Tuesday, January 09, 2007

Gold is not forever with aqua regia

Concentrated hydrochloric acid or nitric acid alone cannot dissolve gold.

Aqua regia, a mixture of hydrochloric acid and nitric acid, can dissolve gold because each of its two component acids carries out a different function. The nitric acid is a good oxidising agent. Chloride ions from the hydrochloric acid from coordination complexes with the gold ions, removing them from solution. Reducing the concentration of the Au3+ ions shifts the equilibrium towards the oxidised form.

Nitric acid/hydrochloric acid with gold on either side, aqua regia with gold in the centre

Au(s) + 3NO3-(aq) + 6H+(aq) ---> Au3+(aq) + 3NO2(g) + 3H2O(l)

Au3+ (aq) + 4Cl-(a) ---> AuCl4-(aq)

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