CHEM 297 INTERMEDIATE CHEMISTRY II LABORATORY
WEEKS OF FEBRUARY 21 AND 28
INORGANIC COMPLEX SYNTHESIS AND ANALYSIS
HEXAMMINECOBALT (III) CHLORIDE
[Co(NH3)6]Cl3
SYNTHETIC PROCEDURE
4CoCl2 + O2 + 4NH4Cl + 20NH3  4[Co(NH3)6]Cl3 + 2H2O
Mix 6 g of cobalt chloride hexahydrate and 4 g of ammonium chloride with 5 mL of water and stir until most of the
salts have dissolved; then add 12.5 mL of concentrated ammonia solution and 0.15 g of activated carbon. Put the
solution into a 125-mL Buchner flask, which is fitted with a one-hole rubber stopper carrying a wide glass (10-mm
bore) tube reaching down to the bottom of the flask. With the aspirator pump attached to the side arm of the flask,
draw air through the solution at a brisk rate. Make sure you have a trap attached between the aspirator pump and
the flask. After an hour and a half or so, the color of the solution should have changed from a reddish color to a
yellowish brown color. The color may be easily observed by swirling the liquid whilst holding the flask up to the
light. Pass air for 10 minutes after all the red color has gone from the mixture; then vacuum filter. The product,
mixed with carbon, remains on the filter paper. Stir the filter cake into 40 mL of water to which has been added 0.5
mL of concentrated HCl. Test the solution to make sure it is acidic with litmus; if necessary, add a few more drops
of HCl. Heat to 50 - 60oC and filter whilst hot to remove the carbon. Add 7.5 mL of concentrated HCl to the hot
solution and set aside to cool slowly. Once at room temperature, cool in an ice-water bath, vacuum filter the fine
crystals, wash with, first 12.5 mL of 60% ethanol, then with 12.5 mL of 95% ethanol and suck as dry as possible.
Then dry on a watch glass in an 80o oven.
ANALYSIS
1. Co as [Co(SCN)4]2Weigh out a sample of the compound in the range of 0.0600 g and place it in a tall form beaker. Do all the
following in the fume hood. Dissolve the sample in several mL of concentrated H 2SO4. Heat the mixture on a hot
plate until the fumes of SO3 are just detected. CAUTION: H2SO4 is a very strong acid. SO3 fumes should not be
inhaled. Let the mixture cool and then add with extreme caution 5 mL of concentrated HNO 3. This step is
potentially dangerous so take adequate precautions against spattering or spilling. Evaporate the mixture slowly to
dryness and avoid charring the organic material that may remain. Do all the above in the hood.
Dissolve the residue in 20 mL of deionized water (heat if necessary), transfer to a 100 mL volumetric flask and
dilute to 100 mL total volume. Take a 10-mL aliquot (by pipette) and add 8 mL of 6M HCl (make sure the HCl
solution is colorless). Now add 1 drop of 50% NH 4SCN solution. If a red color develops see note 1 before
continuing. Add 10 mL of 50% NH4SCN solution, 50 mL of water, and dilute to 100 mL with acetone. Determine
the absorbance at 600 nm and compare with the calibration curve (see note 2).
Calculate millimoles of Co in the sample taken and the % Co in your original sample.
Note 1 - if a red color develops add 10% tetrasodium pyrophosphate solution until the color just disappears noting
the volume used. Add the NH4SCN solution as above and follow with half of the volume of Na 4P2O7 previously
added. Then dilute as above.
Note 2 - standards for calibration curve. A solution of 0.4936 g Co(NO 3)2.6H2O in 1 L total volume contains 0.1
mg Co/mL. Run the analysis 4 times from the point beginning "take a 10-mL aliquot" but use samples of the
standard solution (measured from a burette) as follows:
1.
2.
3.
4.
5 mL
10 mL
15 mL
20 mL
(0.5 mg Co)
(1.0 mg Co)
(1.5 mg Co)
(2.0 mg Co)
Run the thiocyanate-containing sample as rapidly as possible after work up since the color tends to fade upon
standing.
2. Organic ligand
Obtain the IR spectra of your complex. Assign the major absorptions in the spectrum to the appropriate bond
vibrations.