Final answer:
The reagent used in the Malloy and Evelyn method to form a purple azobilirubin with bilirubin is diazotized sulfanilic acid. This reagent is involved in the direct Van Den Bergh test which is crucial for the diagnosis of obstructive jaundice.
Step-by-step explanation:
In the Malloy and Evelyn method for the determination of bilirubin, the reagent that is reacted with bilirubin to form a purple azobilirubin is diazotized sulfanilic acid. The direct Van Den Bergh test indicates that conjugated bilirubin reacts with a diazo reagent (a combination of sodium nitrite and sulfanilic acid in the presence of HCl as a catalyst) to give a purple-red color almost immediately, without the need for alcohol. This form of bilirubin is known as direct bilirubin. Whereas, in the case of unconjugated bilirubin, which is known as indirect bilirubin, the indirect Van Den Bergh test requires the addition of alcohol to produce the purple-red coloration. This difference in reaction is useful for diagnosing different types of jaundice.