Preparations for Performing Exp. 12
(Determination of the Composition of
Cobalt Oxalate Hydrate)
Special Equipment
and Supplies
Analytical balance
Volumetric flask,
100-mL
Florence flask,
500-mL
Hot plate
Burner
Crucibles
Crucible covers
Crucible tongs
Clay triangle
Desiccator
Sodium oxalate, Na2C2O4 (s)
Potassium permanganate, KMnO4 (s)
Desiccant
Objective
In performing this experiment, the student will prepare one
standard solution and one solution that will need to be
standardized and will also heat crucibles to constant
weight.
Safety
Pay close attention when lighting and adjusting the burner.
Sodium oxalate is a poison. Potassium permanganate is a
toxic, strong oxidizing agent.
Because of the nature of the chemicals used, safety goggles
are mandatory. The wearing of laboratory aprons is
strongly advised.
First Aid
If sodium oxalate gets on your skin, flush with water.
If you ingest, sodium oxalate, drink a large quantity of
water, followed by milk or milk of magnesia. Then see a
doctor.
Aqueous potassium permanganate may leave brown
stains on skin but normally causes no serious health
problems. Rinsing with water should be sufficient
treatment for KMnO4-stained skin. This may be
followed by treatment with aqueous NaHSO3, which
with reduce the KMnO4 or MnO2 to the colorless Mn2+.
Obtaining accurate experimental results often requires a good deal of preparation prior to
the start of the experiment. The intention of this experiment is for the student to consider
some of the factors that must be taken into account so that preparations will be done
efficiently and with the needed degree of accuracy.
PRINCIPLES
If you needed to prepare 100.00 mL of a 0.2000 M solution of potassium dichromate
(K2Cr2O7) in water, you would weigh out 5.8836 g of reagent-grade potassium
dichromate on an analytical balance. You would then dissolve the solid in 25 to 50 mL of
deionized water that you would transfer to a 100-mL volumetric flask. To ensure
complete transfer of the solute, you would rinse the container that originally held the
solution with several small portions of water, carefully transferring each of the rinsings to
the volumetric flask. You would then dilute the solution to the mark on the neck of the
volumetric flask. Once the solution had been diluted, you would cap and invert the
volumetric flask several times to ensure that the mixture it contained had become
homogeneous. The concentration of the solution would be obtained via a calculation,
such as the one shown in Equation 1.
5.8836 g K2Cr2O7 × (1 mol K2Cr2O7/294.181 g K2Cr2O7) = 0.020000 mol K2Cr2O7
(0.020000 mol K2Cr2O7/(0.10000 L) = 0.20000 M K2Cr2O7
(1)
The procedure outlined above is valid because potassium dichromate is a primary
standard. Primary standards are substances that can be obtained in a state of virtually
100% purity and which remain pure for long periods of time, whether they are in their
original states or they have been put into solution. Solid primary standards either have
little tendency to absorb moisture from the air or can be oven-dried (to remove any
acquired moisture) without decomposing.
Most substances are not primary standards. Many exhibit such strong tendencies to
decompose that they cannot be obtained in pure form. Their tendency to decompose may
be facilitated by the dissolution process. In some cases, bacteria, light, and other
substances may catalyze their decomposition. When you need to work with a solution of
such a substance and you need to know its concentration accurately, you would take
appropriate steps to slow the decomposition and to remove any products of
decomposition that might interfere with the analysis in which you are employing the
solution. You would also need to standardize the solution, possibly by titrating it against
a solution of a primary standard.
In preparing the solutions needed for Exp. 12, you will make one solution that is a
primary standard and one that is not. In your report, you will describe the procedures
used to prepare the solutions. You will show a sample calculation of the exact
concentration of the primary standard and of the approximate concentration of the other
solution. In your discussion, you will state which solution is the primary standard and
which solution will need to be standardized, giving explanations for your classifications.
While you are preparing your solutions, you will also be heating two crucibles to constant
weight. You will need the solutions and the crucibles when you perform Exp. 12
(Determination of the Composition of Cobalt Oxalate Hydrate).
PROCEDURE
Procedure in a Nutshell
Dissolve a known mass of sodium oxalate in 100.00 mL of solution. Store the solution in a plastic
bottle. Use water that has been boiled and cooled to prepare a solution of potassium permanganate. Store
the solution in the dark in a brown glass bottle. Heat two crucibles to constant weight. Store the crucibles in
a desiccator.
Sodium Oxalate Solution
Transfer 2-3 grams of reagent grade sodium oxalate to a weighing boat. Weigh out
1.34 g of sodium oxalate into a second weighing boat. Record the mass of the sodium
oxalate in the second weighing boat to the nearest 0.1 mg. Dissolve the sodium oxalate in
a small amount of deionized water. Transfer the sodium oxalate solution to a clean, but
not necessarily dry, 100-mL volumetric flask. Rinse the beaker with several small
portions of deionized water and transfer these rinsings to the volumetric flask.
Dilute the solution in the volumetric flask to the mark with deionized water. (Add
water very slowly as the solution begins to fill the neck of the flask. The final drops may
be added with a Pasteur pipet). The volumetric flask contains 100.00 mL of solution
when the bottom of the water’s meniscus just touches the mark on the neck of the flask.
Be sure that you are looking directly at the mark when you determine the position of the
meniscus.
After the solution has been diluted, cap the flask and invert it several times to ensure
complete mixing. Rinse a plastic storage bottle with several small portions of the sodium
oxalate solution. Transfer the solution from the volumetric flask to the rinsed bottle. Keep
this solution until it is needed for the analysis of the cobalt oxalate hydrate synthesized
previously.
Potassium Permanganate Solution
Boil approximately 600 mL of deionized water. Cool the boiled water to room
temperature (containers of boiled water may be placed in ice baths to hasten cooling).
Weigh out approximately 1.6 g of KMnO4 (s). Dissolve the KMnO4 (s) in 500 mL of the
(boiled and cooled) water.
Store the solution in the dark (either in a brown glass bottle or in a colorless glass
bottle that has been wrapped in aluminum foil). Filter the KMnO4 solution through
Pyrex glass wool prior to its use in the analysis of the cobalt oxalate compound.
Preparing Crucibles for Analytical Work
All crucibles to be used in gravimetric analyses must be heated to constant weight.
Crucibles should be weighed on analytical balances and then heated in a burner flame.
After crucibles have been heated, they should be allowed to cool in a Desicooler that has
been charged with Drierite®. When the crucibles have cooled to room temperature, they
should be weighed once again on the analytical balances. The cycle of heating, cooling,
and weighing should be continued until two consecutive weighings agree to ±0.1 mg.
A separate crucible must be prepared for each of the analysis that will be performed in
Exp. 12.
Disposal of Reagents
Place any excess sodium oxalate and potassium permanganate in separate
collection vessels. The solids will be combined with corresponding solutions after
Experiment 3 has been performed.
Date______Name_______________________________Section____Desk Number____
PRE-LAB EXERCISES FOR “PREPARATIONS” EXPERIMENT
These exercises are to be performed after you have read the experiment but before you
come to the laboratory to perform it.
1. What are the criteria by which a substance is classified as a “primary standard”?
2. What mass of potassium dichromate is needed to prepare 250.00 mL of 0.1500 M
K2Cr2O7?