The nitrate/nitrite test strip employs the well-known Griess reaction. Any nitrate present is first reduced to nitrite which then will turn pink due to the Griess reagents present. Both the nitrate and nitrite test pads contain the Griess reagents. Only the nitrate test pad contains the reducing agent. For these reasons, if only nitrate is present in the sample being tested, only the nitrate test pad will turn pink because the reducing agent will reduce the nitrate to nitrite which then reacts to turn pink. The nitrite test pad will remain colorless because there is no nitrite in the sample being tested. If the sample being tested contains nitrite both the nitrate and nitrite test pads will turn pink since the chemicals needed to detect nitrite are present on both test pads.
The nitrate/nitrite test strips are useful in determining if the test solution contains nitrate but not nitrite. They are also useful in testing solutions that contain nitrite only (disregard the nitrate test pad). They are not as useful on solutions that contain both nitrate and nitrite.