A common nitrate test, known as the brown ring test[1] can be performed by adding iron(II) sulfate to the solution, then slowly adding concentrated sulfuric acid (NOT nitric acid, since it gives nitrate ion which gives a positive result anyway) and watching for a brown ring on the test tube, which will indicate the presence of the nitrate ion.[2] Note that the presence of nitrite ions will interfere with this test.[3]The test follows several phases; these have been written as balanced chemical equations:1: 2 Zn(NO3)2 + 2 H2SO4 → 2 ZnSO4 + 4 HNO34 FeSO4 + 2 H2SO4 → 2 Fe2(SO4)3 + 2H2These two reactions happen simultaneously.2: 4 HNO3 → 2 H2O + 3O2 + 4 NOThe Nitric Acid decomposes in the intense heat produced by the high concentrate Sulphuric Acid used.3: 2 Fe2(SO4)3 + 2 H2 + 4 NO → 4 [Fe(H2O)5NO]SO4 + 2 H2SO4The [Fe(H2O)5NO]SO4 forms a brown ring in the middle of the solution produced by the reaction, making it easy to identify the presence of nitrates in the water.
Take about 2-3 ml of dilute nitric acid, and add equal volume of concentrated sulphuric acid. Heat is evolved. Cool well under the tap. Then keeping the test tube as slanting as possible, pour an equal volume of freshly prepared ferrous sulphate solution down the side of the test tube.
A brown ring is formed at the junction of the liquids as shown in figure 6.19.
Instead of nitric acid, aqueous solution of a nitrate salt can also be used. It will react with the sulphuric acid to yield nitric acid. In presence of sulphuric acid, nitric acid oxidizes ferrous sulphate to ferric sulphate and itself gets reduced to nitric oxide.
This nitric oxide reacts with the excess ferrous sulphate to form nitrosoferrous sulphate (FeSO4.NO). This is the brown ring. It is a very unstable compound. On gentle warming, it decomposes and the brown colour disappears.
A paradox is an argument that starts with apparently acceptable assumptions and leads by apparently valid deductions to an apparent contradiction. Since logic admits no contradictions, either the apparently acceptable assumptions are not acceptable, or the apparently valid
deductions are not valid, or the apparent contradiction is not a contradiction. A paradox moves us to reexamine the argument until we find out what is wrong.
Is it that ferrous sulphate solution is poured down the test tube containing the mixture of dilute nitric acid and concentrated sulphuric acid ? Because I've learnt that concentrated sulphuric acid is poured down the test tube containing the mixture of dilute nitric acid and ferrous sulphate solution. Please let me know which one is correct.
Moderation Team
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