金與"酸"之反應

If we mentioned acid, usually it refers to the chemical which can liberate hydrogen ion. The degree of hydrogen ions generated is the measurement of acidity. In this case, gold of course will not react with hydrogen ions since gold is very inert towards the redox reaction with hydrogen ions. In fact, the reaction between "aqua regia" (王水) and gold is due to the generation of chlorine radical which will react instantly with gold to liberate Auric acid, H(AuCl4). The equations are as follows:

HNO3 + Cl- + H+ ---> NO2 + Cl. +H2O

3Cl. + Au + Cl- + H+ ---> H(AuCl4)

So, the main reagent for the dissolution of gold is newly generated chlorine radical. Therefore, the aqua regia must be freshly prepared before the reaction. Otherwise, once the chlorine radical got, this mixture of hydrochloric acid and nitric acid will no longer function.

Since NO2, which is a well-known air-pollutant is generated in the reaction, some gold chemical manufacturers have skipped the nitric acid and use hydrogen peroxide as the oxidant to generate the chlorine radical. In this case, the reaction is cleaner but the cost is more expnsive.

I hope the above information can help.

王水又稱「王酸」，是一種腐蝕性非常強、冒黃色煙的液體，是一種硝酸和鹽酸組成的混合物，其中混合比例為1:3(體積比)，還是少數幾種能夠溶解金和鉑的物質。這也是它的名字的來源。不過一些非常惰性的金屬如鉭不受王水腐蝕。王水中含有硝酸、氯化亞硝基、氯等強氧化劑。王水中也含有高濃度的氯離子，能跟金屬離子形成穩定的絡離子，有利於向金、鉑溶解方向進行。 王水被用在蝕刻工藝和一些分析過程中。王水很快就分解，因此必須在使用前直接製作。 王水在冶金工業和化學分析用於溶解金屬。


[編輯] 歷史
鹽酸是於約800年左右波斯煉金術士賈比爾·伊本·哈楊將食鹽與礬（硫酸）混合到一起時發現的。他將鹽酸與硝酸混合在一起發明了能夠溶解金的王水。


[編輯] 原理
雖然王水的兩個組成部分單一無法溶解金，但它們聯合起來卻可以溶解金，原理是這樣的：硝酸是一種非常強烈的氧化劑，它可以溶解極微量的金，而鹽酸則可以與溶液中的金離子反應，形成氯化金，使金離子離開溶液，這樣硝酸就可以進一步溶解金了：

Au + NO3- + 4H+ → Au3+ + NO↑ + 2H2O
Au3+ + 4Cl- → AuCl4-
取自"http://zh.wikipedia.org/w/index.php?title=%E7%8E%8B%E6%B0%B4&variant=zh-hk"