Gold, very pure 1 cyanide

These are very unreactive metals. Silver exists mainly as silver sulfide, Ag2S (silver glance). The extraction involves treatment of the pulverised ore with sodium cyanide. Zinc is then added to displace the silver from solution. The pure metal is obtained by electrolysis. Silver also exists to a small extent native in the Earth s crust. Gold is nearly always found in its native form (Figure 10.17). It is also obtained in significant amounts during both the electrolytic refining of copper and the extraction of lead. 

The standard potential is shifted by - 2.28 V to - 0.60 V so that gold may be oxidized easily by oxygen and will therefore dissolve in cyanide solution with oxygen access. A similar situation holds when gold is dissolved in a 1 3 mixture of hydrochloric and nitric acid. In this case Au " " is com-plexed by Cl" leading to a very small Au " concentration in solution. With A i,=2.2x 10" for the AuClJ complex, the standard potential of the Au/Au electrode is shifted from 1.42 V to 0.994 V. Apparently, the formation of a thermodynamically stable AUCI4 complex leads to the dissolution of gold, which does not occur in pure nitric acid. 

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