The structure of the commercial polymorph (form II) of 5-amino-2,3dihydro-1,4-phthalazinedione (luminol) used for ages by crime scene investigators to secure blood evidence was determined by powder X-ray diffraction and Rietveld refinement. The "new" form II is a polytype 1 of form I. 2 The two polymorphs only differ in distinct π-stacking motifs. Slurry experiments prove that the commercial material is indeed the metastable polymorph.

Luminol is chemically sufficiently stable to be diprotonated at high proton concentrations as provided by concentrated sulfuric acid. The luminol dication (5‐ammonium‐2‐hydro‐1,4‐phthalzinediol) sulfate was isolated as macroscopic single crystals and its structure was determined and refined from single‐crystal X‐ray data collected at 173 K [cell parameters: a = 8.3994(17) Å, b = 6.9985(14) Å, c = 17.486(4) Å, β = 90.85(3)°, V = 1027.8(4) Å3, space group P21/c]. The structure is comprised of layers stacked along the b axis. Intralayer interactions are accomplished by strong hydrogen bonds of three luminol dications to one central [SO4]2– ion. Interlayer interactions are formed by weak hydrogen bonds of one luminol dication to two [SO4]2– ions in the adjacent layers, respectively, and alternating sandwich and parallel‐displaced π‐π‐stacking of the 1‐hydropyridazine‐3,6‐diol moieties of luminol dications in adjacent layers, respectively.

Abstract.A so far structurally unknown polymorph (form II) of dihydrogenhydrazinium (hydrazine) sulfate(IV) was isolated as macroscopic single crystals and its structure was determined and refined as twin from single-crystal X-ray data collected at 173 K [cell parameters: a = 5.5592 (11)