Background information on this precious metal, and notes on detection methods based on voltammetric analysis, are outlined by Peter Jones, product specialist at Metrohm
Platinum plays a huge role in modern industry - notably in electrochemical and catalytic systems, and is currently being used in fuel and electrolytic cells, pharmaceutical production, petro-chemistry, laboratory equipment, electrical contacts, dentistry and anti-pollution devices in the automotive and water-treatment sectors, as well as surgical and astronomical devices.
Naturally-occurring platinum and platinum-rich alloys have been known for a long time and although the metal was used by pre-Columbian Indians, the first European reference to platinum appears in 1557 in the writings of the Italian humanist Julius Caesar Scaliger as a mysterious metal found in Central American mines between Panama and Mexico "up until now impossible to melt by any of the Spanish arts".
Platinum was discovered by the Spanish astronomer Antonio de Ulloa in 1735 as platina ("little silver") del pinto, the unworkable metal found with gold in Columbia, it was also regarded as an impurity in the silver being mined and was often discarded. Charles Wood independently isolated the element in 1741.
Platinum is often found in a native state and the ore sperrylite (platinum arsenide, PtAs2) is a major source of the metal.
A naturally occurring platinum/iridium alloy is platiniridium and this metal is also found in the mineral cooperite (platinum sulphide, PtS). This metal is often accompanied by small amounts of other platinum family metals which are found in alluvial deposits in Columbia, Ontario, the Ural Mountains, and in certain western American states.
Platinum is produced commercially as a by-product of nickel ore processing.
The huge quantities of nickel ore processed makes up for the fact that platinum makes up only two parts per million of the ore.
Naturally occurring platinum is composed of five stable isotopes and several radioisotopes, one of which, Pt-190, has a very long half-life (over 6 billion years) and the most stable is Pt-193 with a half-life of 50 years.
The metal is silvery-white in its pure form, malleable, ductile, and corrosion-resistant to many chemicals.
Its catalytic properties are excellent (to the point where hydrogen and oxygen spontaneously react in the presence of platinum) and it has several, common oxidation states including +2, +3, and +4.
Platinum does not oxidise in air at any temperature and has excellent high temperature characteristics and stable electrochemical properties.
The metal is insoluble in hydrochloric and nitric acid, but does dissolve when mixed as aqua regia (forming chloroplatinic acid).
This metal has a coefficient of thermal expansion that is almost equal to soda-lime-silica glass and is therefore used to make sealed electrodes in glass systems.
The standard definition of a metre for a long time was based on the distance between two marks on a bar of platinum-iridium housed in Sevres; it is also used in the definition of the Standard Hydrogen Electrode.
It has applications in the military sector where it is used for coating missile nose cones, jet engine fuel nozzles, and other devices that must perform reliably at high temperatures for extended periods of time.
Medical applications include Cis-platin, [Pt-Cl2-(NH3)2], a drug that is effective in treating certain types of cancer which include leukaemia and testicular cancer; as well as, 90/10 platinum/osmium alloy used in the manufacture of pacemakers, replacement valves and other surgical implants such as supports for fractured bones.
Metrohm provide applications for the determination of platinum in biological matrices pre-loaded with the voltammetric analysis (VA) range of instruments and also available as downloads from its website.
Fine platinum wire glows red hot when exposed to methanol vapour, acting as a catalyst that converts the alcohol to formaldehyde.
This phenomenon has been commercially used to make cigarette lighters and hand warmers, while alloys of platinum and cobalt have excellent magnetic properties - one alloy (76.7%, 23.3% Co by mass) forms an extremely powerful magnet.
Metrohm offer a full choice for platinum dependant applications, the 797 VA Computrace has the highest degree of sensitivity due to the unique Multi-Mode-Electrode (MME) and a newly designed potentiostat (with detection limits of 0.1ppt for Platinum) and pre-loaded with over 220 important analytical methods including elementary sulphur in petroleum, and platinum in a variety of matrices VA is rapidly becoming a viable alternative to AAS or ICP, for a fraction of the purchase price and running costs, it is possible to carry out analyses with the same or improved sensitivity - relying on a small amount of reagents and a supply of high-purity nitrogen.
No expensive flammable gases, no specially constructed fume hoods in the laboratory, and no expensive metal vapour lamps make VA extremely cost effective as an analysis solution, and high ionic matrices are not a problem for VA, making it ideal for electroplating baths, waste and sea waters.
