QUALITATIVE ANALYSIS A small portion of a solute sample is placed in a test tube

Question

QUALITATIVE ANALYSIS

A small portion of a solute sample is placed in a test tube and a few drops of dilute hydrochloric acid are added. There is immediate effervescence due to the escape of a gas. A drop of barnium hydroxide held on a glass rod at the mouth of the tube where the gas is escaping becomes turbid due to the formation of a white precipitate. What acid (anion) is present in the sample? You suspect an unknown compound to be a nitrate. You have made an identification test using concentrated sulphuric acid and ferrous sulphate The typical brown ring was formed Would you consider this test as positive proof that the unknown compound is a nitrate? Explain your answer

Explanation / Answer

1. Carbonate anion (CO3)2- is present in the sample. This can be deduced from the fact that a carbonate ion on reacting with HCl produces carbon dioxide gas (CO2) which can be seen as an effervescence. When Barium hydroxide is placed near this escaping gas, a white precipitate in the form of Barium carbonate (BaCO3) is seen.

2. In the brown ring test, we use the oxidising property of the nitrate ions (NO3-) to oxidise Fe from Fe2+ (FeSO4) to Fe3+ (Fe2(SO4)3), while the nitrate ions get reduced to NO- ions. This NO- ions then react with the [Fe(H2O)6]3+ complex to give the brown ring.

Hence, it can be seen that the presence of NO- radical can also interfere in this test and give the presence of nitrate ions. This test alone cannot be taken as a positive proof for Nitrate ions. We need another test such as the Copper turnings test in conjunction with this test.

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