Rhodium is extremely rare metal and is present in around 0. Rhodium is found in free form in nature, as well as alloyed with gold and other metals of the platinum group. Rhodium is also present in form of minerals such as rhodplumsite and bowieite. Commercially, rhodium is extracted from its nickel and platinum ores.
It is also produced as a byproduct during the uranium fission reaction, however, this extraction is complex and makes the commercial level production through this method fairly impossible. The largest producers of rhodium include South Africa, Canada and Russia, where large natural deposits of rhodium metal are present [2].
Rhodium is silvery white transition metal. It has a shiny appearance and is hard in nature. Rhodium is a Nobel metal and belongs to the group of exquisite metals including rhodium, platinum, osmium, ruthenium, osmium and iridium termed as the platinum group metals PGMs.
Rhodium is a durable metal. It does not react with oxygen even at high temperatures. That is why, rhodium is highly resistant to corrosion and tarnishing.
Its density is lower than platinum, around Rhodium is non-reactive, chemically inert element. It is not dissolved in dilute nitric acid, or aqua regia [3]. Rhodium does not react with oxygen and that is why is resistant to tarnishing in air. Compounds of rhodium resemble compounds of platinum. It does not form volatile compounds. It reacts with halogens to form halides with various oxidation states.
Most of the oxides of rhodium are stable. Rhodium is one of the of the six platinum group metals: platinum, palladium, rhodium, osmium, iridium and ruthenium. It is also classified as a noble metal, meaning that it does not react to oxygen easily, acts as a fantastic catalyst and is resistant to corrosion and oxidation.
Some common characteristics of the platinum group metals include high melting points, general non-toxicity, and resistance to wear, oxidation and corrosion, according to Chemistry Libretexts. Rhodium is the rarest of the platinum group, only occurring up to one part per million in the Earth's crust, according to Chemistry Libretexts.
Rhodium has lower density and a higher melting point than platinum. Rhodium it is unaffected by air and water up to 1, degrees Fahrenheit degrees Celsius , according to Lenntech. Rhodium was discovered in by English chemist William Hyde Wollaston shortly after he had discovered the element palladium. Wollaston extracted rhodium from a piece of platinum ore that he had obtained from South America.
Rhodium often occurs with deposits of platinum and is commonly obtained from the mining and refining of platinum. Wollaston was first alerted to the possibility of a new element existing in platinum ore by the French chemist Hippolyte-Victor Collet-Descotils, who believed that the red color of some platinum salts was due to the presence of an unidentified metal.
After a series of chemical reactions, Wollaston was able to remove the platinum and palladium from the sample of platinum ore. He was left with a dark red powder — which turned out to be sodium rhodium chloride, according to Jefferson Lab. The main use for rhodium is in catalytic converters designed to clean vehicle emissions.
This is a reflection of the extreme rareness of these elements, explaining the very high price of the rhodium chloride I bought as a graduate student. They are in fact so rare that annual production is counted in kilos, not tonnes. And yes, the metals from the catalytic converters are recycled, accounting in these days for around 10 per cent of the yearly supply of rhodium, the lion's share of the rest, around 20, kilos, coming from mines in South Africa.
The specific role of rhodium in catalytic converters is to break down nitrogen oxides, the so-called NOX emissions, to give oxygen and nitrogen gas, the main components of the air we breathe.
Chemical industry is, just as my old research group, interested in using rhodium to build molecules. Rhodium was, for example, until recently the prime choice as catalysts in making one of mankind's oldest chemicals, acetic acid.
It supplanted its periodic table upstairs neighbour cobalt in this process in the late s in a prime example of what is now know as green chemistry making the process more energy efficient and generating less by-products. This is important as chemical plants worldwide produced some 5 million tonnes per year of acetic acid. Today, however, rhodium's downstairs neighbour iridium has largely taken over this role. And, if you chew gum you will most likely encounter another result of rhodium catalysis: menthol.
Originally extracted from different species of mint plants, the demand for this substance with its characteristic minty scent far exceeds the natural sources, and it is now produced in several thousands tonnes a year in a process devised by Japanese Nobel prize winner Ryoji Noyori.
So, instead of associating this metal with immense wealth, such as when the Guinness Book of Records awarded Paul McCartney a rhodium-plated disc for being history's all-time best-selling songwriter and recording artist in , chewing gum may be what pops up in your mind the next time someone mentions rhodium. So we have rhodium to thank when our breath is minty fresh. Now next week an element with a grand position in the periodic table. The number is a very significant one for human beings.
It's partly because our number system is based on ten - so ten tens seems to have a special significance. In years, it's around the maximum lifetime of a human being, making a century more than just a useful division in the historical timeline. But in the natural world, isn't quite so important. There's nothing about being element that makes fermium stand out - the periodic table doesn't attach any significance to base But it's hard not to think that fermium must be special in some way.
And to find out if fermium really does have any special qualities, join Brian Clegg in next week's Chemistry in its element. Until then, I'm Meera Senthilingham and thank you for listening.
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