How Is Aqua Regia Used to Purify Gold?

The value of gold is determined by its purity. But how is gold purified? What kind of process is involved in creating this most precious of precious metals? The process of refining gold requires reactions among chemical compounds, and the key to the refining process is a highly corrosive liquid called aqua regia. This combination of acids is integral to the extraction and purification of noble metals, including gold and platinum.

The end result of the chemical refining process can render gold up to 99.99% pure, but the process itself involves potent and dangerous chemicals to achieve that purity. While you could make your own aqua regia (provided you had the proper materials), it is best left to professionals who know how to handle strong acids properly and take the proper laboratory safety precautions.

What Is Aqua Regia?

Aqua regia, also known as “royal water,” was named during the Middle Ages because it was a special liquid that could dissolve even noble metals (like gold and platinum), which did not react well with other solutions. Aqua regia does not dissolve all noble metals, but it is especially well known for its effect on gold, platinum, and palladium.

It is a very strong acid created out of a combination of two other strong acids—concentrated nitric acid and concentrated hydrochloric acid. The resulting solution is extremely corrosive and can be dangerous in the wrong setting. It is primarily used to remove metals from substances, especially in microelectronics labs.  It is also effective in metal etching and cleaning lab equipment, especially in cleaning glassware of organic compounds.

How Does It Purify Gold?

If aqua regia dissolves gold, how can it be used to purify gold? The gold ore itself may contain impurities, which must be removed in order to create the pure gold metal that we recognize in jewelry and use in microchips. Aqua regia dissolves the gold, and the solution can then be filtered and refined to leave behind pure gold once the impurities are removed.

The Refining Process

On a scientific level, the refining process is a series of chemical reactions. While neither hydrochloric acid nor nitric acid will dissolve gold on its own, together they can do the task. This is because each acid performs a different task.

Nitric acid works as an oxidizing agent, dissolving an almost undetectable amount of gold—just enough to create gold ions in the solution. The hydrochloric acid provides chloride ions, which react with the gold ions to produce tetrachloroaurate anions in the solution.

The tetrachloroaurate anions react with the hydrochloric acid and form chloroaurate anions, which remove gold ions from the solution. The reaction will work to keep the solution in equilibrium, so these chemical reactions dissolve the gold to create a compound called chloroauric acid.

Undissolved materials can be filtered away from the resulting solution. If the solution only contains gold, the excess aqua regia can be slowly boiled off, leaving behind gold that can be up to 99.99% pure. Any impurities are removed in the process of dissolving the gold and boiling away the remaining solution.  

What Are the Steps to Make an Aqua Regia Solution?

Creating aqua regia is a chemical reaction that produces some toxic components. If you are going to make your own, it is very important to proceed with care and closely follow safety protocols.

Safety First

If you are making aqua regia, be sure to make and use the solution inside a fume hood with the sash down. This will contain any toxic vapors and protect you against injury in case of splashing or broken glass. Always make sure that you are mixing the chemicals in a well-ventilated area, and that the fan in your fume hood is functioning correctly.

Do not make aqua regia in large batches. Instead, make only the minimum amount required for your purposes. Most of the time, aqua regia is only made in batches of 10 milliliters at a time.

Keep your skin covered. Because you are working with concentrated acids, it can be dangerous if you get any on your skin or clothes. Be sure to wear safety goggles, gloves, and a lab coat or apron to protect yourself.

Make sure the glassware you are working with is very clean. Any organic contaminants on the glass can create a violent reaction. Do not use the finished solution on any material that contains an organic compound.

Remember that the solutions you are working with are highly corrosive. If you accidentally get the solution on your skin, wipe it off immediately and rinse with lots of clean water. If you spill it on your clothing, remove the garment immediately.  

In Case of Eye Contact

Use the emergency eyewash station and immediately seek emergency medical treatment.

In Case of Inhalation

If you inhale any of the vapors, move immediately to fresh air.

In Case of Ingestion

Rinse your mouth with water. Do not attempt to induce vomiting, because this will only cause more corrosion in your esophagus. If you are ever concerned about how your accidental contact with aqua regia will affect your health, contact medical professionals as soon as possible.

Neutralize any spills with sodium bicarbonate or a similar compound. It is best to neutralize strong acids with a weak base, not a strong base.  

Preparation

The usual molar ration between the components of aqua regia (the concentrated hydrochloric acid and concentrated nitric acid) is 3:1. However, the volume ratio is a little different. Since concentrated hydrochloric acid is about 35% and concentrated nitric acid is about 65%, the actual volume ratio is closer to four parts hydrochloric acid to one part nitric acid. A typical final product is usually about 10 milliliters of aqua regia—most applications do not require a large amount.

When combining the acids, it is very important to add the nitric acid to the hydrochloric acid. Do not add hydrochloric acid to nitric acid—this can create a dangerous and volatile reaction. If you’ve done it correctly, the resulting solution should be a fuming red or yellow liquid that smells very strongly of chlorine. Do not inhale the fumes or vapors as they are toxic.  (If you are using a properly functioning fume hood, you should be protected from this.)

Use And Storage

Use the resulting aqua regia while it is still fresh. If you must store it, keep it in a cool location, and do not store it for extended periods of time. The solution can become unstable and decompose. Never put a stopper in the container when you store aqua regia, as this can cause a buildup of pressure and break the container.

 

Leftover Solution

If you have any leftover solution, pour it over ice and neutralize the result with sodium bicarbonate or a 10% solution of sodium hydroxide. Only once it has been neutralized can it be safely poured down the drain. If, however, you have used the aqua regia on heavy metals, you cannot dispose of the solution in this manner. Check your local regulations for guidance on how to properly dispose of the solution.

Aqua Regia In History

Aqua regia is commonly used today when working with the gold components in microelectronics and micro-fabrications. However, it also has its place in history.  

When Nazi Germany occupied Denmark in 1940, German soldiers were given orders to confiscate precious metals for the Third Reich. Jewelry, watches, and other items made from gold and silver were taken from their rightful owners by Nazi soldiers.

Scientists Max von Laue and James Franck, both outspoken opponents of the Nazis, had entrusted their gold Nobel Prize medals to Niels Bohr in Copenhagen. Chemist George de Hevesy, who was working at Bohr’s Institute of Theoretical Physics, hid his colleagues’ Nobel Prize gold medals by dissolving them in aqua regia and placing the jars of solution on a shelf with numerous other jars and bottles. When the Nazi soldiers ransacked de Hevesy’s chemistry lab, the jars of aqua regia, hidden among other jars and bottles of solutions, went unrecognized and were deemed unimportant.

After World War II was over, the jars remained untouched. De Hevesy was able to recover the gold from the acid solution, and the Nobel Foundation recast the medals from the original gold to present the awards back to Von Laue and Franck.

Conclusion

Aqua regia may have earned its name in the Middle Ages as the liquid of golden royalty, but it is still a powerful and useful compound in modern science. It has been used to protect the hard-earned accomplishments of noted scientists as well as purify the precious metals that are used in jewelry and electronic circuit boards.  

If you choose to create your own aqua regia, always handle with care. It is extremely corrosive and can be very dangerous if it comes in direct contact with your skin, clothes, or eyes. Always work in a well-ventilated area when dealing with chemical compounds, especially strong acids. And above all, always follow proper safety protocols when working with chemicals. Aqua regia is potent, to say the least, and following safety protocols can only serve to protect you.