Platinum Rhodium

Platinum-rhodium alloys are alloys of the platinum group precious metals platinum and rhodium.

Surepure Chemetals typically supplies alloy proportions of
94% platinum / 6% rhodium,
90% platinum / 10% rhodium,
80% platinum / 20% rhodium, and
70% platinum / 30% rhodium.

Surepure Chemetals sells both platinum-rhodium and pure platinum conforming to the ASTM E1159 specification for Thermocouple Materials, Platinum-Rhodium Alloys, and Platinum.

A thermocouple is a temperature-measuring device consisting of two different types of conductor wires. Platinum and Platinum-Rhodium thermocouples are a very stable type of thermocouple, are resistant to oxidation in air, and can be used at high temperatures, up to 1800°C (3272°F), depending on the type of thermocouple.

Type B thermocouples are made up of Platinum 70% / Rhodium 30% wire paired with Platinum 94% / Rhodium 6% wire, and can be used up to 1800°C (3272°F).

Type R thermocouples are made up of Platinum 87% / Rhodium 13% wire paired with Pure Platinum wire, and can be used up to 1600°C (2912°F).

Type S thermocouples are made up of Platinum 90% / Rhodium 10% wire paired with Pure Platinum wire, and can be used up to 1600°C (2912°F). Before 1990, Type S thermocouples were used as the standard international thermometers from 630°C to 1064°C.

Surepure Chemetals sells platinum-rhodium wire and pure platinum wire for use in thermocouples.

Whereas platinum has a Young's modulus of 165 GPa, the Young's modulus of rhodium is 372 GPa. As would be expected, platinum-rhodium alloys have substantially higher mechanical strength at all temperatures than chemically pure platinum. Even at temperatures immediately below the melting temperature, the platinum-rhodium alloys demonstrate oxidation resistance that is similar to that of platinum.

Alloys containing about 10 percent of rhodium are used as positive elements in thermocouples, as catalytic gauzes for the oxidation of ammonia to nitric acid, and to some extent as the heating element in resistance furnaces. Alloys containing 3 to 4 percent of rhodium are used for crucibles and similar laboratory ware.