[ Pobierz całość w formacie PDF ]

former Chilean saltpeter mining
communities of Humberstone and Santa
decomposes at 653 K (380
Boiling point
Laura were declared Unesco World
°C)
Heritage sites in 2005.
Density 2.3 ×103 kg/m3
Crystal
Chile still has the largest reserves of
?
structure
caliche, with active mines in such
locations as Pedro de Valdivia, Maria
Solubility 92 g in 100mL water
Elena and Pampa Blanca. Sodium nitrate,
Thermochemistry
potassium nitrate, sodium sulfate and
"fH0liquid -452 kJ/mol
iodine are all obtained by the processing
of caliche.
"fH0solid -468 kJ/mol
S0solid 117 J/mol·K
Sodium nitrate is also manufactured
Safety
synthetically by reacting nitric acid with
soda ash. May cause gastroenteritis
Ingestion
and abdominal pains.
The compound has antimicrobial
Inhalation respiratory irritation
properties when used as a food
Skin May cause irritation.
preservative. It is found naturally in leafy
green vegetables. It has possible health Eyes May cause irritation.
benefits for increasing oxygen to blood, as
More info MSDS
well as known health side effects in
SI units were used where possible. Unless otherwise
particular at high doses.
stated, standard conditions were used.
It can be used in the production of nitric
Disclaimer and references
acid by combining with sulfuric acid and
subsequent separation through fractional distillation of the nitric acid, leaving behind a
residue of sodium bisulfate.
Aqua regia
Aqua regia (Latin for "royal water") is a highly corrosive, fuming yellow liquid, formed
by a fresh mixture of concentrated nitric acid (otherwise known as aqua fortis) and
concentrated hydrochloric acid, usually in the ratio of one to three. It is one of the few
reagents able to dissolve gold and platinum. It was so named because it can dissolve the
so-called royal, or noble metals, although tantalum, iridium, and a few other extremely
passive metals are able to withstand it. Aqua regia is used in etching and in certain
analytic procedures. Aqua regia does not last very long; thus, it has to be mixed
immediately before use.
How it works
Aqua regia works to dissolve gold, even 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 tiny (virtually undetectable) amount of gold,
forming gold ions. The hydrochloric acid provides a ready supply of chloride ions, which
react with the gold to produce gold chloride, also in solution, but far to the right in
equilibrium. This allows further oxidation of gold to take place, and so, the gold is
dissolved. Appropriate equations are:
Au(s) + 3NO3-(aq) + 6H+(aq) Ì! Au3+(aq) + 3NO2(aq) + 3H2O(l)
Au3+(aq) + 4Cl-(aq) + H+(aq) Ì! HAuCl4 (aq)
A similar set of reactions occurs with platinum:
Pt(s) + 4NO3-(aq) + 8H+(aq) Ì! Pt4+(aq) + 4NO2-(aq) + 4H2O(l)
Pt4+(aq) + 6Cl-(aq) + 2H+(aq) Ì! H2PtCl6 (aq)
History
Hydrochloric acid was first discovered around the year 800 by the Islamic alchemist Jabir
Ibn Hayyan (Gaber), 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.
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. After the war, he returned to find the solution
undisturbed and precipitated the gold out of the acid. The Nobel Society then recast the
Nobel Prizes using the original gold. [ Pobierz całość w formacie PDF ]