[′plat·ən·əm ′rō·dē·əm ′al‚ȯi] (metallurgy) An alloy with up to 40% rhodium; as concentration of rhodium increases, so do chemical resistance and hardness (although less hard than for platinum-iridium alloys); used as a catalyst to make nitric acid, in thermocouples, and in rayon spinnerets.