Aqua regia, Latin for “royal water,” is a highly corrosive mixture of concentrated nitric acid (\(\text{HNO}_3\)) and hydrochloric acid (\(\text{HCl}\)), typically mixed in a 1:3 volume ratio. This powerful acid is uniquely capable of dissolving noble metals, such as gold, which neither acid can dissolve alone. When gold is dissolved, it is oxidized into gold ions (\(\text{Au}^{3+}\)) by the nitric acid and then stabilized in the solution as chloroauric acid (\(\text{HAuCl}_4\)) by the chloride ions from the hydrochloric acid. The goal of precipitation is to chemically reverse this process, converting the dissolved gold ions back into solid, metallic gold powder.
Necessary Safety and Solution Preparation
Working with aqua regia requires extreme caution due to the toxic fumes it produces, including chlorine gas and nitrogen oxides. All procedures must be conducted within a properly functioning chemical fume hood to safely manage these harmful vapors. Personal protective equipment is mandatory and includes a face shield over safety goggles, a lab coat, and chemical-resistant gloves worn over a disposable pair.
Before precipitation can occur, the gold-bearing solution must be prepared. Filter the solution to remove any undissolved solids, such as base metals or insoluble silver chloride, which can interfere with the purity of the final product. Next, the solution must be diluted with distilled water, which helps control the precipitation reaction and reduces the concentration of the strong acids.
A particularly important preparation step is the elimination of residual nitric acid, a process often called “denoxing.” Excess nitric acid is a strong oxidizer and will prevent the gold precipitation agent from working correctly by consuming it before it can react with the gold ions. This is typically achieved by gently heating the solution to boil off the nitric acid or by introducing a chemical like urea (\(\text{CO(NH}_2\text{)}_2\)) until the solution stops effervescing.
The acid waste must never be poured directly down the drain. Instead, the solution must be neutralized to a \(\text{pH}\) between 6 and 9, often using sodium bicarbonate or sodium hydroxide, before being disposed of according to local hazardous waste regulations.
Choosing the Right Precipitation Agent
Precipitating the gold requires a chemical reducing agent. This agent supplies electrons to the gold ions, changing the gold from its dissolved ionic state (\(\text{Au}^{3+}\)) back into neutral, solid metal (\(\text{Au}^{0}\)). Sodium Metabisulfite (\(\text{Na}_2\text{S}_2\text{O}_5\)), commonly referred to as SMB, is the preferred agent for this task due to its effectiveness.
When SMB is added to the acidic solution, it breaks down to produce sulfur dioxide (\(\text{SO}_2\)) gas, which acts as the reducing agent. The sulfur dioxide reacts with the gold ions, causing the gold to drop out of the solution in its metallic form. Alternatives like ferrous sulfate (\(\text{FeSO}_4\)) can also be used, but SMB is favored for producing a cleaner and higher-purity gold powder.
Step-by-Step Guide to Gold Precipitation
Once the solution is filtered and free of excess nitric acid, the precipitation process can begin. The precipitant must be added very slowly while stirring the chloroauric acid solution to ensure the reaction is controlled. A successful reaction will cause the solution to change color and exhibit effervescence as the sulfur dioxide gas is released.
As the gold ions are reduced, a dark, powdery material will appear and begin to settle at the bottom of the container. This material is the gold metal. Adding too much precipitant too quickly can result in an overly vigorous reaction or the precipitation of unwanted base metals, which reduces the final purity of the gold.
The addition of the SMB solution should continue until no more gold powder appears to be forming and the effervescence has ceased. To confirm that all the dissolved gold has been recovered, a small sample of the remaining liquid is tested using a solution of stannous chloride. If this test shows a dark purple, brown, or black color, it indicates that gold ions are still present, and more precipitant is needed.
Once the test confirms the absence of dissolved gold, the entire solution must be left undisturbed for several hours, often overnight, to allow the gold powder to settle out of the liquid. Rushing this settling period can result in the loss of valuable gold powder when the excess liquid is removed.
Processing the Recovered Gold Powder
After the gold powder has settled, the liquid remaining above it, known as the “waste solution,” must be removed through decantation. This acidic liquid, even without the gold, still requires proper neutralization and disposal as hazardous waste. The gold powder is not yet pure enough for consolidation.
The powder must be washed multiple times with hot distilled water to remove any residual acid, salts, and chemical byproducts from the precipitation reaction. Each wash involves adding hot water, stirring the powder gently, allowing it to settle, and then decanting the wash water. This washing process is repeated until the wash water is confirmed to be neutral, indicated by a \(\text{pH}\) strip.
Once the gold powder is washed, it is dried, often in a low-temperature oven or on a hot plate, resulting in gold dust. The final step is to melt this purified gold powder, typically in a crucible with a flux like borax, using a high-temperature torch or furnace. Smelting the powder consolidates it into a solid, marketable gold button or bar, completing the refining process.