Aqua regia reaction with gold

Velocity Most metals and alloys are protected from corrosion, not by nobility [a metal s inherent resistance to enter into an electrochemical reaction with that environment, e.g., the (intrinsic) inertness of gold to (almost) everything but aqua regia], but by the formation of a protective film on the surface. In the examples of film-forming protective cases, the film has similar, but more limiting, specific assignment of that exemplaiy-type resistance to the exposed environment (not nearly so broad-based as noted in the case of gold). Velocity-accelerated corrosion is the accelerated or increased rate of deterioration or attack on a metal surface because of relative movement between a corrosive fluid and the metal surface, i.e., the instability (velocity sensitivity) of that protective film. 

Hydrochloro-Auric Acid. Treat gold with aqua regia. When the metal dissolves, add 1 ml of concentrated hydrochloric acid and evaporate the solution until crystals appear. What is the composition of the obtained substance Dissolve several crystals of the compound in water, add to one half of the solution several drops of a soluble cesium salt, and to the other half a 0.1 N potassium hydroxide solution. What forms Write the equations of the reactions. 

There are, however, certain chemical reactions by which platinum may be readily distinguished and separated from the gold in solution. Sulphate of iron and oxalic acid, which precipitate gold, do not precipitate platinum. When the latter is dissolved in aqua regia, and the acid neutralized by carbonate of soda, it is deposited as a black powder, if the mixture be boiled with tartaric acid and sodar—the ingredients of a Seid-litz powder. Further, the addition of chloride of ammonium and alcohol te a strong solution of platinum, causes the deposition of a yellow crystalline precipitate, which is charactsristtc of this metal. These and other reactions may be summarily stated as follows... 

Application of the Jkagmts,—Having thus described the properties and characteristic reactions of the different metals which it is desirable to look for as being frequently associated with gold, lot it now be assumed that the substance to be examined is a piece of. auriferous quartz, TbiB must be first reduced to powder, and then boiled for some time in an earthenware or glass dish with aqua regia. The solution ie then diluted with water, passed through a filter, and allowed to cool. If any silver bo present, it will remain in the filter as a white precipitate, mixed with the quartz. 

Dry test all platinum compounds when fused with sodium carbonate upon charcoal are reduced to the grey, spongy metal (distinction from gold). The residue is insoluble in concentrated mineral acids, but dissolves in aqua regia. The solution is evaporated almost to dryness, dissolved in water, and reactions 1, 2, 5, 9 or 10 applied. 

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