19. Copper

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

$Cu^{2+}$ $+$ $S^{2-}$ $\longrightarrow$ $CuS$ - Black precipitate - Soluble in $HNO_3$ - Insoluble in $(NH_4)_2S$

2. $KOH$

$Cu^{2+}$ $+$ $2OH^-$ $\longrightarrow$ $Cu(OH)_2$ - Blue, gelatinous precipitate - Soluble in dilute mineral acids - Insoluble in excess reagent

3. $NH_3$

$Cu^{2+}$ $+$ $2OH^-$ $\longrightarrow$ $Cu(OH)_2$ - Blue precipitate - Soluble in an excess of reagent: $Cu(OH)_2$ $+$ $4NH_3$ $\longrightarrow$ $Cu(NH_3)_4^{2+}$ $+$ $2OH^-$ - Tetramminecopper(II) - Soluble - Sky Blue - "Schweitzer liquor " - masks 2.,6.,7., not 1.

4. $KCN$

$Cu^{2+}....$ $\longrightarrow$ $CuCN....$ (spontaneous reduction) - Yellow precipitate - Soluble in an excess of reagent: $CuCN$ $+$ $3CN^-$ $\longrightarrow$ $Cu(CN)_4^{3-}$ - Tetracyanocuprate(I) - Soluble - yellowish - masks everything - the reduction is accompanied by a release of the toxic cyanogen gas !

5. Potassium sodium tartrate

- formation of a blue chelate:

- masks 2., not 1. and 6.

6. $K_4[Fe(CN)_6]$

$2Cu^{2+}$ $+$ $[Fe(CN)_6]^{4-}$ $\longrightarrow$ $Cu_2[Fe(CN)_6]$ - Brown precipitate - Insoluble in dilute mineral acids - Insoluble in $NH_3$

7. $SCN^-$

$Cu^{2+}$ $+$ $2SCN^-$ $\longrightarrow$ $Cu(SCN)_2$ - Black precipitate - $SO_2$ reduces this precipitate to purple $Cu(SCN)$