Hydrogen phosphate and dihydrogen phosphate salts of 4-amino­azobenzene

4-Amino-trans-azobenzene {or 4-[(E)-phenyl­diazen­yl]aniline} can form isomeric salts depending on the site of protonation. Both orange bis{4-[(E)-phenyl­diazen­yl]anilinium} hydrogen phos­phate, 2C₁₂H₁₂N₃⁺·HPO₄²⁻, and purple 4-[(E)-phenyl­diazen­yl]­anilinium dihydrogen phosphate phosphoric acid solvate, C₁₂H₁₂N₃⁺·H₂PO₄⁻·H₃PO₄, (II), have layered structures formed through O—H...O and N—H...O hydrogen bonds. Additionally, azobenzene fragments in (I) are assembled through C—H...π inter­actions and in (II) through π–π inter­actions. Arguments for the colour difference are tentatively proposed.