Aqua regia is a highly corrosive mixture of hydrochloric acid and nitric acid. The compound is so named because it dissolves "royal" or noble metals such as gold and platinum. The formula for aqua regia is usually a 1:3 ratio of nitric acid to hydrochloric acid.
To make aqua regia, do the following steps:
1. First put on the appropriate safety equipment including gloves, safety goggles, and lab coat.
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2. Measure the amount of hydrochloric acid in a glass or plastic container. To avoid dangerous splashing and possible heat generation, pour the acid into the water, not the other way around.
3. Add the appropriate amount of nitric acid to the hydrochloric acid. The ratio of nitric acid to hydrochloric acid can be varied depending on the desired strength of the mixture.
4. Mix HCL or HNO3 thoroughly. The resulting mixture may be yellow or orange in color and will emit noxious fumes.
Note- This mixture is a very strong acid and can cause serious injury if not handled properly. It should only be used by trained professionals in a proper laboratory with appropriate safety precautions.
Properties of Aqua Regia
Formula: HNO₃ + 3HCl
Boiling Point: 108 °C
Melting point: -42 °C
Vapor pressure: 2.1Kpa
Density: 1.21 g/cm³
ChemSpider ID: 56437
Solubility in Water: Soluble in water
Aqua Regia Reaction with Gold
Aqua regia is a powerful acid that also dissolves gold, which is why it is often used in precious metal purification and analysis. The reaction between aqua regia and gold consists of several steps which are as follows:
Hydrochloric acid in aqua regia reacts with gold to form gold chloride (AuCl3) and hydrogen gas (H2).
Au + 3HCl → AuCl3 + 3H2
Nitric acid present in aqua regia reacts with gold chloride to form nitrosyl chloride (NoCl) and regenerate hydrochloric acid:
AuCl3 + 4HNO3 → 3HCl + Au(NO3)4 + 4NO2
A single reaction can be written as, the net reaction between aqua regia and gold:
Au + 3HNO3 + 4HCl → AuCl3 + 3NO2 + 4H2O
The gold chloride formed in the first step of the reaction is soluble in water and is separated from the insoluble residue by filtration. The gold is then separated from the chloride solution by various methods.
Aquaregia Reaction with Platinum Equation
The reaction between platinum and aqua regia is similar to that of gold, as platinum is also a noble metal that can be dissolved by aqua regia.
Pt + 4HNO3 + 6HCl → H2[PtCl6] + 4NO2 + 4H2O
In this reaction, hydrochloric acid and nitric acid in aqua regia react with platinum to form hexachloroplatinate(IV) acid, also known as chloroplatinic acid, and nitric oxide gas and water.
