Redetermination of 2-[4-(2-hydroxy­ethyl)piperazin-1-ium-1-yl]ethanesul­fonate at 100 K

The crystal structure of the title compound (common name HEPES), C(8)H(18)N(2)O(4)S, has been redetermined at 100 K in order to properly elucidate the protonation state of the HEPES molecule. The piperazine ring has a chair conformation and one of the N atoms in the ring is protonated, which was not previously reported [Gao, Yin, Yang, & Xue (2004). Acta Cryst. E60, o1328–o1329]. The change of protonation state of the nitrogen atom significantly affects the intermolecular interactions in the HEPES crystal. The structure is stabilized by N—H⋯O and O—H⋯O hydrogen bonds and ionic inter­actions, as the title compound in solid state is a zwitterion. HEPES mol­ecules pack in layers that are held together by ionic and weak inter­actions, while a hydrogen-bonded network connects the layers.