Which chemical is commonly used to detect cleaned blood evidence?

Luminol is a chemical compound that is widely recognized for its ability to detect trace amounts of blood, even after it has been cleaned or removed from a scene. When luminol is sprayed in a dimly lit area, it reacts with the iron present in hemoglobin, which is found in blood. This reaction produces a chemiluminescent glow, allowing forensic investigators to visualize areas where blood may have been present.

The other chemicals listed do not serve the same broad purpose as luminol. Phenolphthalein is primarily used in colorimetric tests to indicate the presence of blood through a color change when it reacts with hemoglobin, but it requires a more significant amount of blood and does not illuminate it in the dark. Hydrogen peroxide has its uses in forensic investigations, but it acts as a bleaching agent and is not specifically used for detecting cleaned blood. Benzidine was formerly used as a reagent for detecting blood, but its use has declined due to its carcinogenic properties and is not commonly utilized in modern forensic procedures. Thus, luminol remains the primary choice for detecting previously cleaned blood due to its sensitivity and the visual indication it provides.