An unknown solution can contain only Al(H_2O)_6^3+, Mg(H_2O)_6^2+, Ca^2+, and NO

Question

An unknown solution can contain only Al(H_2O)_6^3+, Mg(H_2O)_6^2+, Ca^2+, and NO_3^- ions Suggest two reagents (Include concentration) where each of them, if used carefully, could precipitate all of the metallic ions? Considering each of your suggested reagents one at a time, would it be useful by itself in detecting and/or distinguishing between Ca^2+ and Al(H_2 O)_6^3+ if you knew that only one of these ions was present in the solution? How? (Make sure that you discuss each reagent that you have suggested in enough detail.)

Explanation / Answer

6. Unknown solution with Al3+, Mg2+, Ca2+ and NO3- ions

a) Adding 5 M NH4OH (aq) would precipitate Al(OH)3 and Mg(OH)2. Ca(OH)2 is sligthly soluble.

Al(OH)3 being the least soluble of all three cations. This precipitation is achieved in basic medium.

excess NaOH would dissolve Al(OH)3 by forming Al(OH)4- ion.

K2Cr2O7 addition forms precipitate of Mg2+ and Ca2+. With precipitate of Ca2+ least soluble of the two ions.

b) Separation and identification of Al3+ and Ca2+ ions

1) addition of NH4OH precipitates the two Al3+ and Mg2+ as hydroxides. Addiiton of more base would dissolve Al(OH)3 precipitate whereas, Mg(OH)2 remains insoluble in solution.

2) Mg2+ forms precipitate with dichromate ion added to the solution.

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