Why can heated palladium tubes be used to purify hydrogen?

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Heated palladium tubes are effective for purifying hydrogen because the metal acts as a semipermeable membrane. At high temperatures, hydrogen gas can pass rapidly through the palladium metal, while other gases remain blocked and cannot penetrate the tube walls.1

This process relies on the unique ability of palladium to absorb large volumes of hydrogen, forming a metallic or alloylike hydride. Because hydrogen can be easily removed from this hydride state through the application of heat and reduced pressure, the metal serves as an efficient medium for isolating pure hydrogen from gas mixtures.1