Preparation and Analysis of a Cobalt Complex

PURPOSE OP EXPERIMENT Prepare an unknown cobalt complex with ammine and/or chloro ligands, and analyze the complex for anionic chloride and for charge to determine its identity. 

In this experiment you will start with an aqueous solution of cobalt(II) chloride, C0CI2. Ammonium chloride, NH4CI, will be added to provide a source of additional Cl , and aqueous ammonia, NH3, will be added to provide potential NH3 ligands. Finallly, hydrogen peroxide, H2O2, will be added to oxidize Co(II) to Co(III). After the reaction is complete, you will filter crystals of your cobalt-ammine complex. It is not uncommon in chemical syntheses that several products can be formed from the seuae set of reactants. In this experiment you should consider [Co(NH3)6]Cl3, [Co(NH3)5Cl]Cl2, and [Co(NH3)4Cl2]Cl as possible products (See PRELABORATORY QUESTION 1). Formation of one of the products can often be favored by subtle changes in the mole ratios of the reactants, the particular catalyst used, or conditions such as temperature. After purification of the primary product, its identity can be determined by analyses or by instrumental techniques. In this experiment, you will use two independent analyses for this purpose. 

The first analysis is for the chloride ion which serves to balance the charge of the complex ion. Many complex ions are stable enough that ligand chlorides attached directly to the central metal ion (inside the bracket in the formula) are not precipitated as silver chloride, AgCl, upon the addition of silver nitrate, AgN03, solution, whereas anionic chlorides (outside the bracket in the formula) are precipitated quantitatively by this technique. For example, only one Cl precipitates from the following complex. 

The AgCl can be filtered, dried, and weighed. From the mass of AgCl you can calculate the moles of AgCl and Cl and the mass of Cl. This also represents the mass of Cl present as an anion in the complex since it all came from that source. Thus, using the original mass of your sample, you can calcu- 

Note that the number of moles of H displaced from the resin is equivalent to the number of moles of positive charge on the complex cation being exchanged. Stated another way, the ratio of the moles of H+ liberated per mole of complex cation exchanged is a direct measure of the charge on the cation. 

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