Rhodium (lat. Rhodium; denoted by the symbol Rh) is an element of a secondary subgroup of the eighth group of the fifth period of the periodic system of chemical elements of D. I. Mendeleev, atomic number - 45. In appearance, compact rhodium is a beautiful silvery metal with a bluish tint.
Rhodium belongs to the noble metals of the platinum group. Compared to other metals in this group, it is brighter and harder - it becomes plastic only when heated.
The name of the element, translated from ancient Greek, means "rose". He got this name from the color of his typical compounds - they are red, like roses.
Rhodium is a very rare and trace element. Its average content in the earth's crust is 1 • 10-7% by weight. It does not have its own minerals. Contained in nickel and platinum ores as a simple compound. It is also contained in isomorphic admixtures of minerals of the osmous iridium group (up to 3.3%), in copper-nickel ores. The rarest variety of osmist iridium is also known - the native nevyanskite. It contains up to 11.3% rhodium. It is the richest mineral in rhodium.
Rhodium was discovered in England in 1803 by the remarkable scientist of his time, William Hyde Wollaston. While studying native South American platinum, he drew attention to a bright pinkish-red filtrate, which he obtained from a solution of native platinum in aqua regia. The solution acquired this color after the precipitation of platinum and palladium. From this solution, Wollaston isolated a dark red powder, calcined it in a hydrogen atmosphere and obtained a heavy white metal - rhodium.
PHYSICAL PROPERTIES OF RHODIUM
In appearance, compact rhodium is a beautiful silvery metal with a bluish tint. It has a face-centered cubic lattice, the unit cell parameter is a = 0.3803 nm. Density at 20 ° C 12.41 g / cm3. Melting point 1963 ° C, boiling point 3727 ° C. Unlike gold and platinum, rhodium is difficult to machine. Therefore, it can be rolled or pulled into a wire only at 800 ... 900 ° C. Shows the properties of a paramagnet. The reflectivity of the surface of rhodium is 80% for the visible spectrum.
- heat of fusion - 21.8 kJ / mol;
- heat of vaporization - 494 kJ / mol;
- specific heat - 0.244 J / (K • mol);
- thermal conductivity - 150 W / (m • K).
Rhodium occurs naturally in the form of a stable isotope, 103Rh. Its longest-lived isotopes with half-lives are: 101Rh (3.3 years), 102Rh (207 days), 102mRh (2.9 years), 99Rh (16.1 days).
Compact rhodium is extremely resistant to any kind of chemical attack. However, if you take an alloy of rhodium with zinc or cadmium and dissolve it in hydrochloric acid, and then filter it off, you get a fine rhodium precipitate that can explode in air.
CHEMICAL PROPERTIES OF RHODIUM
Rhodium is a noble metal, superior to platinum in chemical resistance in most corrosive environments. Neither acids nor alkalis act on compact rhodium. Only finely crushed rhodium dissolves slowly in hot aqua regia or concentrated sulfuric acid. Rhodium is very resistant to the action of halogens: it reacts with chlorine, bromine and even fluorine only after prolonged heating. In this case, depending on the reaction temperature, halides of various compositions are obtained. In particular, chlorine forms chlorides of one-, two- and trivalent rhodium RhCl, RhCl2, RhCl3. At high temperatures, rhodium slowly reacts with sulfur, turning into sulfides RhS, RhS2, Rh2S5.
Finely ground rhodium is only slowly oxidized at temperatures above 600 ° C:
4Rh + 3O2 = 2Rh2O3.
Rhodium, when heated in air, has a peculiar behavior. At temperatures from 600 to 800 degrees, an oxide film - Rh2O3 - forms on its surface. When the temperature rises from 800 to 1000 degrees, the oxide film on the surface of rhodium decomposes again to metal.
Deposits and mining of rhodium
Less than 30 tons of rhodium are mined in the world annually. The main exporter of this metal is South Africa (approximately 80%). Industrial mining of rhodium is difficult as the metal occurs in ores mixed with other metals such as palladium, silver, platinum, and gold.
From the mines, crude platinum goes to a refinery, where it is loaded into porcelain kettles and processed with aqua regia. The process takes place when heated during the day. As a result, rhodium goes into solution.
APPLICATION OF RHODIUM
The price of rhodium is high, not only because the element is precious, but also because there are more industries where it is needed than the metal itself. However, it is used only where there is no cheaper and more widespread alternative to it.
The lion's share of the rhodium mined is consumed by the automotive industry. Excellent catalytic properties of the metal make it possible to manufacture exhaust gas neutralizers from its alloys.
Rhodium-platinum alloy is a very effective catalyst in the production of nitric acid by oxidizing ammonia with air. It is in this area that rhodium as a metal catalyst has no analogues.
Rhodium in alloys with iridium and / or platinum is used in the manufacture of thermocouples, which are used to effectively measure high temperatures up to 2200 degrees Celsius.
A large consumer of rhodium is the glass industry. From an alloy of rhodium with platinum (usually 7% Rh), vessels are made for melting glass melt and obtaining the finest glass and quartz threads. Rhodium is also practically irreplaceable here.
The surface of rhodium is highly reflective (80%) to the visible spectrum. The reflectivity of rhodium is less than that of silver (95%), but its resistance to the action of corrosive gases and high temperatures is much greater. Rhodium-plated surfaces do not tarnish even in a voltaic arc atmosphere. Therefore, the reflectors of searchlights and technical mirrors of precision measuring instruments of various purposes are coated with rhodium.
Metallic rhodium is used for the production of mirrors subject to strong heating (incandescence) for high-power laser systems (for example, hydrogen fluoride lasers), as well as for the production of diffraction gratings for devices for the analysis of matter (spectrometers).
Precious metal and LCD monitor filters. The need for them is growing every year. More and more rhodium is required for this industry.
Rhodium detectors are used in reactors to measure neutron flux.
There are precious minerals whose crystals grow under artificial conditions only on filters made of platinum and rhodium alloy.
Rhodium has a beautiful silvery-white hue, for which it is highly prized in the jewelry industry. Its reflectivity is very high, the metal sparkles and shimmers, its brilliance is magnificent. Therefore, rhodium plating is becoming an increasingly popular procedure in jewelry workshops.
Rhodium plating is the application of a thin layer of rhodium (0.1 - 0.25 microns) to the surface of jewelry. The coating improves not only the appearance of metals, giving the surface a platinum luster, but also their chemical and mechanical properties. In addition, rhodium does not tarnish over time.
Advantages of rhodium plating:
- brilliant platinum color;
- resistance of the product to mechanical damage, scratches, etc.
Disadvantages of rhodium plating: - Do not clean the jewelry with abrasives, as you can damage the coating;
- after each repair of the product, it will have to be re-plated with rhodium.
Rhodium plating does not last forever, but its lifespan depends on several factors, the main of which is thickness. According to the experience of use, the renewal of the rhodium layer on rings, earrings, chains is required every five years and less often - depending on the intensity of use of the product. In many cases, high-quality rhodium plating has adorned the metal for decades.
PRICE OF RHODIUM ON THE WORLD MARKET
Everyone knows that precious metals cannot be cheap. And since rhodium also belongs to this category, the price per gram of metal, which is also quite rare, is also quite high.