How to Make an Acidic Aqua Regia Solution

Aqua regia is a strongly oxidizing mixture prepared by mixing two strong concentrated mineral acids, namely concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO ₃ ). Given the acidic and oxidizing characteristics of aqua regia, it is capable of dissolving a wide variety of minerals, including minerals that contain gold, platinum, and other noble metals.

Composition of aqua regia

Aqua regia is more than just a mixture of two strong acids. In this mixture, hydrochloric and nitric acid react with each other to generate some highly reactive chemical species. For starters, although nitric acid is a strong acid, it is also a strong oxidizer, and is capable of oxidizing hydrochloric acid to chlorine gas and water, reducing itself to nitrosyl chloride. The reaction is the following:

Gaseous chlorine is an irritant and highly corrosive gas due to its strongly oxidizing nature. For its part, nitrosyl chloride is an unstable species that undergoes a disproportionation reaction (it oxidizes and reduces itself) to produce nitric oxide and another chlorine molecule. Nitric oxide can then react with oxygen in the air to produce nitrogen dioxide.

The reactions described are presented below:

The overall reaction is as follows:

As we can see, the reaction requires a stoichiometric ratio of 3 moles of HCl for every 1 mole of HNO ₃ . In this way, it is capable of generating three moles of chlorine gas, which can easily oxidize most organic and inorganic compounds.

Uses of aqua regia

Aqua regia has many uses including:

• Engraving of different surfaces.
• The refining of some noble metals such as gold and platinum.
• Chlorine refining.
• They are also used in some laboratories for cleaning glassware.

Safety measures for the preparation and handling of aqua regia

The characteristics of aqua regia make it a very toxic and corrosive mixture. For this reason, it must be handled with extreme care. Some essential safety measures for the safe handling of this substance are:

• Just prepare the aqua regia and open containers that contain it inside a fume hood.
• Only prepare the amount that is going to be used at the time it is going to be used, since the mixture is unstable.
• Always wear safety glasses, nitrile gloves, and lab coat when handling.
• If it will be handled outside of a hood, it is preferable to use a gas mask to filter out chlorine and the various nitrogen oxides generated by the reaction between hydrochloric and nitric acid.
• In case of contact with the skin, neutralize with a sodium bicarbonate solution and wash with plenty of water as soon as possible.

Steps to prepare an acid solution of aqua regia

Concentrated nitric acid has a concentration of about 14.4 molar, while concentrated hydrochloric acid has a molarity of about 12 molar. Since the reaction requires a 3:1 HCl:HNO ₃ ratio , this implies that a minimum of 3.6 mL of concentrated hydrochloric acid will be required for every 1 mL of concentrated nitric acid. This ratio could be increased a little to 4:1 to compensate for HCl evaporative losses in the concentrated acid.

With this information in hand, below are the steps for preparing aqua regia in the laboratory:

Step 1: Determine the amounts of each acid to mix

The first step requires, first of all, knowing how much aqua regia you want to produce. Then, assuming that the volumes of both solutions are additive, and that the sum of 4 mL of HCl plus 1 mL of HNO ₃ gives 5 mL of aqua regia, we can establish the following proportions to determine the amount of each component that we will need to prepare the desired amount of aqua regia:

• For every 5 mL of aqua regia, 4 mL of concentrated HCl is required.
• _{For every 5 mL of aqua regia, 1 mL of concentrated HNO 3} is required .

Thus, if we want to prepare 100 mL of aqua regia, then we will need (100 mL aqua regia)*(4 mL HCl/ 5 mL aqua regia) = 80 mL of concentrated HCl. The missing 20 mL correspond to concentrated nitric acid.

Step 2: Measure the amounts of each acid

Using a graduated or volumetric pipet, first measure the volume of HCl (80 mL in our previous example) and HNO ₃ (20 mL in the same example).

Step 3: Mix by first adding the HCl

Once both acids have been measured, we proceed to add the amount of HCl first in a glass container, be it a volumetric flask, a vial or a beaker. Then, slowly and with constant stirring, the nitric acid is added.

Step 4: Use immediately and do not store the leftover sealed

Aqua regia is not very stable, so it is advisable to use it immediately. In the event that we have aqua regia left over when we finish using it, it can be stored in a glass container as long as it is not hermetically sealed. This is because gases can accumulate and build pressure until the container ruptures. If you wish to dispose of excess aqua regia instead, never dispose of it directly down the drain, but rather into the acidic waste container in the laboratory.

References

F&Q exercises. (2019, May 5). Molarity and normality of a commercial hydrochloric acid – Exercises F&Q . https://ejercicios-fyq.com/Molaridad-y-normalidad-de-un-acido-clorhidrico-comercial

Chemistry.ES. (nd). Agua_regia . https://www.quimica.es/enciclopedia/Agua_regia.html

SGS Latin America. (nd). Gold using aqua regia . https://www.sgs-latam.com/es-es/mining/analytical-services/chemical-testing/precious-metals/gold-by-aqua-regia