Brown-ring test

By the brown ring test with iron(II) sulphate and cold concentrated sulphuric acid. 

Qualitative Analysis. Nitric acid may be detected by the classical brown-ring test, the copper-turnings test, the reduction of nitrate to ammonia by active metal or alloy, or the nitrogen precipitation test. Nitrous acid or nitrites interfere with most of these tests, but such interference may be eliminated by acidifying with sulfuric acid, adding ammonium sulfate crystals, and evaporating to alow volume. 

Nitric oxide forms a brown ring in cold ferrous sulfate solution (brown ring test for nitrates). Tbe reaction involves the replacement of a water molecule by nitric oxide in bydrated ferrous ion ... 

Nitrous oxide can be analyzed by GC on a molecular sieve column using a thermal conductivity detector and helium as carrier gas. It may be identified by GC/MS from its mass spectra. The molecular ion is 44. Also, it can be oxidized to NO and identified by the brown-ring test, using FeS04 solution, (see Nitric Oxide.)... 

Certain of the salts in solution have a stronger colour than the acid, and in some cases are more stable thus, a deep blue solution of the copper salt may be obtained by the reduction of nitrosulphonic acid (in sulphuric acid) by mercury in the presence of copper. A suggestion has been made7 that the colour in the brown ring test for a nitrate is due to the formation of the ferrous salt of purple acid, but this is improbable.8... 

The brown-ring test for nitrates involves the formation of a dark-brown unstable complex of nitric oxide and hydrated ferrous ion. The composition of the complex19 is probably [Fe(OH2)8NO]"f+. Let us... 

NC, NG, DEGDN, etc)(Ref 27, pp l347-48)(See also Ref 18a and Charts A B given further in this section). A blue ring test, which is similar to brown ring test described under Reagent G may be used... 

A sensitive test for nitric oxide is based upon its coordination with Fe2+ ion, giving a dark brown cation Fe(NO)2 f. This cation is the ion which appears in the well-known brown ring test for nitrate after the latter is reduced with excess Fe2+. Nitric oxide forms a large number of coordination compounds, but its behavior does not fit into the same pattern as do the common coordinating agents. Thus, the chemistry of the complexes of... 

Iron (11) sulphate solution When the nitrite solution is added carefully to a concentrated (25%) solution of iron(II) sulphate acidified with dilute acetic acid or with dilute sulphuric acid, a brown ring, due to the compound [Fe,N0]S04, is formed at the junction of the two liquids. If the addition has not been made cautiously, a brown colouration results. This reaction is similar to the brown ring test for nitrates (see Section IV.18, reaction 3), for which a stronger acid (concentrated sulphuric acid) must be employed. 

Iron(II) sulphate solution and concentrated sulphuric acid (brown ring test) This test is carried out in either of two ways (a) Add 3 ml freshly prepared saturated solution of iron(II) sulphate to 2 ml nitrate solution, and pour 3-5 ml concentrated sulphuric acid slowly down the side of the test-tube so that the acid forms a layer beneath the mixture. A brown ring will form where the... 

Nitrate in the presence of nitrite The nitrite is readily identified in the presence of a nitrate by treatment with dilute mineral acid, potassium iodide, and starch paste (or potassium iodide-starch paper). The nitrate cannot, however, be detected in the presence of a nitrite since the latter gives the brown ring test with iron(II) sulphate solution and dilute sulphuric acid. The nitrite is therefore completely decomposed first by one of the following methods ... 

The nitrate can then be tested for by the brown ring test. 

Nitrate in the presence of bromide and iodide The brown ring test for nitrates cannot be applied in the presence of bromides and iodides since the liberation of the free halogen with concentrated sulphuric acid will obscure the brown ring due to the nitrate. The solution is therefore boiled with sodium hydroxide solution until ammonium salts, if present, are completely decomposed powdered Devarda s alloy or aluminium powder (or wire) is then added and the solution gently warmed. The evolution of ammonia, detected by its smell and its action upon mercury(I) nitrate paper (see Section III.38, reaction 1) and upon red litmus paper, indicates the presence of a nitrate. 

Nitrate in the presence of chlorate The chlorate obscures the brown ring test. The nitrate is reduced to ammonia as described under 3 the chlorate is at the same time reduced to chloride, which may be tested for with silver nitrate solution and dilute nitric acid. 

Nitrite, (i) Brown ring test with dilute acetic acid or with dilute sulphuric acid (IV.7, 2). (ii) Thiourea test (IV.7, 9). (iii) Sulphanilic acid-a-naphthylamine reagent test (IV.7, 11). 

Nitrate, (i) Brown ring test with concentrated H2S04 (IV.18, 3), if bromide, iodide, and nitrite absent, (ii) Ammonia test with Devarda s alloy (IV.18, 4). 

Nitrate in the presence of nitrite Dissolve 10 mg of the solid in 1 ml of water. Remove 1 drop with a glass stirring rod and spot on Kl-starch paper moistened with very dilute H2S04 a blue colour is obtained, due to the iodine liberated by the nitrous acid. Add 20-30 mg sulphamic acid and stir until effervescence ceases (Section IV.7, 6). Test for absence of nitrite by spotting on Kl-starch paper. Then apply the brown ring test (Section VI.6). 

IV.45, 3), dropwise until precipitation is complete. Centrifuge. Add excess concentrated H2S04 cautiously to the centrifugate and apply the brown ring test. 

The brown ring test for nitrate can then be applied. 

All are readily hydrolyzed by water. The ion is isoelectronic with CO and its complexes are discussed in Section 9-10. The compound responsible for the brown ring test for nitrates is [Fe(H20)5N0]2+. 

The chloride dimer [Fe(NO)2Cl]2 also exists, and can be reduced in THF solution to Fe(NO)2 , which is most likely [Fe(NO)2(THF) ]. The brown color of the familiar, so-called, Brown Ring Test for nitrate in which the sample is treated with ferrous sulfate and sulfuric acid is due to the formation of the nitric oxide complex [Fe(H20)5N0] +. 

Nitric acid and the nitrates the manufacture of nitric acid from ammonia the fixation of nitrogen as nitric oxide the brown-ring test for nitrates. 

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