0.1N HCl is used to acidify the culture grown on phenylalanine agar before ferric chloride reagent (10% ferric chloride dissolved in water) is added for completion of the phenylalanine deaminase test. The test determines whether the microbe has produced phenylpyruvic acid from phenylalanine.
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.