Ferric chloride reagent consists of 10% ferric chloride dissolved in water. It is used in the phenylalanine deaminase test to determine the presence of phenylpyruvic acid, the product of phenylalanine deaminase.
After inoculating phenylalanine agar and incubating overnight, the culture is acidified by the addition of 0.1N HCl (necessary for the color change to take place). Then, several drops of ferric chloride are added. Development of a green color indicates the presence of phenylpyruvic acid.