Miscellaneous: Dissolving gold
Gold belongs to the group of noble metals, which means that it is fairly resistant to corrosion, i.e. oxidation. Gold doesn't react with individual acids; however, mixtures of nitric and hydrochloric acid can oxidize and dissolve gold. This type of mixture is known as aqua ragia. The most common molar ratio used is 1:3, for nitric acid and hydrochloric acid respectively.
Aqua regia was prepared by taking into account the mentioned molar ratio. Carefully and slowly 10 mL of 70% nitric acid was mixed with 60 mL of 20% hydrochloric acid.
After that, the aqua regia was added to a larger lab beaker. Then, the gold containing material was added. Here we used a processor and other electronic parts taken from an old computer. Because of high electrical conductance of gold, it can commonly be found on contact surfaces of such parts.
Gold is dissolved in two main steps:
Au + 3NO3- + 6H+ -› Au3+ + 3NO2 + 3H2O
Au3+ + 4Cl- -› AuCl4-.
As one can see, first gold(III) ions are produced. These ions later react with chloride ions to form tetrachloroaurate(III) ions, which are a part of tetrachloroauric(III) acid, which can be found in the solution, in a dissociated state.
This way, one can oxidize and dissolve gold. The dissolved gold can later be obtained by reduction.
First, the nitric acid is neutralized by adding urea. This is described by the following reaction:
(NH2)2CO + HNO3 -› (NH2)2COHNO3
Urea is added until no further reaction between urea and nitric acid occurs. After that, potassium or sodium metabisulphite is added. The following reactions take place:
K2S2O5 + H2O -› 2KHSO3
3KHSO3 + 2HAuCl4 + 3H2O -› 3KHSO4 + 2Au + 8HCl
As one can see, potassium metabisulphite first reacts with water, thereby forming sodium bisulphite which finally reduces the gold. The reduced gold comes in form of very small particles which settle to the bottom of the beaker.
After that, the upper part of the solution is decanted, so that only very little solution remains, together with the gold powder. This is then filtered in order to obtain pure gold powder.
Gold obtained in this way comes in form of an orange to brown colored powder, which doesn't show the characteristic shiny golden appearance. In order to obtain a larger single piece of gold, the powder can be heated to temperatures high enough to melt the gold, after which it can be cast to a desired shape.
Napomena: Before exposing the electronic parts to the aqua regia, it is useful to first let them react with hydrochloric acid and with nitric acid afterwards. This helps to remove impurities, which would otherwise be dissolved by the aqua regia, together with the gold. That would finally lead to an impure product, which is, of course, not desired.