English to Italian Tech/Engineering Engineering (general)

ISO/TR 15916
Basic considerations for the safety of hydrogen systems

Flammability limits
Hydrogen, by itself in bulk, does not undergo a chemical reaction when exposed to an ignition source.
For combustion to occur, hydrogen has to be mixed with sufficient quantities of an oxidizer to form a flammable mixture.
The greatest part of the discussion in this paper here is about combustion of premixed gas clouds.

In una precedente traduzione di questo testo, "hydrogen, by itself in bulk" è stato tradotto con "l'idrogeno, per sé stesso molecolare"", ho visto però che si trova l'espressione "idrogeno in bulk".... Ma cosa si intende e qual è il corrispondente termine in italiano?