Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org Gold & Silver Coatings By A . T . Kakhia 1 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org 2 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org Part One General Knowledge 3 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org 4 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org Aqua Regia ( Royal Acid ) Freshly prepared aqua regia is colorless, Freshly prepared aqua but it turns orange within seconds. Here, regia to remove metal fresh aqua regia has been added to these salt deposits. NMR tubes to remove all traces of organic material. Contents 1 Introduction 2 Applications 3 Chemistry 3.1 Dissolving gold 3.2 Dissolving platinum 3.3 Reaction with tin 3.4 Decomposition of aqua regia 4 History 1 - Introduction Aqua regia ( Latin and Ancient Italian , lit. "royal water"), aqua regis ( Latin, lit. "king's water") , or nitro – hydro chloric acid is a highly corrosive mixture of acids, a fuming yellow or red solution. The mixture is formed by freshly mixing concentrated nitric acid and hydro chloric acid , optimally in a volume ratio of 1:3. It was named 5 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org so because it can dissolve the so - called royal or noble metals, gold and platinum. However, titanium, iridium, ruthenium, tantalum, osmium, rhodium and a few other metals are capable of with standing its corrosive properties. IUPAC name Nitric acid hydro chloride Other names aqua regia , Nitro hydrochloric acid Molecular formula HNO3 + 3 H Cl Red , yellow or gold Appearance fuming liquid 3 Density 1.01–1.21 g / cm Melting point − 42 °C Boiling point 108 °C Solubility in water miscible in water Vapor pressure 21 mbar 2 – Applications Aqua regia is primarily used to produce chloro auric acid, the electrolyte in the Wohl will process. This process is used for refining highest quality (99.999 %) gold. Aqua regia is also used in etching and in specific analytic procedures. It is also used in some laboratories to clean glassware of organic compounds and metal particles. This method is preferred over the "traditional" chromic acid bath for cleaning NMR tubes, because no traces of paramagnetic chromium can remain to spoil spectra.[3] While chromic acid baths are discouraged because of the high toxicity of chromium and the potential for explosions, aqua regia is itself very corrosive and has been implicated in several explosions due to mishandling. Due to the reaction between its components resulting in its decomposition , aqua regia quickly loses its effectiveness ( yet 6 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org remains a strong acid ) , so its components are usually only mixed immediately before use. While local regulations may vary , aqua regia may be disposed of by careful neutralization , before being poured down the sink. If there is contamination by dissolved metals , the neutralized solution should be collected for disposal . 3 – Chemistry 3 – 1 - Dissolving gold Pure gold precipitate produced by the aqua regia chemical refining process Aqua regia dissolves gold, though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions (Au3+). The hydrochloric acid provides a ready supply of chloride ions (Cl−), which react with the gold ions to produce tetra chloridoaurate (III) ion anions, also in solution. The reaction with hydro chloric acid is an equilibrium reaction which favors formation of chloro aurate anions − (AuCl4 ). This results in a removal of gold ions from solution and allows further oxidation of gold to take place. The gold dissolves to become chloro auric acid. In addition, gold may be dissolved by the free chlorine present in aqua regia. Appropriate equations are: Au (s) + 3 NO −3 (aq) + 6 H+ (aq) → 3 + Au (aq) + 3 NO2 (g) + 3H2O (l) and Au 3 + (aq) + 4 Cl−(aq) → Au Cl −4 (aq). 7 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org The oxidation reaction can also be written with nitric oxide as the product rather than nitrogen dioxide : Au (s) + NO−3 (aq) + 4 H+(aq) → 3+ Au (aq) + NO (g) + 2 H2O (l). If the aqua regia solution only contains gold, solid tetra chloro auric acid may be prepared by boiling off excess aqua regia, and removing residual nitric acid by repeatedly heating with hydro chloric acid. That step reduces nitric acid . If elemental gold is desired, it may be selectively reduced with sulfur dioxide , hydrazine, oxalic acid, etc. 3 – 2 - Dissolving platinum Platinum being dissolved in aqua regia Similar equations can be written for platinum. As with gold, the oxidation reaction can be written with either nitric oxide or nitrogen dioxide as the nitrogen oxide product. −3 + Pt (s) + 4 NO (aq) + 