Why is the cylinder kilogram made up of platinum-iridium alloy?
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There are lot of reasons :-
1. The most important reason is that platinum and iridium both are inertia metals. Means they don't react with any other elements , compounds etc.. it means mass of the artefact made up of platinum- iridium remains constant in any environment.
2. Its high density ( approximately 21.5 g/cm³) , means the artefact has smalp surface are so the potential for surface contamination is minimised.
3.platinum is malleable and ductile. Addition of 10% iridium to platinum makes it hard.
1. The most important reason is that platinum and iridium both are inertia metals. Means they don't react with any other elements , compounds etc.. it means mass of the artefact made up of platinum- iridium remains constant in any environment.
2. Its high density ( approximately 21.5 g/cm³) , means the artefact has smalp surface are so the potential for surface contamination is minimised.
3.platinum is malleable and ductile. Addition of 10% iridium to platinum makes it hard.
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Answer:
Because it’s made of platinum group metals, which don’t oxidize under normal everyday conditions, the mass is more stable. Oxidization would add the mass of some oxygen atoms to the mass of the metal chunk. Because it’s alloyed, it’s harder than pure metals, the mass is more stable because handling doesn’t remove as much metal as it would with a softer chunk.
Even so, the mass of the standard and its replicas (they made more than one so they could be shared and compared) has drifted over time, mostly in a downward direction due to handling wear, and there is a need to figure out a mass standard that doesn’t depend on a chunk of metal in a vault somewhere.
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