How can ortho-hydrogen change into para-hydrogen?

Ortho-hydrogen and para-hydrogen are two forms of molecular hydrogen that differ based on the spin alignment of their protons. In ortho-hydrogen, the spins are parallel, while in para-hydrogen, they are antiparallel.1

While these forms typically do not interconvert, they can reach equilibrium under specific conditions. Methods to facilitate this transformation include:1

• Catalysts: Introducing substances such as activated charcoal or various paramagnetic materials.1
• Electrical Discharge: Applying an electrical discharge to the gas.1
• Temperature Adjustment: Heating the gas to a high temperature.1

For example, bringing a mixture into contact with charcoal at the temperature of liquid hydrogen can convert ortho-hydrogen into para-hydrogen, resulting in essentially pure para-hydrogen. Conversely, because the concentration of para-hydrogen in an equilibrium mixture never drops below 25 percent, pure ortho-hydrogen cannot be prepared directly from the mixture.1