Qualitative analysis of Group I ions is a classic analytical scheme used to separate and identify the ions in a known mixture of Group I cations. The ions are separated by selective precipitation, which involves the addition of a carefully selected reagent to an aqueous mixture of ions, resulting in the precipitation of one or more of the ions, while leaving the rest in solution. Once each ion is isolated, its identity can be confirmed by using a chemical reaction specific to that ion .
Cations are typically divided into groups, where each group shares a common reagent that can be used for selective precipitation. The classic qualitative analysis scheme used to separate various groups of cations is shown in the flow chart below. Note that Ag+, Pb2+, and Hg2+2+ are called the Group I cations since they are the first group separated from the larger mixture. Since these ions all form insoluble chlorides, their separation from all other ions may be accomplished by the addition of 6 M HCl (aq) resulting in the precipitation of AgCl (s), PbCl2 (s), and Hg2Cl2 (s) .
The sample must then be centrifuged (spun rapidly), which separates the solid precipitates from the ions still in solution. The precipitates are then washed with water to remove any remaining soluble ions, and then dissolved in a small amount of dilute nitric acid (HNO3) .
The next step is to test for the presence of Ag+, Pb2+, and Hg2+2+ ions. This is done by adding a small amount of 6 M NH3 (aq) to the solution. The formation of a white precipitate indicates the presence of Ag+ ions, while the formation of a yellow precipitate indicates the presence of Pb2+ ions. The formation of a reddish-brown precipitate indicates the presence of Hg2+2+ ions .
The next step is to test for the presence of Cu2+ ions. This is done by adding a small amount of 6 M NH3 (aq) to the solution. The formation of a