Dissolving and Purifying Gold

Gold is a member of the group of inert metals called the noble metals. Very few chemicals react with these metals. The first reagent found that reacts with gold was a mixture of hydrochloric and nitric acids called aqua regia, or royal water in Latin. Tiny amounts of gold react with the nitric acid on the surface of the metal, and the products of this reaction react with the hydrochloric acid and are dissolved in solution.

Au + 6 HNO3 ---> Au(NO3)3 + 3 NO + 3 H2O

And then:

Au(NO3)3 + 3 HCl ---> AuCl3 + 3 HNO3
AuCl3 + HCl ---> HAuCl4

Once the gold is all dissolved it can then be slowly evaporated and crystals of pure chlorauric acid will form (HAuCl4).

Warnings:
Aqua regia is very corrosive!
Aqua regia will also slowly release NO, NO2, NOCl and Cl2 gasses, these are all toxic and corrosive. This experiment was done outside.
Chlorauric acid is toxic and will stain skin purple.















(This was from cleaning the glassware after the experiment. A single drop which was immediatly drowned in water)

Materials:
31.45% hydrochloric acid (pH down for pools)
Potassium nitrate (Stump remover, was substituted for nitric acid)
Alaskan gold dust

August 3, 2006
6.15g of 31.45% HCl was added to a test tube containing 1.15g gold dust and 1.34g KNO3.

KNO3 + HCl ---> KCl + HNO3

The bottom of the solution turned green and then the color was lost as the solution turned yellow. It began to bubble slowly. A faint smell of chlorine was noticed coming from the test tube. The gold began to turn black. Water was added until all KNO3 dissolved.

August 4
In the morning the gold was surrounded by a gray dust. The gold had stopped bubbling, however, it was not all consumed. A similar quantity of HCl and KNO3 was readded to the gold after the solution was decanted.

~5 hours later
The gold had again stopped bubbling. The test tube was decanted and a similar quantity of reagents was again added. The test tube was heated this time in a beaker of water. The test tube was again decanted and a similar amount of reagents added. The solution stopped bubbling while it was somewhat uncolored.















Left in the test tube was a gray metallic solid and a white dust. My guess is that the solids were composed of metallic lead or silver as well as ionic lead or silver, both have insoluble chlorides. The solution was filtered and the solids discarded. 72.7 Ml clear golden liquid was recovered. The solution was set out to evaporate but since the solution contained large amounts of KCl it was annoying to separate the two crystals. The solution was redissolved in water and the gold was recovered by adding zinc powder, displacing the gold in solution. The excess zinc was dissolved with HCl and the resulting gold powder was washed, dried, and stored.

The text presented here is for informational purposes only. The author is not liable for actions taken by the reader.