P-Anisidine

para-Anisidine (p-anisidine), a grey-brown solid, is the most toxic of the three isomers of anisidine and causes blood damage upon oral ingestion, inhalation or skin contact. If heated strongly, it may release very toxic fumes of nitrogen oxides.
p-Anisidine reacts with secondary oxidation products such as aldehydes and ketones in fats and oils to form products that absorb at 350 nm wavelength of light; therefore, it is used as an official method for detecting them by the American Oil Chemists' Society. It is particularly good at detecting unsaturated aldehydes, which are the ones that are most likely to generate unacceptable flavors, making it particularly useful in food quality testing.