Gold Purification... Auric Compounds???
* Pardon the train of thought, it goes to the gold topic eventually.
I had been researching the uses of a nitrated diamide compound produced from plastic bottles recently. So far, apart from ferric ions
(Fe3+), it shows the peculiar property of forming water insoluble compounds with the ions of two elements of the periodic table silver
(Ag+) and copper (Cu2+). Since those elements are those in group 11 I wondered if the same properties would be exhibited with
the next element in the series gold, so I set out on finding a way to create a solution containing auric (Au3+)ions.
I searched through my house to look for possible sources of gold, and I found an old gold chain, but knowing that gold jewelry is usually an alloy of
base metals such as copper or iron with gold that brought up another issue, the separation of the different elements. Since gold is quite unreactive I
decided to go all out and "dissolved" the jewelry in aqua regia. The solution took on a yellow green colouration after the jewelry was dissolved.
Going on intuition, (expecting that the colour of the solution which would be yellow/golden if it contained only auric ions), I reasoned that the
green colouration exhibited must have been due to the presence of copper ions. As such I though of a quick way to remove copper ions from the
solution.... by adding sodium carbonate, which should have precipitated out the copper as insoluble copper carbonate and neutralizing any excess acid
present. I then left the mixture to stand for approx. 2-3 weeks in which time beautiful blue crystalline deposits were observed. The solution's green
colour faded significantly however it was still not golden yellow. Thinking of solubility I decided to add 70% isopropyl alcohol to the mixture and
placing the vessel in a freezer to force any residual copper carbonate to "crash" out of solution. To my surprise upon allowing the solution to thaw
this is what awaited me.
The alcohol and the sodium carbonate laden mixture formed a biphasic system, with the alcohol taking on the golden colour expected of gold containing
compounds. Going on a hunch (not very scientific of me) I decided to play it out as a liquid-liquid extraction and replenished the system with fresh
alcohol, froze the mixture and repeated the removal of the layers until the alcohol layers were clear and I separated the assumed gold laden organic
phase from the excess salt containing aqueous phase.
Afterwards since I wanted to ensure that there were only gold compounds in the alcohol (or that the gold was in majority to other ions present) I
decided to concentrate the solution and remove any excess sodium carbonate that might have been dissolved in the organic phase. To do this I employed
the use of calcium chloride, which I knew also forms a biphasic layered system with isopropyl alcohol and received the following observations.
*The layering in the above picture is due to the settling out of calcium carbonate giving the appearance of three layers.
The organic layer was once again removed and this time placed into a separate container.
A small sample of the liquid was then added to glucose in addition to small amounts of ammonium hydroxide and heated. The solution went dark as a fine
particulate formed in the testing vessel. The particulate was filtered off, dried, mixed with boric acid and placed in a capillary tube and heated
leading to the following observation (aided by microscope).
With that test it was confirmed that the organic layer did indeed contain gold compounds, and a test on the lower aqueous layer
no reaction therefore showing that most of the gold resides in the organic phase.
Questions:-
I would expect that, due to the slightly basic environment, that the auric ions in solution after addition of sodium carbonate would become aurate
ions most likely in the form of sodium aurate. However, is it known whether or not this compound shows solubility in organic solvents?
What would chemically be the form taken by the gold at the end of the processing with calcium chloride?
Thank You......
"Discoveries are not made by idly sitting around and hoping something interesting might happen; they are made
by getting out there and doing something to push the results and odds in your favour." "Chemistry always works... just not always in the way you
want."
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