Separation and identification of cations in solution

8 SEPARATION AND IDENTIFICATION OF CATIONS IN SOLUTION The preliminary tests and tests for anions may have indicated the presence of certain elements (e.g. As, Cr, Mn) which are normally identified as cations. This information should always be kept in mind when the separation and identification of cations is to be attempted. 

Before describing the general scheme for the separation of the cations into groups, the student should take note of the following facts  

The analysis should not be conducted with large quantities of the substance because much time will be spent in filtering the precipitates and difficulty may be experienced in washing and dissolving them. It is therefore recommended that about 5-20 ml be employed for the analysis. After a little experience the student will be able to judge from the relative sizes of the precipitates the relative quantities of the various components present in the mixture. 

The ppt. may contain PbCl2 -white. Hg2Cl2 -white. AgCl -white. Group I present. 

This must give no further precipitate with a few drops of dilute HC1. Add 1 ml of 3 per cent H202 solution (3). Adjust the HC1 concentration to 0-3m (4). Heat to boiling and pass H2S through the solution until precipitation is complete (S). Filter and wash (6). 

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