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At 120 K, crystals of pure laboratory-grown uric acid dihydrate [7,9-dihydro-1H-purine-2,6,8(3H)-trione dihydrate], C5H4N4O3.2H2O, are monoclinic, P21/c, with a = 7.237 (3), b = 6.363 (4), c = 17.449 (11) Å, β = 90.51 (1)°, Z = 4, wR2 (all data) = 0.1094, R1 = 0.0406 for data with I > 2σ(I). The crystals exhibit pseudo-orthorhombic twinning with refined twin fractions of 0.89 and 0.11. Within each twin, disorder about a noncrystallographic twofold exists with refined occupancies of 0.83 and 0.17. Packing consists of layers of hydrogen-bonded uric acid separated by layers of hydrogen-bonded water. The epitaxy of uric acid dihydrate with its anhydrous counterpart is readily explicable from the refined model. Treatment of the structure as twinned and disordered in space group P21/c fully accounts for well documented violations of systematic absences in orthorhombic space groups.
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