Flow chart for Identifying cations
The identity of cations present in a solution can be established by qualitative analysis. This depends on observable differences in the acid-base reactions of these cations with water, sodium hydroxide, aqueous ammonia and various reagents that cause precipitation of some of the ions.
Most cations (except Group I and NH4+) produce insoluble precipitates with hydroxide solution.
Many of these precipitates can be identified by their colour - Cu2+ - blue; Fe2+ - green; Fe3+ - orange; Ag+ - brown.
Even the white precipitates can be identified because of their different behaviour when solutions of hydroxide and ammonia are added. These differences result from the formation of soluble complex ions (covered in the Ionic Equations module) but can be summarised by flow diagrams such as shown below.
Most cations (except Group I and NH4+) produce insoluble precipitates with hydroxide solution.
Many of these precipitates can be identified by their colour - Cu2+ - blue; Fe2+ - green; Fe3+ - orange; Ag+ - brown.
Even the white precipitates can be identified because of their different behaviour when solutions of hydroxide and ammonia are added. These differences result from the formation of soluble complex ions (covered in the Ionic Equations module) but can be summarised by flow diagrams such as shown below.
Cation Flow chart
| white ppt Al3+ Zn2+ Pb2+ Mg2+ Ba2+ |  | add excess dilute NaOH | | ppt disappears Al3+ Zn2+ Pb2+ | ||||
 |  |  | ||||||
| coloured precipitate blue (Cu2+) green (Fe2+) orange (Fe3+) brown (Ag+) |  | START: Add 2 drops dilute NaOH to unknown | ppt remains Mg2+ Ba2+ | add 2 drops, then excess dilute NH3(aq) | | white ppt Al3+ Pb2+ | ||
| |  | | |||||
| no ppt Na+ NH4+ | add dilute H2SO4 | add dilute H2SO4 | ||||||
 |  |  | ||||||
| blue litmus turns red | no change to litmus | white ppt | colourless solution | white ppt forms then disappears | white ppt | colourless solution | ||
| NH4+ | Na+ | Ba2+ | Mg2+ | Zn2+ | Pb2+ | Al3+ | ||