Advanced Inorganic Chemistry Dr. M. Lindbeck 7 December, 2000
Introduction The purpose of this experiment was tosynthesize a 6-coordinate cobalt(III) compound from CoCl2 •6H2 O. This is made difficult by the fact that Co2+ ion is more stable than Co3+ for simple salts. There are only a few salts of cobalt(III), suchas CoF3 , that are known. However, cobalt(III) can be made stable when in octahedrally coordinated compounds.. Originally, it was questioned whether the chlorine atoms in hexaammine cobalt(III)chloride were coordinated or ionic. It has since been determined that the chlorine atoms are indeed ionic. There are many ways to ascertain this, the most successful of which is to complex the cobaltiodometrically and titrate the liberated iodine with sodium thiosulfate solution. A difficulty in this experiment is the oxidation of cobalt(II) to cobalt(III). This could be accomplished through theaddition of hydrogen peroxide, but this method is not suitable for this experiment. A more suitable method is the air oxidation of cobalt with carbon as a catalyst. An additional benefit of carbon as acatalyst is its ability to shift the equilibrium in favor of the desired product.
Theory As was indicated in the introduction, cobalt(II) is oxidized to cobalt(III) via the following reaction: CoCl2•6H2 O + 5NH3 + NH4 Cl ‡ [Co(NH3 )6 ]Cl3 + 6H2 O + H+ in the presence of air and carbon. The UV-Visible spectrum may then be used to judge the quality of the product. (1)
Experimental A vacuumfiltration apparatus was assembled and connected. 6.0065 g of CoCl 2 •6H2 O was massed and placed into a 150 mL beaker. Then, 4.0164 g of NH 4 Cl was massed and added to the 150 mL beaker. To this beakerwas added 5 mL nanopure water, and the solution was stirred until most of the crystals dissolved. This solution was a dark redpurple color. Next, 13.0 mL of concentrated ammonia was added to the...