However, when using platinum equipment it must be remembered that even platinum is not a universal wonder material which is resistant to everything. For instance, damage can result from elements which form a low-melting phase with platinum, from very aggressive chemical media or from evaporation.
Handling with Platinum
The excellent resistance of platinum and the platinum group metals (Ir, Rh and Pd) to acids and oxidation at high temperatures, their high menting points and low vapour pressures make these metals indispendable materials for apparatus in the chemistry laboratory.
The formation of alloys with most metals leads to a reduction in the melting point of platinum, especially in the case of the low-melting metals tin, antimony and bismuth. Thus, it is possible that even with low concentrations and at moderate temperatures the melting point can be exceeded in localized areas which results in the destruction of the apparatus (see table "Melting Themperatures of Low-Melting Precious Metal Alloys").
It should be noted that, due to thermal decomposition and especially under reducing conditions, chemical compounds can dissociate and release these detrimental elements. Therefore, to avoid reduction, heating processes and ignitions shoud be carried out in oxidizing atmospheres, i.e. in open cruicibles. In this context it should be especially noted that hydrogen may be picked up by platinum at 400°C, diffuse through the walls of equipment at higher temperatures and can reduce the material contained within.
Apart from reducing conditions in the enviroment, the presence of carbon or organic substances can also result in the reduction of chemical compounds and thus to the release of elements which can demage platinum. Carbon itself can also cause damage to the structural integrity of platinum. Please take especial care to adjust the flame when working with bunsen burners and ensure that gas-heated fluxing equipment is adjusted to an oxygen-rich flame.
A platinum alloy with a low melting point is formed when the elements silicon, lead or arsenic are present in minute quantities. The alloy formation occurs preferentially at the grain boundaries. The embrittlement which is associated with this process can lead to the formation of cracks. The effects are particulary dangerous when organic matter containing phosphorus is ignited, e.g. in flour ignition. Damage to apparatus by silicon corrosion can occur during heat treatment in furnaces with silicon carbide heating elements. Spalling of the exposed silicon carbide rods can lead to silicon being deposited on the platinum equipment or on the furnace floor which then diffuses into the platinum equipment.
Sulphur can also cause platinum corrosion. This problem is encountered, for example, when preparing fused tablets for XRF from samples containing high levels of sulphur in the form of sulphides.
A further potential hazard is contamination of the crucible on its external surface, for instance by placing it on a dirty surface. At elevated temperatures the contamination can then lead to one of the damage mechanisms described above. We recommend, therefore, that only crucible tongs or tweezers whose tips are protected with platinum should be used when handling hot crucibles. The crucible tongs must not be immersed into acids and alkalis beyond the platinum shoes, because of the danger that liquid could penetrate between the tongs and the shoe resulting in corrosion from within. Naturally, care must also be taken that the triangles on which the laboratory equipment is usually heated are not contaminated by corrosive materials such as heavy-metal salts, phosphates, etc. Unprotected iron triangles or wire gauzes should not be used, but only those with platinum/iridium buttons, or those made of platinum wires. Alternatively non-precious metal wire triangles which are protected by oxide ceramic tubes at contact points may be used. Contact with ferrous materials should be generally avoided.
Less critical than the damage mechanisms described above is corrosion due to salts, halogen compounds or acids. Normally the advantages of using platinum crucibles are greater than the risk of damage by corrosion. At room temperature platinum only dissolves slowly in aqua regia. Amongst the most severe effects are the melting of alkali metal hydroxides and alkali cyanides at high temperatures. Potassium compounds react more strongly than sodium compounds in such fusion preparations. Alkalis have the effect of being oxygen carriers and oxidise platinum to yellow-brown platinum oxide. For this reason molten salt preparations, above all in soda and soda-potash fusions, should always be carried out in covered crucibles. In this way the carbon dioxide released during the fusion can be retained as a protective gas over the melt and prevents the crucible from oxidation.
It is not always appreciated that a thin oxide film forms on platinum in air at room temperature and evaporates at elevated temperatures. The platinum loss which occurs as a result can lead to significant damage over very long operating periods, e.g. at 900°C in air. This effect can be counteracted to a limited extent by alloying with a few percent of rhodium. Platinum-iridium alloys with higher iridium contents, on the other hand, suffer from evaporation losses which are very much greater than for other platinum alloys when exposed to air for long periods.
Crucibles and dishes of platinum or platinum alloys are cleaned by boiling in a suitable solvent. Platinum utensils can be cleaned very thoroughly by melting potassium pyrophosphate in them. For the removal of substances which have alloyed with the surface we recommend that the apparatus be scoured with alumina powder. The use of grinding media containing carbon (e.g. SiC) should be avoided at all costs. Any remaining alumina residues should be removed with a hydrofluoric acid treatment. Contaminated utensils may not, under any circumstances, be cleaned by heating, because the impurities might thereby diffuse into the platinum.
The dissolution of electrolytically deposited metal layers from platinum electrodes is achieved with analytically pure acids. Burning off gauze electrodes over an open flame is not to be recommended because of the risk that impurities remain embedded in the corners and that these then form an alloy. Furthermore this heating causes an undesirable softening of the wire gauze and thus reduces its resistance to deformation. Clean electrodes should be stored in a desiccator. The stability of the remaining metals of the platinum group (Ir, Rh, Pd, Os,Ru) in aggressive media is shown in the Table "Resistance of the Platinum Group Metals in Corrosive Media". Should problems arise in the course of using platinum equipment or should the solution to a specific problem be required, we are pleased to be at your service with further information.
Should problems arise in the course of using platinum equipment or should the solution to a specific problem be required, we are pleased to be at your service with further information.