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 test has also been widely used for the detection of trace explosives containing nitro groups. It is usually necessary to release the nitrite ion from the organic explosive before analysis. This is normally done by alkaline hydrolysis. The Griess diazotization reaction on which the Griess reagent relies was first described in 1858 by Peter Griess.

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 sulphanilic acid is added (in the picture its sulphonamide is shown instead), the nitrites form a diazonium salt. When the azo dye agent (N-alpha-naphthyl-ethylenediamine) is added a pink colour 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.^[1]

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 the molecule of nitroglycerine to produce nitrite ions. The concentration of this caustic soda is crucial to the test.

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

Due to the ability of many substances to produce nitrite ions, the test is not conclusive and eventually proved of limited value. British police forces had practically stopped using it by the mid-1980s.

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

Precautions

The Griess reagent should only be used for simple chemical tests or reaction and not be used as a drug. One should read all the information about the reagent before taking any type of important step

Storage

Griess reagent is stored in powdered form at room temperature. The reagent should not be stored more than three months

References

1. Mick Hamer (1991-11-09). "Forensic science goes on trial: Even senior judges can be blinded by science". NewScientist. Retrieved 2007-08-07.