SULPHANILIC ACID 0.8%

Sulphanilic Acid 0.8% is an analytical reagent employed along with α-Naphthylamine Solution for the detection of nitrate reduction by bacterial strains, particularly members of Enterobacteriaceae. The test is used as an aid in identifying bacteria by their nitrate reduction.
During the reaction, microorganisms producing nitrate reductase reduce nitrate (NO₃) to nitrite (NO₂). The nitrite and sulphanilic acid react to form a diazonium salt. This intermediate is subsequently coupled with α-Naphthylamine to yield a water soluble azo dye. Development of a distinct pink-red to red colour in 1-2 min signals nitrate reduction to nitrite.
The test should be conducted with appropriate low-nitrite media, like Nitrate Agar, Nitrate Broth, to avoid the influence of nitrite that is present already in the media. Confirm a negative nitrite reaction by addition of zinc dust to check if nitrate has been reduced to a stage further than nitrite.
Excess zinc should be avoided as it can reduce unreduced nitrate in its entirety and give false results. The nitrate reduction test should be used as an aid in identification and confirmed with other biochemical tests using an isolate from a pure culture for full identification.

Works with α-Naphthylamine Solution to detect nitrate reduction through formation of a characteristic pink-red colour.