syn and anti conformations in 2-hy­droxy-5-[(E)-(4-nitro­phen­yl)diazen­yl]benzoic acid and two related salts

The comparison of the crystal structures of 5-(4-nitro­phenyl­diazen­yl)salicylic acid and two related salts shows that the intra­molecular O—H...O hydrogen bonds in the anions are stronger than that in the neutral mol­ecule. The nitro­phenyl­diazenyl and carb­oxy­lic acid/carboxyl­ate groups in the neutral molecule and the sodium salt are located anti to each other, whereas in the ammonium salt, the anion adopts a syn conformation.