Chemistry - Salt Analysis Question with Solution | TestHub

Solution:

Both $As{O}_4^{3-}$ and $P{O}_4^{3-}$ react with ammonium molybdate to give a yellow precipitate. To specifically detect $P{O}_4^{3-}$, $As{O}_4^{3-}$ must first be removed. This is achieved by passing $H_{2}S$ gas through the solution in an acidic medium (hot HCl), which precipitates $As{O}_4^{3-}$ as $A{s}_2{S}_5$. The filtrate, now free of arsenate, can then be tested for $P{O}_4^{3-}$ using the ammonium molybdate test.