The yellow solution of Gold dissolved in the Aqua Regia was added to Tin dissolved in Hydrochloric Acid. The formation of a purple precipitate (Purple of Cassius) is a positive test that gold was in the solution.
A few drops of Aqua Regia were placed in a test tube and heated with a pellet of Potassium Hydroxide. A strip of damp red litmus paper was placed on the tube, which turned blue, indicating a base’s vapors. There was also the smell of Ammonia. Both these tests are positive that an Ammonium Salt is present in the Aqua Regia.
The solution in the test tube above was heated to ensure the complete expulsion of Ammonia. This solution was allowed to cool and acidified with dilute Nitric Acid to neutralize the Potassium Hydroxide. A few drops of a solution of 2% Silver Nitrate were added to the test tube with the formation of a curd white precipitate (left). This precipitate was washed with distilled water and allowed to settle (center). The water was pipetted off, and to the damp precipitate, a few drops of concentrated Ammonium Hydroxide were added (right). The precipitate dissolved, proving the presence of Chlorine.
Potassium and Sodium are elements that are difficult to analyze by wet analytical methods because their salts are mostly soluble. The presence of these elements can be detected by saturating a piece of asbestos with an unknown solution and placing it into a flame. Potassium colors the flame violet (left). The sample was taken from the distilled Aqua Regia, showing the presence of Potassium in the acid.
Because Sodium is a common contaminant and imparts a yellow-orange color to the flame in extremely low concentrations, it is often difficult to see the presence of other elements that would impart color to the flame. In this case, it is helpful to use a Bunsen spectroscope to determine the presence of other elements.
Several distillations were carried out using Pyrex test tubes with smaller quantities of reactants. The salt in the bottom of the test tube was removed and analyzed. The salt is very soluble in water. It is neutral to red and blue litmus paper. The solution of the salt is positive for Chlorine. The brown gas in the upper portion of the tube is Nitrogen Dioxide, NO2.
1. The dry distillation of Ammonium Chloride, NH4Cl, and Potassium Nitrate, KNO3, forms Nitric Acid and Water.
2. The Nitric Acid also contains Chlorine probably in the form of Ammonium Chloride, which excess, sublimes from the reaction.
3. The distilled Aqua Regia is positive for Ammonia, reinforcing #2 above.
4. The Aqua Regia also contains Potassium. This may have volatilized with an Ammonia compound.
5. The Ammonium Chloride and the Nitric Acid form an Aqua Regia because it dissolves Gold. Typical formulations of Aqua Regia are combinations of Hydrochloric Acid and Nitric Acid, commonly in the ratio of 3:1.
6. The salt remaining in the test tube is white, soluble in water, neutral, and contains Chlorine; and is therefore Potassium Chloride.
7. Potassium Nitrate (saltpeter), which is one of the reactants, loses monatomic Oxygen (O) at 400 ° C and forms Potassium Nitrite (KNO2). Monatomic oxygen is a highly reactive species and oxidizes the Ammonium Chloride (sal ammoniacum) into a Hydrogen-Nitrogen-Oxygen compound and Chlorine. The Chlorine combines with the Potassium Nitrite forming Potassium Chloride and Nitrogen Dioxide (NO2). The Nitrogen Dioxide and the Hydrogen-Oxygen-Nitrogen compound form Nitric Acid (HNO3) and water (H2O). Excess Ammonium Chloride sublimes into the acid.
8. The Aqua Regia prepared by this method contains Salts of Ammonia. Although Gold will dissolve in this acid, making the solution alkaline will produce Fulminating Gold.
“Fulminating Gold does not cause a Transmutation. Fulminating Gold causes an Explosion!” – Anonymous
But the proceedings to obtain Aqua Regia are slightly different when described by real ancient chemists. For example, Nicolas Lemery dedicates numerous pages of his Cours de la Chimie to this topic. Se my post Lemery & Aqua Regia First Step: Sal Armoniac or the conclusive one at Aqua Regia and Fulminating Gold According to Lemery .