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) + Cl₂ (g) + 2 H₂O (l)

Nitrosyl chloride can further decompose into nitric oxide and chlorine. This dissociation is equilibrium-limited. Therefore, in addition to nitrosyl chloride and chlorine, the fumes over aqua regia contain nitric oxide.

2 NOCl (g) → 2 NO (g) + Cl₂ (g)

History

Hydrochloric acid was first discovered around the year 800 by the alchemist Abu Musa Jabir ibn Hayyan (Geber) by mixing common salt with vitriol (sulfuric acid). Jabir's invention of gold-dissolving aqua regia, consisting of hydrochloric acid and nitric acid, contributed to the effort of alchemists to find the philosopher's stone.^[3]

When Germany invaded Denmark in World War II, the Hungarian chemist George de Hevesy dissolved the gold Nobel Prizes of Max von Laue and James Franck into aqua regia to prevent the Nazis from stealing them. He 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 who recast the medals and again presented them to Laue and Franck

Cuprates. High Temperature Superconductors (hts)

Yttrium barium copper oxide, often abbreviated YBCO, is a crystalline chemical compound with the formula YBa₂Cu₃O₇. This material, a famous "high-temperature superconductor", achieved prominence because it was the first material to achieve superconductivity above the boiling point of nitrogen.

History

In April 1986 (seventy-five years after the discovery of superconductivity in 1911), Georg Bednorz and Karl Müller, working at IBM in Zurich, discovered that certain semiconducting oxides became superconducting at the then relatively high temperature of 35 K. In particular, the lanthanum barium copper oxides, an oxygen deficient perovskite-related material, proved promising. In 1987, Bednorz and Müller were jointly awarded the Nobel Prize in Physics for this work.

Building on that, Maw-Kuen Wu and his graduate students, Ashburn and Torng at the University of Alabama in Huntsville in 1987, and Paul Chu and his students at the University of Houston in 1987 (see superconductor page for info), discovered YBCO had a T_c of 93 K. (The first samples were Y_1.2Ba_0.8CuO₄.) Their work led to a rapid succession of new high temperature superconducting materials, ushering in a new era in material science and chemistry.

YBCO was the first material to become superconducting above 77 K, the boiling point of nitrogen. All materials developed before 1986 became superconducting only at temperatures near the boiling points of liquid helium or liquid hydrogen (T_b = 20.28 K) - the highest being Nb₃Ge at 23 K. The significance of the discovery of YBCO is the much lower cost of the refrigerant used to cool the material to below the critical temperature.