Question

Why we should use specific PH=9 in identification of presence of Al cations in NaCl solution

Answer 1

Answer on Question#37435 - Chemistry - Inorganic Chemistry

The Al cations identification is based on formation of white gel-like precipitate of $\mathrm{Al(OH)_3}$:

$\mathrm{Al}^{3+}(\mathrm{aq}) + 3\mathrm{OH}^-(\mathrm{aq}) \rightarrow \mathrm{Al(OH)}_3(\mathrm{s})$

Aluminium hydroxide is formed as a result of interaction with hydroxyl ions (e.g. sodium hydroxide solution). That is why pH should be basic (pH > 7).

However, addition of more sodium hydroxide solution results in aluminium precipitate dissolution, due to the following reaction

$\mathrm{Al(OH)_3(s)} + \mathrm{OH}^-(\mathrm{aq}) \rightarrow \mathrm{Al(OH)_4}^-(\mathrm{aq})$

That is why in case of too basic pH (pH > 9) the precipitate is not formed:

$\mathrm{Al}^{3+}(\mathrm{aq}) + 4\mathrm{OH}^-(\mathrm{aq}) \rightarrow \mathrm{Al(OH)_4}^-(\mathrm{aq})$

and the Al cations, thus, cannot be identified.

That is why only specific pH = 9 should be used.