Nitrate test
Encyclopedia
A nitrate test is a chemical test
used to determine the presence of nitrate
ion in solution. Testing for the presence of nitrate via wet chemistry is generally difficult compared with testing for other anions, as almost all nitrates are soluble in water. In contrast, many common ions give insoluble salts, e.g. halides precipitate with silver, and sulfates precipitate with barium.
The nitrate anion is an oxidizer, and many tests for the nitrate anion are based on this property. Unfortunately, other oxidants present in the analyte may interfere and give erroneous results.
to a solution of a nitrate, then slowly adding concentrated sulfuric acid
such that the sulfuric acid forms a layer above the aqueous solution. A brown ring will form at the junction of the two layers, indicating the presence of the nitrate ion. Note that the presence of nitrite ions will interfere with this test.
The overall reaction is the reduction of the nitrate ion by iron(II) which is oxidised to iron(III) and formation of a nitrosyl complex.
It should be noted here that Fe is not in its usual +2 oxidation state but is in a +1 Oxidation state and the NO is also in a +1 oxidation state.
. When reacted with nitrate in sodium hydroxide solution, ammonia
is liberated. The ammonia formed may be detected by its characteristic odor, and by moist red litmus — very few gases other than ammonia evolved from wet chemistry are alkaline.
Aluminium is the reductant in this reaction.
may be used as a wet chemical test
for the presence of the nitrate
ion. In this test, a solution of diphenylamine and ammonium chloride
in sulfuric acid
is used. In the presence of nitrates, diphenylamine is oxidized, giving a blue coloration. This reaction has been used to test for organic nitrates as well, and has found use in gunshot residue kits detecting nitroglycerine and nitrocellulose
.
Other oxidants such as chlorate, bromate, etc. interfere by similarly oxidizing diphenylamine. They may be removed by reduction with sodium sulfite
. Where nitrite is present, a false negative result may be observed due to sulfite reducing nitrate in the presence of nitrite.
The diphenylamine test may be selective for nitrate by reducing nitrite with sodium azide
, prior to treatment with sodium sulfite. Other derivatives have been reported as well.
Chemical test
In chemistry, a chemical test is a qualitative or quantitative procedure designed to prove the existence of, or to quantify, a chemical compound or chemical group with the aid of a specific reagent...
used to determine the presence of nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
ion in solution. Testing for the presence of nitrate via wet chemistry is generally difficult compared with testing for other anions, as almost all nitrates are soluble in water. In contrast, many common ions give insoluble salts, e.g. halides precipitate with silver, and sulfates precipitate with barium.
The nitrate anion is an oxidizer, and many tests for the nitrate anion are based on this property. Unfortunately, other oxidants present in the analyte may interfere and give erroneous results.
Brown ring test
A common nitrate test, known as the brown ring test can be performed by adding iron(II) sulfateIron(II) sulfate
Iron sulfate or ferrous sulfate is the chemical compound with the formula FeSO4. Known since ancient times as copperas and as green vitriol, the blue-green heptahydrate is the most common form of this material...
to a solution of a nitrate, then slowly adding concentrated sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
such that the sulfuric acid forms a layer above the aqueous solution. A brown ring will form at the junction of the two layers, indicating the presence of the nitrate ion. Note that the presence of nitrite ions will interfere with this test.
The overall reaction is the reduction of the nitrate ion by iron(II) which is oxidised to iron(III) and formation of a nitrosyl complex.
- NO3- + 3Fe2+ + 4H+ → 3Fe3+ + NO + 2H2O
- [Fe(H2O)6]2+ + NO → [Fe(H2O)5(NO)]2+
It should be noted here that Fe is not in its usual +2 oxidation state but is in a +1 Oxidation state and the NO is also in a +1 oxidation state.
Devarda's test
Devarda's alloy (Cu/Al/Zn) is a reducing agentReducing agent
A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...
. When reacted with nitrate in sodium hydroxide solution, ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
is liberated. The ammonia formed may be detected by its characteristic odor, and by moist red litmus — very few gases other than ammonia evolved from wet chemistry are alkaline.
- 3 + 8 Al + 5 + 18 → 3 + 8
Aluminium is the reductant in this reaction.
Diphenylamine test
DiphenylamineDiphenylamine
Diphenylamine is the organic compound with the formula 2NH. It is a colourless solid, but samples are often yellow due to oxidized impurities.-Preparation and reactivity:...
may be used as a wet chemical test
Chemical test
In chemistry, a chemical test is a qualitative or quantitative procedure designed to prove the existence of, or to quantify, a chemical compound or chemical group with the aid of a specific reagent...
for the presence of the nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
ion. In this test, a solution of diphenylamine and ammonium chloride
Ammonium chloride
Ammonium chloride NH4Cl is an inorganic compound with the formula NH4Cl. It is a white crystalline salt that is highly soluble in water. Solutions of ammonium chloride are mildly acidic. Sal ammoniac is a name of natural, mineralogical form of ammonium chloride...
in sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
is used. In the presence of nitrates, diphenylamine is oxidized, giving a blue coloration. This reaction has been used to test for organic nitrates as well, and has found use in gunshot residue kits detecting nitroglycerine and nitrocellulose
Nitrocellulose
Nitrocellulose is a highly flammable compound formed by nitrating cellulose through exposure to nitric acid or another powerful nitrating agent. When used as a propellant or low-order explosive, it is also known as guncotton...
.
Other oxidants such as chlorate, bromate, etc. interfere by similarly oxidizing diphenylamine. They may be removed by reduction with sodium sulfite
Sodium sulfite
Sodium sulfite is a soluble sodium salt of sulfurous acid. It is a product of sulfur dioxide scrubbing, a part of the flue gas desulfurization process...
. Where nitrite is present, a false negative result may be observed due to sulfite reducing nitrate in the presence of nitrite.
The diphenylamine test may be selective for nitrate by reducing nitrite with sodium azide
Sodium azide
Sodium azide is the inorganic compound with the formula NaN3. This colourless azide salt is the gas-forming component in many car airbag systems. It is used for the preparation of other azide compounds. It is an ionic substance and is highly soluble in water. It is extremely...
, prior to treatment with sodium sulfite. Other derivatives have been reported as well.