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I'm not an expert teacher or lecturer of chemistry. I was only a student from SMA NEGERI 15 SURABAYA who had been one of the Bronze Medalist Participants of Olimpiade Sains Nasional X (2011) of Chemistry In Manado, North Sulawesi, 11 - 16 September 2011 and graduated in 2012. Now, I'm studying at Universitas Airlangga in Surabaya, Indonesia. I do love chemistry and I would like to help them who had difficulties in studying chemistry. That's why, please understand me if you found some misconcepts in my entries. Suggestions are always necessary in order to develop this blog. And I'm sorry because my English isn't so well.

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## Sunday, July 08, 2012

### Analyzing Gold and Aqua Regia

Once, I went to a gold shop.
In there, my grandmother wanted to sell her earrings that is made with 23 carat of gold.
Then, I saw the shopkeeper tested the purity of that gold by reacting it with some solution.

Can you guess what solution it is?
Remember, gold is one of the most inert metal in this world. Only aqua regia that can dissolve it perfectly.

In my country, the solution is called, "Air Keras". You usually call it "Strong Water" or "Aqua Fortis".
It is a solution that consist of concentrated strong acid such as Sulfuric acid, Hydrochloric Acid, and Nitric Acid.

Because gold is inert. We can test it with those solutions above. Don't worry, they won't dissolve your gold easily.

If your gold is pure (24 Carat). There will be no reaction when you try to test it. But if you have a 23 Carat gold, some gas (Hydrogen Gas) may be produced. Just think what makes it happened?

Aqua Regia is a mixture of concentrated HCl and HNO3 (Ratio of volume: 3:1).
Nitric acid is a strong oxidizer, this makes the gold to be oxidized.
Then, the Aurum ions are stabilized by HCl.

Furthermore, you can read these copied information from Wikipedia:

Aqua regia dissolves gold, though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions (Au3+). The hydrochloric acid provides a ready supply of chloride ions (Cl), which react with the gold ions to produce chloroaurate anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction which favors formation of chloroaurate anions (AuCl4). This results in a removal of gold ions from solution and allows further oxidation of gold to take place. The gold dissolves to become chloroauric acid. In addition, gold may be dissolved by the free chlorine present in aqua regia. Appropriate equations are:

Au (s) + 3 NO3(aq) + 6 H+ (aq)Au3+ (aq) + 3 NO2 (g) + 3 H2O (l) and
Au3+ (aq) + 4 Cl (aq)AuCl4 (aq).

The oxidation reaction can also be written with nitric oxide as the product rather than nitrogen dioxide:

Au (s) + NO3 (aq) + 4 H+ (aq)Au3+ (aq) + NO (g) + 2 H2O (l).

You have to know this too.
Aqua regia can also dissolve platinum!

Similar equations can be written for platinum. As with gold, the oxidation reaction can be written with either nitric oxide or nitrogen dioxide as the nitrogen oxide product.

Pt (s) + 4 NO3 (aq) + 8 H+ (aq) → Pt4+ (aq) + 4 NO2 (g) + 4 H2O (l)
3Pt (s) + 4 NO3 (aq) + 16 H+ (aq) → 3Pt4+ (aq) + 4 NO (g) + 8 H2O (l)

The oxidized platinum ion then reacts with chloride ions resulting in the chloroplatinate ion.
Pt4+ (aq) + 6 Cl (aq) → PtCl62− (aq)

Experimental evidence reveals that the reaction of platinum with aqua regia is considerably more complex. The initial reactions produce a mixture of chloroplatinous acid (H2PtCl4) and nitrosoplatinic chloride ((NO)2PtCl4). The nitrosoplatinic chloride is a solid product. If full dissolution of the platinum is desired, repeated extractions of the residual solids with concentrated hydrochloric acid must be performed.

2Pt (s) + 2HNO3 (aq) + 8 HCl (aq) → (NO)2PtCl4 (s) + H2PtCl4 (aq) + 4 H2O (l)
(NO)2PtCl4 (s) + 2 HCl (aq) $\rightleftharpoons$ H2PtCl4 (aq) + 2 NOCl (g)

The chloroplatinous acid can be oxidized to chloroplatinic acid by saturating the solution with chlorine while heating.
H2PtCl4 (aq) + Cl2 (g) → H2PtCl6 (aq)

Dissolving platinum solids in aqua regia was the mode of discovery for the most dense metals, iridium and osmium, both of which are found in platinum ore and will not be dissolved by the acid, instead collecting on the base of the vessel.

Dissolving Platinum

Thanks
Source: Wikipedia