Tetrazolium Red(2,3,5-Triphenyltetrazolium chloride; TPTZ) is used to visualize dehydrogenase enzyme activity; initially the tetrazolium solution is colorless but changes to red when it comes into contact with hydrogen. Tetrazolium red is used in a biochemical viability test for seeds. The test relies on dehydrogenase enzymes to release hydrogen ions which subsequently reduce the colorless tetrazolium salt solution to a red compound called formazan. Living cells turn red while dead cells remain colorless.