filter it through a weighed crucible with suction, keeping most of the precipitate in the beaker. NOTE: Always break the suction on the flask before turning off the water flow on the aspirator.
6. Test the filtrate in the flask for complete precipitation again using a few drops of silver nitrate solution. If your filtrate remains clear, dispose of the filtrate in the appropriate waste container.
7. Wash the precipitate with three 25-mL portions of 0.01 M nitric acid (2 drops of concentrated HNO3 in 100 mL of water). The washings are poured through the filter and the precipitate is left in the beaker.
8. Stir the precipitate up in a small volume of 0.01 M HNO3 and quantitatively transfer the precipitate to the crucible.
9. After filtering, place the crucibles in a beaker covered with a watch glass and dry at 120-140 °C for 2 hours. You can leave the crucibles overnight, if you return the next day and put them in your desiccator.
10. Cool in a desiccator and weigh.
11. Return them to the oven for 20 minutes.
Then cool in the desiccator for 20 minutes and reweigh. Repeat this step until
mass of a same crucible agrees to within 0.4 mg.
Gravimetric calculations
Calculate the % chloride in the sample.
The gravimetric factor is:
The mass of chlorine in each sample is:
mCl
= mAgCl x gravimetric factor
The percent Cl in each sample is:
Note: Avoid contact of AgNO3
with skin. Decomposition of AgNO3 on skin gives black
spots.
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