13. Aluminium

1. $H_2S$

no precipitation in acid medium

2. $(NH_4)_2S$

$2Al^{3+}$ $+$ $3S^{2-}$ $\longrightarrow$ $Al_2S_3$ $Al_2S_3$ $+$ $6H_2O$ $\longrightarrow$ $2Al(OH)_3$ $+$ $3H_2S$ - White gelatinous precipitate - Soluble in dilute mineral acids

3. $KOH$

$Al^{3+}$ $+$ $3OH^-$ $\longrightarrow$ $Al(OH)_3$ - White gelatinous precipitate - Soluble in dilute mineral acids - With an excess of reagent a complex ion is formed: $Al(OH)_3$ $+$ $OH^-$ $\longrightarrow$ $[Al(OH)_4]^-$ - tétrahydroxoaluminate - Soluble, colorless

4. $NH_3$

$Al^{3+}$ $+$ $3OH^-$ $\longrightarrow$ $Al(OH)_3$ - White gelatinous precipitate - Soluble in dilute mineral acids - Insoluble in excess reagent

5. Eriochromcyanine

It must be operated at pH 5 (acetic buffer). There is formation of a violet chelate.

6. Morin

It must operate at pH = 5 (acetic buffer) or in a neutral medium. With an alcoholic solution of Morin, there is formation of a fluorescent complex.