15. Mercury(II)

1. $H_2S$ ou $(NH_4)_2S$

$Hg^{2+}$ $+$ $S^{2-}$ $\longrightarrow$ $HgS$ - Black precipitate - Insoluble in dilute nitric acid - Soluble in $HNO_3$ - $HCl$

2. $KOH$

$Hg^{2+}$ $+$ $2OH^-$ $\longrightarrow$ $H_2O$ $+$ $HgO$ - Yellow precipitate

3. $NH_3$

$Hg^{2+}$ $+$ $NH_3+OH^-$ $\longrightarrow$ $H_2O$ $+$ $HgCl(NH_2)$ - "amido-chloride" - White precipitate

4. $SnCl_2$

$2Hg^{2+}$ $+$ $2Cl^-$ $+Sn^{2+}$ $\longrightarrow$ $Hg_2Cl_2$ $+$ $Sn^{4+}$ $Hg_2Cl_2$ $+$ $Sn^{2+}$ $\longrightarrow$ $2Hg^o$ $+$ $2Cl^-$ $+$ $Sn^{4+}$ - Black precipitate

5. $KI$

$Hg^{2+}$ $+$ $2I^-$ $\longrightarrow$ $HgI_2$ - Red precipitate - Soluble in an excess of reagent: $HgI_2$ $+$ $2I^-$ $\longrightarrow$ $HgI_4^{2-}$ - Tetraiodomercurate (II) - Colorless - Masks 4

6. $KCN$

$Hg^{2+}$ $+$ $4CN^-$ $\longrightarrow$ $Hg(CN)_4^{2-}$ - Tetracyanomercurate (II) - Soluble, colorless - Masks 2. and 5. but not 1. and 4.