Nitrate test


Nitrate test

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.

Contents

Brown ring test

A common nitrate test, known as the brown ring test[1] can be performed by adding iron(II) sulfate 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.[2] Note that the presence of nitrite ions will interfere with this test.[3]

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 agent. 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.

3 NO
3
+ 8 Al + 5 OH + 18 H2O → 3 NH3 + 8 [Al(OH)4]

Aluminium is the reductant in this reaction.

Diphenylamine test

Diphenylamine 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,[4] and has found use in gunshot residue kits detecting nitroglycerine and nitrocellulose.[5]

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.[6] Other derivatives have been reported as well.[7]

See also

References

  1. ^ Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN 0-12-352651-5
  2. ^ Mascetta, Joseph A. Barron's How to Prepare for the SAT II: Chemistry, 7th edition. Barron's Educational Series, Inc., 2002. p208.
  3. ^ Holltzclaw, H; Robinson, W. College Chemistry with qualitative analysis, 8th edition, D. C. Heath and Company:Lexington, MA, 1988, p. 1007.
  4. ^ http://nvl.nist.gov/pub/nistpubs/jres/049/3/V49.N03.A05.pdf
  5. ^ Pamela K. Walker and Philip J. Rodacy (2002). Field Test Kit for Gunshot Residue Detection. Sandia National Laboratories. http://prod.sandia.gov/techlib/access-control.cgi/2001/013942.pdf. 
  6. ^ Suter, Hans A.; Suter, Patricia H. (1956). "A selective spot test for nitrate ion". Microchimica Acta 44 (7–8): 1136. doi:10.1007/BF01257445. 
  7. ^ e.g. http://article.pubs.nrc-cnrc.gc.ca/ppv/RPViewDoc?issn=1480-3291&volume=36&issue=4&startPage=652