Platinum as a Metallic Material
Examples of Application in Chemical Analysis
The use of crucibles, dishes and other laboratory utensils made of platinum for sample preparation has achieved exceptional significance in chemical analysis. This can be attributed to the near-perfect oxidation resistance of the platinum materials and their excellent resistance to corrosive attack by many chemical media. It can thus be ensured that, on the one hand, the samples for analysis are not contaminated by dissolved metal and, on the other hand, that the mass of the crucibles and dishes remains stable over a large number of preparations, ensuring the high precision of gravimetric determinations.
Particularly in X-ray fluorescence analysis (XRF), samples are commonly dissolved in a molten oxidic flux, e.g. in lithium tetraborate. The fusion process is usually carried out in a crucible of platinum because of its resistance to oxidation and corrosion by the flux and the metal oxides in the sample and has the purpose of achieving a homogeneous distribution of the sample in the lithium tetraborate glass melt. The melt is then cast to a button in a platinum dish that has an extremely smooth, flat base. The base of the sample (i.e. the button) thus attains the same smoothness and flatness and therefore offers an ideal surface for conducting the analysis. The fusion and casting process is commonly carried out in fully or semiautomatic units specially designed for the purpose.
The crucibles and dishes can be made of chemically pure platinum. It is, however, very advantageous to use the alloy PtAu95/5 because its favorable wetting characteristics make it possible to pour the complete sample out of the crucible and to release the button sample easily from the mold. In particular in laboratories analyzing a large number of samples, the higher mechanical strength of the oxide dispersion strengthened version of the alloy (PtAu95/5 DPH) increases the service life.
When using platinum, the danger of contamination of the material by platinum poisons must be considered. These are elements (e.g. arsenic, bismuth, phosphorus, silicon, sulfur, lead) that are harmless in the form of their oxides. However, when they are present in the elemental state, they form low melting eutectics with platinum and can thus cause the rapid failure of platinum equipment. It is therefore of prime importance to avoid reducing conditions when using platinum equipment.