5. Barium

1. Ammonium carbonate

$Ba^{2+}$ $+$ $CO_3^{2-}$ $\longrightarrow$ $BaCO_3$ - White precipitate - soluble in $HCl$ - Soluble in acetic acid

2. Potassium bichromate

$2Ba^{2+}$ $+$ $Cr_2O_7^{2-}$ $+$ $H_2O$ $\longrightarrow$ $2BaCrO_4$ $+$ $2H^+$ - lemon precipitate - soluble in HCl $ $ - insoluble acetic acid - To make the complete precipitation must be buffered by barium acetate - At pH = 5, $Ba^{2+} $ precipitates quantitatively, $Sr^{2+}$ barely - $K_2CrO_4$ precipitates $Ba^{+2}$, but also $Sr^{2+}$

3. Diluted sulfuric acid

$Ba^{2+}$ $+$ $SO_4^{2-}$ $\longrightarrow$ $BaSO_4$ - White precipitate - insoluble in acids

4. Calcium sulfate

$Ba^{2+}$ $+$ $SO_4^{2-}$ $\longrightarrow$ $BaSO_4$ - White precipitate - $CaSO_4$ is poorly soluble in water (0.2 g in 100 mL). Since the solubility of barium sulfate is very small, the precipitation takes place anyway.