This is also known as the P test. It uses a 1–5% ethanolic solution of para-phenylenediamine (PD), made by placing a drop of ethanol (70–95%) over a few crystals of the chemical; this yields an unstable, light sensitive solution that lasts for about a day. An alternative form of this solution, called Steiners solution, is much longer lasting although it produces less intense colour reactions. It is typically prepared by dissolving 1 gram of PD, 10 grams of sodium sulfite, and 0.5 millilitres of detergent in 100 millilitres of water; initially pink in colour, the solution becomes purple with age. Steiners solution will last for months. The phenylenediamine reacts with aldehydes to yield Schiff bases according to the following reaction:
Products of this reaction are yellow to red in colour. Most β-orcinol depsidones and some β-orcinol depsides will react positively. The PD test, known for its high specificity towards substances that yield K+ yellow or red reactions, has largely replaced the simpler yet less conclusive K test. PD is poisonous both as a powder and a solution, and surfaces that come in contact with it (including skin) will discolour.
Some common and widely distributed lichens that have lichen products with a positive reaction to P include Parmelia subrudecta, which is PD+ (yellow) because of the didepside atranorin, and Hypogymnia physodes, which is PD+ (orange) because of the depsidone physodalic acid.