Griess test

The Griess test is an analytical chemistry test which detects the presence of nitrite ion in solution. One of its most important uses is the determination of nitrite in drinking water. The Griess diazotization reaction on which the Griess reagent relies was first described in 1858 by Peter Griess. The test is also been widely used for the detection of nitrates, which are converted to nitrites prior to applying the Griess test. Nitrates are common components of explosives. [1]

Method

Nitrite is detected and analyzed by formation of a red pink colour upon treatment of a NO2-containing sample with the Griess reagent.

When Sulfanilamide is added (or sulphanilic acid), the nitrite ion reacts to form a diazonium salt. When the azo dye agent (N-alpha-naphthyl-ethylenediamine) is formed a pink color develops. This diamine is used in place of the simpler and cheaper alpha-naphthylamine because this latter is a potent carcinogen and moreover the diamine forms a more polar and hence a much more soluble dye in acidic aqueous medium.[2]

A typical commercial Griess reagent contains 0.2% naphthylethylenediamine dihydrochloride, and 2% sulphanilamide in 5% phosphoric acid.

Forensics

The test was used in forensics for many years to test for the traces of nitroglycerine.

Caustic soda is used to break down sample containing nitroglycerine to produce nitrite ions.

The test involves the taking of a sample with ether and its division into two bowls. Sodium hydroxide is added to the first bowl followed by the Griess reagent; if the solution turns pink within ten seconds, this indicates the presence of nitrites. The test itself is positive if, after adding only Griess reagent to the second bowl, the solution there remains clear

The convictions of Judith Ward and the Birmingham Six were assisted by Frank Skuse's flawed interpretation of Griess test results.

References

  1. Moorcroft, M.; Davis, J.; Compton, R. G. (2001). "Detection and determination of nitrate and nitrite: A review". Talanta. 54: 785–803. doi:10.1016/S0039-9140(01)00323-X. PMID 18968301.
  2. Mick Hamer (1991-11-09). "Forensic science goes on trial: Even senior judges can be blinded by science". New Scientist. Retrieved 2007-08-07.
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