Question

a solution that may contain any of the group three cations. Treatment of the solution with clo- in alkaline medium yields a yellow solution and a colored precipitate. The Acidified solution is unaffected by treatment with nh4oh. The colored precipitate dissolves in nitric acid addition; addition of excess nh4oh to this acidic solution produces only a blue solution.
Which group 3 ions are present?
which are absent?
which are in doubt?
how would you remove all doubt?

Answer 1

Group 3 captions are Cr3+, Al3+, Fe3+ and Ni2+. .

As per information provided above, cr3+and Ni2+are present. These can be further confirmed by confirmatory tests. Let us go through step by step analysis:

Part A. Treatment with alkaline and chlorite divides above cations into two groups. One group which which either react with hydroxide or chlorate ions to form soluble complexes or to form insoluble complexes

• Cr3+ oxidizes with chlorate ions to form Cr(6)-this is soluble.
• Al3+ reacts with only hydroxide to form soluble Al(OH)4-.
• Fe3+ and Ni2+ do not react with oxidising agent chlorate and reacts with hydroxide to form insoluble hydroxide .

So till now we are in doubt that yellow solution is either due to Cr3+ o Al. And coloured ppt is due to Nickel hydroxide- greencolor or iron hydroxide complex-red color.

• Further rection witn acids wil only affect Al, asit breaks aluminium hydroxide complex. On other hand cr3+solution remain unchanged to acid affect.... it inicates presence of Cr3+ in solution.
• For Nickel/iron -Further addition of acid to colored ppt of first step wil dissolve these ions . In the next step addition of ammonia solution forms nickel complex--blue solution .. this indicates presence of Ni2+