Nitrate B reagent consists of sulfanilic acid dissolved in acetic acid. It is used with Nitrate A reagent (dimethyl-naphthylamine or alpha-naphthylamine dissolved in acetic acid) to detect the presence of nitrite produced by the reduction of nitrate in nitrate broth.
After inoculating nitrate broth and incubating overnight, five drops of each reagent are added to the culture. Development of a red color indicates the nitrate was reduced to nitrite.