A simple nitrite test can be performed by adding 4 M sulfuric acid to the sample until acidic, and then adding 0.1 M iron(II) sulfate to the solution.  A positive test for nitrite is indicated by a dark brown solution, arising from the iron-nitric oxide complex ion. This test is related to the brown ring test for the nitrate ion, which forms the same complex in a ring. In contrast, nitrites turn the whole solution brown and therefore interfere with that test.