1-5 The standard of mass The SI standard of mass is a platinum-iridium cylinder and assigned as a mass of 1 kilogram (prototype) Secondary standards are sent to standardizing laboratories in other countries Atomic mass unit1-5 The standard of mass • The SI standard of mass is a platinum-iridium cylinder and assigned as a mass of 1 kilogram (prototype). • Secondary standards are sent to standardizing laboratories in other countries. • Atomic mass unit