Aqua Regia: A Different Way to Store Your Gold​!

Aqua Regia: A Different Way to Store Your Gold!

Although it is not easy to save today, there are still people who keep their gold for investment purposes. There are several storage areas these people would prefer. Those who adapt to modern life prefer banks, while more traditional ones find it safe to keep them under the pillow.

So, if you were a scientist living during World War II, where would be the safest place to keep your gold?

Of course, in a beaker where you mix two of the most well-known strong acids!

So let’s find out the story of these acid and mad scientists!

Aqua Regia: Royal Water (?)

When we say royal water, many people think of a regal drink. A drink that is so expensive that it could only grace the table of kings. However, the facts are not like that.

Aqua Regia is the common name given to a mixture of Hydrochloric Acid (HCl) and Nitric Acid (HNO3) .

So what makes this blend so important to call it “Royal Water“?

King Juice Visual

Nobel Metals of Chemistry: Gold, Silver, and Platinum

When we look at precious items, we see that they often contain gold, silver, or platinum. The most important reason for the high interest in these elements is this: just like the noble gases in group 8A in the periodic table, these elements are also reluctant to react. These metals, called noble metals, are so resistant that even the strongest acids cannot afford to corrode them. For this reason, it is especially used in jewelry making and can remain like the first day for years. The fact that gold does not lose its value as a result of external influences has been the subject of proverbs in Turkish:

“Gold is still gold even if it falls into the mud”

However, the power of gold and other noble metals ends with aqua regia. What makes this mixture special is that it can even dissolve gold.

Gold, Silver and Platinum Image

The Craziest Way To Hide Gold: Throw It Into Acid Mixture!

Let’s go back to our scientists at the beginning of our article. In the 1930s, Hitler saw moving gold from the country as treason. Nobel Prize-winning chemist George de Hevesy and Nobel Prize-winning physicist Niels Bohr also wanted to flee the country at that time. But taking their gold medals with them meant committing crimes against the state. Our scientists, who initially thought of burying these gold medals, later thought it was predictable and gave up. Then, they had that crazy idea: throw it in the acid mixture!

Knowing the existence and effect of aqua regia, scientists immediately prepared a three-to-one solution of HCl and HNO3 and poured it into a large glass beaker. Then they threw their gold medallions into this beaker. Encountering aqua regia, the gold dissolved completely within a short time and turned into a more intense orange solution. From the outside, there was no trace of gold anymore. Only a glass beaker with an orange solution remained in the laboratory.

Laboratory Image

Our scientists fled the country, leaving the beaker in the laboratory. Shortly after, the Nazis came to scour the entire building and loot the laboratory. But they found nothing tangible. They didn’t even come near the beaker of an obscure orange solution in the corner of the lab.

Hevesy fled from Stockholm in 1943 and returned to his laboratory. The beaker was still on the corner like it was on the first day. By subjecting it to several chemical treatments, he soon precipitated the gold and remolded the medals.

How Can Aqua Regia Dissolve Gold?

Aqua Regia is formed by combining hydrochloric acid and nitric acid in a 3:1 ratio. The reaction that takes place is as follows:

3 HCl + HNO3 → NOCl (aq) + Cl2 (g) + 2 H2O (l)

When we throw gold into this mixture, the following reaction takes place:

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

While talking about noble metals, we mentioned that these metals are not soluble in acids. However, this is not entirely true. Nitric acid manages to dissolve an almost immeasurable amount of gold and turns them into gold ions. Gold ions in the environment combine with chlorine ions from hydrochloric acid. This combination shifts the reaction equilibrium and causes more gold to dissolve and combine with chlorine. Thus, all of the gold dissolves in the mixture and allows the formation of [AuCl4]ions.

In the last stage, we need to precipitate the gold solid, that is, turn it into a solid state. Sulfur dioxide (SO2) is often used for this:

2 AuCl4  (aq)  + 3SO2 (g)  + 6 H2O(l) → 2Au (s)  + 12H + (aq)  + 3SO4 2− (aq)  + 8 Cl (aq) 

HCl and HNO3

WARNING: We should never attempt to create aqua regia, except for laboratory uses. Since the acid formed as a result of these two substances will be very strong, even the slightest contact will be dangerous. Also, as can be seen in the first reaction, chlorine gas is formed. Even breathing traces of chlorine gas can be fatal. So even if you don’t have direct contact, just inhaling the gas will cause you irreversible damage.

References and Further Readings

Kean, S. (2011). The disappearing spoon and other true tales of madness, love, and the history of the world from the periodic table of the elements. London: Black Swan.

Wang, H. (2017, September 1). Noble Metals. Membrane-Based Separations in Metallurgy.

Wikimedia Foundation. (2021, June 3). Aqua regia Wikipedia, the free encyclopedia.

Images not cited are used through Canva Pro with a royalty payment.

The proofreading has been done by Asu Pelin Akköse and Mete Esencan.

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Mete Esencan

Hello everyone! I'm Mete Esencan. I am a graduate student in the Department of Chemistry at METU. I was planning to establish a platform by combining the research knowledge I gained during my basic science education and the management experience I gained in the METU Chemistry Society, which I was in charge of for three years. For this purpose, in February of 2021, I took the first step and established the OkButWhy, a platform where we can write articles as if to chat about science, art and philosophy. I wish everyone a pleasant reading!

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