Aqua Regia in Chemistry Complete Guide to Properties and Reactions

Aqua regia is a mixture of  nitric and hydrochloric acid concentrations, usually one portion of the former by volume to three portions of the latter.  The alchemists gave this mixture its name (literally, "royal water") due to its ability to dissolve gold and other so-called noble metals.

Aqua regia is a fuming liquid which is yellow, orange and sometimes even red in color. Aqua regia which is freshly made is colorless but it ends up turning orange in a matter of few seconds.

In analytical procedures for the solution of certain iron ores, phosphate rocks, slags, nickel-chromium alloys, antimony ,selenium, and some of the less soluble sulfides, such as those of mercury, arsenic, cobalt, and lead, Aqua regia, and other similar mixtures are used.

These metals are classified as noble metals, so the title 'royal water' was granted to the solution that could dissolve them. Aqua regia is not a single chemical, but rather a combination of two acids, hydrochloric acid (HCl) and nitric acid (HNO3). The product is more effective in the case of aqua regia than the number of its pieces, which is what makes it so beneficial.

Here, I will study aqua regia solution, aqua regia gold, aqua regia chemical name and aqua regia acid in detail.

Manufacturing of Aqua Regia

When concentrated nitric acid and concentrated hydrochloric acid are mixed, chemical reactions happen. Nitrosyl chloride and chlorine gas are the two explosive products of how these reactions result. The yellow color and the fuming nature of the Aqua regia are the evidence of this theory. When the explosive products escape from the solution, the potency of Aqua regia gets diminished. Nitrosyl chloride then decays into chlorine and nitric acid. 

The fumes over Aqua regia because of the addition of chlorine and nitrosyl chloride consist of nitric oxide. The gasses formed consist of nitrogen dioxide, it is because the nitric oxide easily reacts with the atmospheric oxygen.

Uses of Aqua regia

Aqua regia is basically used in the production of chloroauric acid, which is an electrolyte in the process of wohlwill to refine the gold of highest quality. It is also used in some procedures that are analytical. 

In some laboratories, it is used to clean the glassware of metal particles and organic compounds. When it comes to other traditional chromic acid baths to clean the NMR tubes, this method is more likely to be used because to stop the spectra from spoiling, no traces of paramagnetic chromium should be present. 

The high toxicity of chromium and the potential for explosions, chromic acid baths are not welcomed even though Aqua regia when mishandled, can be very corrosive. 

Careful neutralization should be done, when disposing Aqua regia, before pouring it in the basin. If the contamination is there because of the dissolved metals, the collection of neutralized solutions should be done for disposal. 

Acid that Dissolves Gold

There can be several metals in either an elemental form or ionic form. The basic form, solid gold, is uncharged to illustrate this lack of charge, chemists add a superscript "0" to the chemical symbol: for gold, this would be Au0.   Anything called an oxidizer can cause the electrons to be lost to the Au0 molecules, converting them into Au3+, a positively charged ion. Nitric acid is an example of a strong oxidizer which, when combined, can turn Au0 into Au3+. The positive gold ions then dissolve into the liquid solution and become a part of it. On its own, before the reaction enters balance, nitric acid can only dissolve a very tiny amount of the gold atoms into Au3+.

The hydrochloric acid separates into H+ and Cl- ions once the hydrochloric and nitric acids are combined in aqua regia. Due to the attraction of opposite ions, the Cl- ions can then respond to Au3+ to form AuCl4- (also called tetrachloro gold). Because the Au3+ is now part of the tetrachloro gold, the solution is removed. This disrupts the balance and enables the nitric acid to continue to dissolve more Au0 into Au3+. This is an example of the Le Chatelier Principle, which essentially shows that if a reaction's products are consumed, it allows the reaction to continue making more products. This is also known as aqua regia gold. This is one of the uses of Aqua Regia.

Acid that Dissolves Platinum

For gold, the oxidation reaction can be made by using nitric oxide or nitrogen dioxide as a nitrogen oxide product. You can write the same equation for platinum as well. The platinum ion which is oxidized reacts with chloride ions and gives chloroplatinate ions. 

Experimental evidence shows that it is considered more complicated when Aqua regia reacts with platinum. Initially, these reactions make a mixture of nitroso platinic chloride and chloroplatinic acid. The nitroso platinic chloride is a product which is solid in nature. 

If one desires to dissolve the platinum fully, extractions of the residual solid with hydrochloric acid ( concentrated) should be done, repeatedly. When the solution is saturated with chlorine while heating, the chloroplatinic acid can be oxidized to chloroplatinic acid. By dissolving platinum solids in Aqua regia, the discovery of the densest metals were made, that is iridium and osmium. They are both found in platinum ore and instead of dissolved by the acid, they were collected on the water base. 

Precipitation of Dissolved Platinum

When the purification of platinum group metals is done through dissolution in Aqua regia, the precipitation of gold is done by treating it with iron (II) chloride. Platinum in the filtrate, which is available as hexachloroplatinate (IV), gets converted to ammonium hexachloroplatinate by adding ammonium chloride in it. 

This ammonium salt is highly soluble and can be filtered easily. When heated strongly, it converts to platinum metal. 

By using elemental zinc, the un precipitated hexachloroplatinate (IV) is reduced. The recovery of platinum on a small scale from the laboratory residues, the same method is applied. 

Aqua Regia Handling

• Glass (preferably Pyrex) containers should always be used. Some plastics are melted by aqua regia and most metals are corroded. 

• Never store solutions of  aqua regia. Mix up only what you need, then after each use, destroy. 

• With the sash between you and the solution, mix the solution into a hood. Wear splash gloves, face shield, lab coat, and appropriate gloves for chemical splashes. 

• Always add the nitric acid to the hydrochloric acid slowly when preparing the aqua regia solution. 

• Toxic gasses are released by dissolving metals in aqua regia, always working with aqua regia in a fume hood. 

• The solution of Aqua regia is very energetic and potentially explosive. It is very likely that it will get hot, over 100 0C. Handle with care.

• The exothermic reaction will be accelerated by adding any acids or bases to Aqua Regia or spraying it with water. 

• Till it cools, leave the hot aqua regia solution in an open container. 

• Never store Aqua Regia in a container that is closed. Over time, it will oxidize to form toxic nitrosyl chloride, chlorine gasses, and nitrogen dioxide. This will create pressure in the container, probably causing an explosion to occur. 

• An explosion may occur by mixing aqua regia with organic compounds.

Did You know?

Do not store aqua regia. Owing to the oxidation of its reactive components, aqua regia quickly loses its effectiveness. For each use, blend a fresh solution. With sodium bicarbonate, excess solutions should be neutralized and disposed of via the drain, followed by flushing with large amounts of water.

The spent solution should be neutralized with sodium bicarbonate after the substance has cooled and disposed of through the drain, followed by flushing with plentiful quantities of water. The neutralized solution should be collected as a toxic waste if the solution is polluted with heavy metals (i.e. silver, chromium).