The chemicals that dissolve gold, specifically enabling it to be absorbed into a solution, are generally those that can form complexes with gold ions. The most notable are halides in the presence of oxidizing agents.
Chemicals that Dissolve Gold
Several chemicals can dissolve gold, most often by forming stable complexes in solution. Here's a breakdown:
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Aqua Regia: This is a highly corrosive mixture of concentrated nitric acid and hydrochloric acid, optimally in a molar ratio of 1:3. The nitric acid acts as an oxidizing agent and is used to form gold ions (Au3+). The hydrochloric acid is then used to react with the gold ions to form tetrachloroaurate(III) anions ([AuCl4]−), also in solution. The reaction with hydrochloric acid is an equilibrium reaction that favors formation of tetrachloroaurate(III) anions. This reaction results in the removal of gold ions from solution and allows the further oxidation of gold to take place. The relevant reaction equations are:
Au + 3 HNO3 + 4 HCl ⇌ [AuCl4]− + 3 NO2 + H3O+ + 2 H2O
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Cyanide Solutions: Gold is extracted commercially using cyanide solutions, typically sodium cyanide (NaCN) or potassium cyanide (KCN). An oxidizing agent, such as oxygen from the air, is required. This process is known as cyanide leaching. The net reaction is:
4 Au + 8 CN− + O2 + 2 H2O → 4 [Au(CN)2]− + 4 OH−
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Halogens:
- Chlorine: Gold dissolves readily in aqueous chloride solution in the presence of an appropriate oxidizing agent (such as chlorine, nitric acid, or ferric ions) to form the tetrachloroauric ion (
[AuCl4]−). - Bromine: Similar to chlorine, bromine can also dissolve gold in the presence of suitable conditions.
- Iodine: While less common, iodine can also react with gold under specific conditions.
- Chlorine: Gold dissolves readily in aqueous chloride solution in the presence of an appropriate oxidizing agent (such as chlorine, nitric acid, or ferric ions) to form the tetrachloroauric ion (
Factors Affecting Gold Dissolution
The rate and effectiveness of gold dissolution depend on several factors:
- Concentration of chemicals: Higher concentrations generally lead to faster dissolution.
- Temperature: Increased temperature usually accelerates the reaction.
- Oxidizing agent: The presence and strength of an oxidizing agent are crucial for the reaction to proceed.
- pH: The pH of the solution can significantly impact the stability and reactivity of the chemicals involved.
In summary, gold is not "absorbed" in the traditional sense. It is dissolved into a solution through chemical reactions, most commonly with aqua regia, cyanide solutions, or halogens (especially chlorine) in the presence of oxidizing agents. These chemicals facilitate the formation of gold complexes that are soluble in the solution.
