ELEMENT OF THE MONTH
Each month we will explore elements of interest from the periodic table, with a brief history of discovery and development, and a review of uses and applications.
GOLD – ATOMIC NUMBER: 79 – CHEMICAL SYMBOL: AU
Gold is a soft, dense, and extremely malleable yellow metal that can be found in its pure state as dust, flakes, or small nuggets. Low concentrations are found in igneous rocks, and recovery of gold from alluvial deposits in streams and rivers has taken place for thousands of years.
It does not react with air or water so will not tarnish over long periods of time, and is unaffected by most acids but can be dissolved in aqua-regia (a 3:1 molar ratio of hydrochloric and nitric acid). An alkaline cyanide solution can also dissolve gold, and this is a basis for refining or electroplating the metal.
Gold was known to man over 5000 years ago, and its lustrous appearance and workability has led to widespread use of the metal for decorative and ceremonial objects. It can be drawn into wires, or hammered into very thin sheets that can be easily worked into complex shapes and designs, or cast into larger objects.
Electronic manufacturing Industries exploit the high electrical and thermal conductivity of gold in many applications across domestic, industrial, and military markets.
Gold will readily form harder, more wear resistant alloys with other metals such as copper and silver, and a wide range of colours can be produced ranging from white, to red, to green-yellow.
Colours of gold-silver-copper alloys: Different metal colours that can be produced by alloying different amounts of gold, silver, and copper. Image by Metallos, used here under a GNU Free Documentation License
WHERE IS GOLD USED?
As a Pure Element
High purity gold at 99.9% to 99.999% is widely used as part of an investment portfolio, usually in the form of bullion bars weighing from 1 gram to 1 kilogram. Pure gold is also utilised in high reliability electronic connectors and wire bonds to meet aerospace requirements.
Pure or ‘soft’ gold can be electroplated on to copper or nickel-iron alloys to provide high electrical conductivity in contacts, especially in space applications.
Pure gold is deposited at around 50 nanometres thickness in the Electroless Nickel Immersion Gold (ENIG) process to protect a solderable nickel layer on printed circuit board (PCB) tracks and pads. This represents a large proportion of gold usage in the electronics industry worldwide.
As an Alloy
Approximately 75% of gold production is used for jewellery making, and alloys containing 37.5% to 75% gold (9 to 18 karat) are commonly used, although 95.8% gold (23 karat) is favoured in some markets. Alloying elements range from the most common: – silver and copper, to the less common: -iron to give a blue-white appearance, cadmium to give a green hue, or aluminium to push the colour towards purple.
White gold alloys are generally produced with a higher proportion of silver than copper, and at higher karat values they may also include some palladium.
Gold alloys with nickel or cobalt can be electroplated to provide a harder wear resistant conductive coating for electronic connector pins and contacts, especially where multiple insertions are specified.
As a Compound
Gold compounds can be formed with halides such as fluorine, chlorine, bromine, and iodine, and can occur naturally as ores such as Calaverite (gold telluride).
Here are some key parameters for gold, quantifiable using instruments from Helmut Fischer GmbH
- Thickness of very thin gold and palladium coatings on copper-iron integrated circuit lead-frames, in the range 10 to 50 nanometres, measurable with a non-destructive XRF based instrument such as the Fischerscope XDV-SDD
- Composition analysis of gold based precious metal alloys to verify fineness prior to hallmarking, using an XRF based instrument such as the Goldscope SD550
- ENIG and ENEPIG thickness testing in PCB manufacture, automated using XRF pattern recognition technology
- Gold, nickel, cobalt, and other metal concentrations in electroplating or other solutions from grams/litre down to low ppm, using a range of XRF based instruments such as Fischerscope XDAL-SDD
- Authenticity of gold bullion bar and coin investments using a Sigmascope Gold instrument to easily detect frauds and fakes!