N 1650 (OTTO) Nitrazine yellow, indicator Cas 5423-07-4 - used to test vaginal pH during late pregnancy to determine the breakage of the amniotic sac. To perform a fecal pH test for diagnosing intestinal infections or other digestive problems. In civil engineering, to determine the carbonatation spread in concrete structures and therefore assess the state of the rebar's passivation film.