Acid Test For Gold

All that glitters is not gold .Assaying determines just how much of that glitter is gold.

The simplest way in which jewellers roughly estimate the purity of gold is by using a touchstone.This method has been in use since ancient times.A touchstone is a small tablet of dark stone such as fieldstone, slate, or lydite which has a finely grained surface on which soft metals like  gold leave a visible trace. Because different alloys of gold have different colours , depending on the different metals added to the gold and their quantities , the test sample can be compared to samples of known purity to estimate the gold content of the sample.Additionally, the fact, that the trace from the test sample will react differently to specific concentrations of nitric acid or aqua regia, can be used to estimate the quality of the gold.Unfortunately using a touchstone involves scratching the sample and each time the test is performed a tiny bit of gold is lost.And the results obtained by this method are not considered conclusive.

To avoid scratching gold,especially finished jewellery, we can use  X-Ray Fluorescence Spectroscopy (XRF) which is a non-destructive technique used to identify and determine the concentrations of elements.This is the technology used in the carat meters,like the picture on the right, found in places like the Tanishq showrooms. But carat meters only assesses the metal 5 mm from the surface . So they can detect only surface adulterants.Also the machines do not identify a metal unless a sample has been fed (calibrated) into them.This is why for a long time iridium and ruthenium adulteration went undetected in India.

 The only fool-proof method of determining the purity of gold is to go in for a fire assay. This is the method used in various BIS certified labs to conclusively establish the purity of gold.Unfortunately it is a destructive test. 

First, extremely fine slivers of gold are carved out from the sample. Then these slivers are melted and cooled.

The resulting tiny shapeless piece of gold is then pressed into a strip.The strip is then cut into two. One part is placed on silver foil and rounded into a bead. The second strip is for repeating the test at another lab to be doubly certain of the results.

The bead is then placed on a cupel (a porous pot made from magnesium oxide) in a muffle furnace, with good airflow at a temperature of approximately 950 degrees Celsius.

The cupel has an affinity for several metals including lead and copper, and acts as a filter, absorbing the molten lead oxide or copper oxide. Silver and gold remain in the flux that is covered by a foil.Iridium,ruthenium,and other platinum group metals, if present, remain in the gold.The foil is then put into another furnace. Boiling dilute nitric acid separates the silver and gold. The silver gets dissolved in the acid, and the gold is removed, dried, and cooled to prevent it from becoming brittle. But the iridium and ruthenium if present do not get isolated even at this stage.

To separate the iridium and ruthenium from gold, we need to use aqua regia (a mix of hydrochloric acid and nitric acid)—The Acid Test.Gold dissolves in aqua regia but iridium and ruthenium remain insoluble in it.

The dissolved gold can be recovered from the aqua regia by using  ferrous sulphate solution or  sulphur dioxide or  hydrazine hydrate.By noting the quantities of different metals during the fire assay process, it is possible to accurately arrive at the purity of gold.