Observation of 'Rhumann's purple' is a confirmatory test for the presence of: starch, reducing sugar, protein, cupric ion

Ninhydrin (2,2-dihydroxyindane-1,3-dione) is a chemical used to detect ammonia or primary and secondary amines. When reacting with these free amines (proteins), a deep blue or purple color known as Ruhemann's purple is produced.