Sudan IV is a reddish-brown fat soluble chemical dye used in research purposes to stain triglycerides and lipids in frozen paraffin samples. Sudan IV belongs to a group known as lysochromes, along with Sudan III, Oil Red O, Sudan Black B, which are all used for Sudan staining.
Oil Red O is much more popular than Sudan IV for research purposes, as the Oil Red O produces a brighter red stain compared to the more orange color from Sudan IV. However, Sudan IV has many industrial uses, including as a dye for oils, greases, fats, fuels and waxes.
It is used in the United Kingdom as a dye for low-tax heating oil, which is why it is sometimes referred to as Oil Tax Red. Similarly, Sudan III is used in the United States to differentiate between automotive diesel fuel and low-tax heating oil. The U.S. Environmental Protection Agency also uses Sudan III as a dye in fuels than have a higher than average sulfur content.
Sudan IV and the rest of the lysochromes are also categorized by the International Agency for Research on Cancer as Class 3 carcinogens. These dyes have been known to potentially cause serious long-term effects on the body and are prohibited from being used as food dyes.