Archive for July, 2016

Science Makes Sense-Week41:Noble metals

July 11, 2016

In many Indian households, a family’s worth/status is measured by the amount of gold or silver jewelry the women wear.  For many years, actual gold coins were used as monetary currency till other metal coins were produced.  The U.S. Treasury printed the first paper money in 1861.  By the mid 1800’s most of the trading countries wanted to standardize transactions in the booming world market.   They adopted the gold standard.  This meant that governments could redeem paper money for its value in gold.(Ref.1)

Gold,Au, silver,Ag, platinum,Pt, along with other elements  like ruthenium, rhodium,palladium, osmium, iridium, are considered to be the noble metals since they do not oxidize or react easily under normal conditions like most other elements.   These eight elements form a rectangle block in the transition metals group.  The field of physics restricts the definition to include only three elements: gold, silver and copper,Cu, because these three elements have filled electronic ‘d’ bands.  However, in chemistry, copper is known to react quite well and form a series of well-known compounds, hence the former collection of noble metals makes more sense.(Ref.2)

These noble metals make excellent catalysts , especially platinum, since they are not easily oxidized or react with other chemicals that quickly.  Since Noble metals do not react so easily, they are used to clean car exhausts besides being perfect as catalytic  agents.  They are specifically used as catalysts in the manufacture of nitric acid from ammonia. (Ref.3)

Extraction of noble metals: Some of the Noble metals like platinum, iridium and rhodium are found in mines (Check Nuggets) located in some regions all over the world.  The sponge, which is what the raw material from the mines is called, is heated and turned into grains and then big bars or ingots.  After that it is heated again and hammered, it is turned into coils that are further heated to be drawn into very thin wires.  These thin wires are weaved or knitted into fine meshes.  These meshes are used as catalysts in reactions indicated above.(Ref.3)

Other uses of Noble Metals:

Ruthenium,Ru, is used to strengthen alloys with platinum,Pt and palladium, Pd, for electroplating electrical contact materials to be corrosion-resistant, on tips of fountain pen nibs, as a catalyst to split hydrogen sulfide, H2S, and in catalytic converters in automobiles. Rhodium,Rh, is used as a 3-way catlaytic converter in industry and in the production of acetic acid, CH3COOH, from methanol, CH3OH.  It is also used in the electroplating of platinum, Pt and white gold.  Palladium,Pd, is used mainly in jewelry, dentistry, watchmaking, spark plugs for aircraft, surgical instruments.  Iridium,Ir, is a very hard metal and has many applications in industry.  Because of its great resistance to corrosion and a high melting point, it is used in specialized spark plug manufacturing, pen nibs,  as a catalyst and for electrodes.  Machinery parts like spinnerets, balances and high temperature crucibles in the laboratory are also made with iridium, Ir.   The metals, platinum, Pt, gold, Au, and silver,Ag are used as bullion metals, i.e., in making coins as well as jewelry.  Ag is used as an anti- bacterial  medicine, and Pt is used extensively as a catalyst and for electrodes. Meanwhile Au has been used for years  and years to make coins and as a currency.(Ref.4)  Osmium,Os, is used as an alloy ( 90%Pt and 10%Os) for making surgical instruments and the manufacture of pacemakers.  Osmium tetroxide, an oxide of osmium, OsO4, is used in microscopy as a stain for fatty tissue and in fingerprint detection. (Ref.5)

The noble metals group play an important role in our lives, even though they are not as reactive as other metals in the Periodic Table.  Perhaps their low reactivity makes them unique in their applications.

Activities for Middle School Teachers:

What a wealth of fields one can study here: history, geography, economics, science and several cultures!  Let students look at a map and focus on the area once known as Lydia and find out how they fashioned gold.  Where was gold concentrated in the Americas? Study history and geography as the students look at the Aztecs, the Incas and the arrival of Columbus, the conquistadores and the fight for gold.   How did the English colonials in India plunder the gold and wealth there?   Study the wealth in Europe and the Americas especially in some of the old palaces, churches in France, Austria., Germany, Spain, Central America.  Look at the timeline of how gold has been considered such a valuable asset in so many ecomomies.  How do different cultures use gold, silver and the other noble metals today?  Students who come from India can talk about how people wear gold jewelry especially during weddings and special occassions.  Use pictures and take photographs of Indian weddings if possible.

Nuggets of Information:

Sometimes mercury and rhenium are included in the list of noble metals.(Ref.2)

By 560, the Lydians ,in the region between the Mediterranean, Aegean and Black Seas where modern Turkey is situated, succeeded in separating silver from gold and created the first gold coins.(Refs.2,6)

During the days of exploration, the wealthiest country had the most gold.  This explains the reasons why countries like Spain, Portugal and England vied with each other to send explorers like Columbus and others to places like the New World ( the Americas) and the east in search of gold.(Ref.2)

The gold rush of 1948-49 in California initiated the unification of the western part of the United States of America.(Ref.2)

Sometimes the people who work in the manufacture of noble metals from mines use special brushes to recover noble metals from their shoes.  This is called sponge.  One ounce of sponge is obtained from 10-40 tons of rock.(Ref.3)

Noble metals are found in mines that are in South Africa, Canada, Russia  and Zimbabwe. Processing of platinum takes 6 weeks while rhodium takes almost 6 months.(Ref.3)

Palladium is used for blood sugar testing strips and is one of the 4 bullions used by ISO (International Standards Organization) for currency codes.

Rhodium is used as a filter in mammography systems, neutron detectors in combustion systems. Ruthenium is used as a biological stain; staining pectin and nucleic acids.(Ref.4)  A biological stain is used to highlight biological tissue details. (Ref.7)  

When crude platinum was dissolved in aqua regia (a mixture of concentrated nitric and hydrochloric acid: HNO3+HCl) a black metallic powder was formed.  In 1803, the English chemist Tennant recovered  osmium and iridium from this mixture.   The name ‘osmium’ comes from the Greek word ‘osme’ which means unpleasant odor. Os and Ir are very dense; Os is the densest element known.   OsO4, osmium tetroxide is highly toxic and causes lung, skin and eye damage.(Ref.5)