The Polymorphism of C.I. Pigment Yellow 5

Department of Physics Brunei University Uxbridge Middlesex UB8 3PH By interpretation of X-ray diffraction patterns it has been found that C. I. Pigment Yellow 5 (α-[1-hydroxy-ethylideneacetanilide-α-azo-2′;-nitrobenzene]) is dimorphic, both forms being available commercially. The single-crystal data and X-ray powder pattern are reported for one form (termed α) and the powder pattern has been indexed from the single-crystal unit cell dimensions. The second form (β) has only been obtained as a powder, and the powder pattern is reported. This form changes to the α-form by recrystallisation from toluene.