For the ultra trace determination of platinum, formaldehyde and hydrazine can be condensed to form the corresponding hydrazone which complexes with Pt(II).
This complex is then adsorbed onto the hanging mercury drop electrode (HMDE, one operating mode of the MME) where it reduces the hydrogen over-potential.
The measured signal of the hydrogen reduction is proportional to the concentration of the platinum complex.
Owing to the catalytic effect of platinum the determination is extremely sensitive.
In the catalytic industries, platinum is used in its finely divided form in the automotive industry and sulphuric acid production.
It has extensive uses in the chemical industry where it is used, either on its own, or with as a rhodium alloy (in the form of a gauze) to catalyse the partial oxidation of ammonia, to yield nitric oxide - which is the precursor to ammonium nitrate fertilisers, explosives and nitric acid.
Platinum-supported catalysts are used in the refining, and reforming of crude oil, also in the production of high-octane gasoline and aromatic compounds for the petrochemical industry.
The metal itself can absorb huge amounts of hydrogen, which is released on heating - it is therefore of much interest as both a catalyst and a storage medium in the rapidly expanding field of fuel cell technology.
However, despite its general inert behaviour, platinum can be corroded by exposure to cyanides, halogens, sulphides and caustic alkalis.
Platinum in the electrochemical cell industry.
An electrolytic cell utilises energy to drive chemical change in the cell electrolytes and is used in a variety of chemical production and energy storage applications.
A fuel cell works on the principle of hydrolysis, whereby hydrogen and oxygen are combined, with the aid of a catalyst, to produce water with energy as a by-product: 2H2 + O2 = 2H2O (exothermic).
This clean technology, coupled with fossil fuel depletion and the need to reduce global atmospheric emissions, is the driving force behind environmentally friendly power generation systems.
One of the most promising technologies of the fuel cell industry is the proton exchange membrane fuel cell (PEMFC).
Costs.
Much of the cost of a PEMFC is the platinum, and latest estimates suggest that by the time fuel cell cars are in commercial-scale production, each will require eight to ten grams of platinum.
The fuel for PEMFCs is produced mainly from reformed hydrocarbons, which means that, the platinum anode may be exposed to undesirable by-products, such as carbon monoxide, ammonia, and more importantly hydrogen sulphide, all of which poison the platinum causing the hydrogen chemisorption capacity to decrease and electrodes to become extremely brittle, powdering by touch.
One study showed that exposure of a fuel cell's membrane electrode assembly (MEA) with Pt catalyst to 8ppm of H2S considerably degraded the cell's electrical performance.
The decrease in current flow, observed in poisoned electrodes, may be due to H2S dissociation producing a sulphur block that sterically prevents hydrogen adsorption on Pt.
These poisoning effects could also be assigned to strong sulphide adsorption on Pt causing the Pt sites to be inaccessible to hydrogen according to the following mechanism proposed by Mathieu and Primet.
5 H2S + Pt = Pt-S + H2 Pt-H + H2S = Pt-S + 3/2H2.
During the H2S adsorption, Najdeker and Bishop suggested that Pt-S (platinum sulphide) and Pt-S2 (platinum disulphide) could also be formed electrochemically exacerbating the problem.
Pt + H2S = Pt-S + 2H+ + 2e- (E0 = 0.30 V) Pt-S + H2S = Pt-S2 + 2H+ + 2e- (E0 = 0.01 V). VA with Metrohm means reproducibility and accuracy and to that end the electrodes used are automatically checked before each determination, and if problems occur the faulty electrode is identified and displayed on screen, this test may also be run manually to test the system.
Full automation is available with the 813 compact autosampler or 766 sample processor.
The software automatically checks the validation intervals of the analysis system and informs the user.
Each report shows whether the analysis is still valid.
The GLP wizard guides the user step-by-step through the various validation tests and automatically evaluates the results.
The software also has a built in diagnosis program that allows the individual components of the instrument to be checked.
The diagnosis program runs as part of the validation wizard, but can also be run on its own. Thus for the analysis of platinum, or more importantly contaminants poisoning the metal - such as sulphur, cyanide and halides, the VA Computrace offers control and protection of important and expensive catalyst material and can be used to monitor degradation of platinum systems with remarkable and unparalleled sensitivity.
