The expression chemically pure platinum is generally understood to describe the commercial platinum grade 99.95% according to ASTM[i] (ASTM B561-94(2005) Standard Specification for Refined Platinum).
The expression effectively designates the quality of platinum that is yielded by chemical purification and precipitation of metallic platinum. Furthermore, considerable quantities of recycle material are added when processing the pure platinum. The recycle material is derived both from internal production processes (e.g. sheet offcuts) and from used platinum components that have been subjected to a metallurgical purification process.
The attainment of the chemical purity specification is ensured by chemical analysis (e.g. ICP[ii]). In practical application it is usual to distinguish between the various types of impurities.
Firstly, the further platinum metals: ruthenium, rhodium, palladium, osmium and iridium together with gold have a great similarity to platinum and are considered to give no problems within the specified concentration ranges (100-300ppm).
Secondly, experience has shown that elements such as iron, nickel, copper and molybdenum cause no detrimental effects within the specified concentration ranges (50-100ppm).
The third type of impurity comprises elements such as silicon, arsenic, tin, antimony, lead and bismuth which form low-melting eutectics with platinum and thus can lead to considerable damage when used. They are therefore commonly designated "platinum poisons". Although the specification for these elements permits contents up to 50 or 100ppm each, the manufacturer normally takes great care to ensure that the sum of the concentrations of these elements does not exceed 50ppm.
