9. Nitrate test with Durham tube. This test is used to determine whether the organism can convert nitrate to nitrite, and/or N2 gas. With reagents, nitrite will show up as red. Nitrogen gas will show up as a bubble in the Durham tube. If nitrogen gas was produced by change to nitrate first, nitrate will not appear unless zinc is applied. After application of zinc, reagents can be applied to test for nitrite. Reagents:sulfanilic acid (A) and dimethyl-alpha-napthylamine (B).