Separation and identification of Group IV cations

Some textbooks recommend the removal of ammonium salts simply by sublimation. This can be done by first evaporating the acidified filtrate on a water bath, and then heating the dry residue at a higher temperature, until the white fumes of ammonium salts cease to appear. It is much easier however to remove ammonium salts with concentrated nitric acid, as described in Table V.12, when a chemical decomposition takes place  

This reaction proceeds at a much lower temperature than is needed for the volatilization of ammonium salts. 

A small amount of ammonium ions must however be present to prevent the precipitation of magnesium again. Thus a small amount of ammonium chloride is added after the removal of ammonium salts. 

These operations are all incorporated into Table V.12, describing the general separation scheme of cations into groups. 

It may be noted that the precipitation with ammonium carbonate takes place in hot solution, due to the fact that the carbonates are thus obtained in a more granular form and are therefore more easily filtered and washed. The solution must not be boiled as this decomposes the reagent and may result in some redissolution of the carbonates  

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In the laboratory you will most likely carry out one or more experiments involving the separation and identification of cations present in an "unknown" solution. A scheme of analysis for 21 different cations is shown in Table A. As you can see, the general approach is to take out each group (I, II, III, IV) in succession, using selective precipitation. 

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