8 H (aq) → 4+ Pt (aq) + 4 NO2 (g) + 4 H2O (l) 3Pt (s) + 4 NO−3 (aq) + 16 H+ (aq) → 4+ 3Pt (aq) + 4 NO (g) + 8 H2O (l) The oxidized platinum ion then reacts with chloride ions resulting in the chloro platinate ion. 4+ − −6 Pt (aq) + 6 Cl (aq) → PtCl2 (aq) 8 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org Experimental evidence reveals that the reaction of platinum with aqua regia is considerably more complex. The initial reactions produce a mixture of chloro platinous acid (H2PtCl4) and nitroso platinic chloride ((NO)2PtCl4). The nitroso platinic chloride is a solid product. If full dissolution of the platinum is desired, repeated extractions of the residual solids with concentrated hydrochloric acid must be performed. 2Pt (s) + 2HNO3 (aq) + 8 HCl (aq) → (NO)2PtCl4 (s) + H2PtCl4 (aq) + 4 H2O (l) (NO)2PtCl4 (s) + 2 HCl (aq) H2PtCl4 (aq) + 2 NOCl (g) The chloro platinous acid can be oxidized to chloro platinic acid by saturating the solution with chlorine while heating. H2PtCl4 (aq) + Cl2 (g) → H2PtCl6 (aq) Dissolving platinum solids in aqua regia was the mode of discovery for the most dense metals, iridium and osmium, both of which are found in platinum ore and will not be dissolved by the acid, instead collecting on the base of the vessel. As a practical matter, when platinum group metals are purified through dissolution in aqua regia, gold (commonly associated with PGMs) is precipitated by treatment with iron (II) chloride. Platinum in the filtrate, as hexa chloro platinate (VI), is converted to ammonium hexa chloro platinate by the addition of ammonium chloride. This ammonium salt is extremely insoluble, and it can be filtered off. Ignition (strong heating) converts it to platinum metal : 3 (NH4)2PtCl6 → 3 Pt + 2 N2 + 2 NH4Cl + 16 HCl Un precipitated hexa chloro platinate (IV) is reduced with elemental zinc, and a similar method is suitable for small scale recovery of platinum from laboratory residues. 9 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org 3 – 3 - Reaction with tin Aqua regia reacts with tin to form tin (IV) chloride, containing tin in its highest oxidation state: 4 HCl + 2 HNO3 + Sn → SnCl4 + NO2 + NO + 3 H2O 3 – 4 - Decomposition of aqua regia Upon mixing of concentrated hydrochloric acid and concentrated nitric acid , chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine as evidenced by the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, the aqua regia loses its potency. HNO3 (aq) + 3 HCl (aq) → NOCl (g) + 2[Cl] (g) + 2 H2O (l) Nitrosyl chloride can further decompose into nitric oxide and chlorine. This dissociation is equilibrium - limited. There fore , in addition to nitrosyl chloride and chlorine, the fumes over aqua regia contain nitric oxide. 2 NOCl (g) → 2 NO (g) + Cl2 (g) Because nitric oxide reacts readily with atmospheric oxygen, the gases produced also contain nitrogen dioxide, NO2. 2 NO (g) + O2 (g) → 2 NO2 (g) 4 – History Aqua regia first appeared in the work of medieval European alchemist Pseudo - Geber, dating from the 14th century. The third of Basil Valentine’s keys shows a dragon in the foreground and a rooster eating a fox eating a rooster in the background. The rooster symbolizes gold (from its association with sunrise and the sun’s association with gold), and the fox represents aqua regia. The repetitive dissolving, heating, and re dissolving ( the rooster eating the fox eating the rooster ) leads to the buildup of chlorine gas in the 11 Copyright © Tarek Kakhia. All rights reserved. http://tarek.kakhia.org flask. The gold then volatilizes in the form of gold chloride, whose red crystals were known as dragon’s blood. The reaction was not reported in modern chemical literature until 1890. Antoine Lavoisier called aqua regia nitro - muriatic acid in 1789. When Germany invaded Denmark in World War II, Hungarian chemist George de Hevesy dissolved the gold Nobel Prizes of German physicists Max von Laue (1914) and James Franck (1925) in aqua regia to prevent the Nazis from confiscating them. The German government had prohibited Germans from accepting or keeping any Nobel Prize after jailed peace activist Carl von Ossietzky had received the Nobel Peace Prize in 1935. De Hevesy placed the resulting solution on a shelf in his laboratory at the Niels Bohr Institute. It was subsequently ignored by the Nazis who thought the jar — one of perhaps hundreds on the shelving — contained common chemicals. After the war, de Hevesy returned to find the solution undisturbed and precipitated the gold out of the acid. The gold was returned to the Royal Swedish Academy of Sciences and the Nobel Foundation. They re - cast the medals and again presented them to Laue and Franck.