How
to Make Aqua Regia Acid Solution
By Anne Marie Helmenstine, Ph.D.
Aqua regia is an
extremely corrosive mixture of nitric and hydrochloric acid, used as an
etchant, for some analytical chemistry procedures, and to refine gold.
Aqua regia dissolves
gold, platinum, and palladium, but not the other noble metals.
Here's what you need
to know to prepare aqua regia and use it safely.
Fast Facts: Aqua
Regia
Aqua regia is a
corrosive acid mixture made by combining nitric acid and hydrochloric acid.
The usual ratio of
acids is 3 parts hydrochloric acid to 1 part nitric acid.
When mixing the
acids, it is important to add the nitric acid to the hydrochloric acid and not
the other way around.
Aqua regia is used to
dissolve gold, platinum, and palladium.
The acid mixture is
unstable, so it is usually prepared in small amounts and used immediately.
Reaction to Make Aqua
Regia
Here is what happens
when nitric acid and hydrochloric acid are mixed:
HNO3 (aq) + 3HCl (aq)
→ NOCl (g) + 2H2O (l)
+ Cl2 (g)
Over time, nitrosyl
chloride (NOCl) will decompose into chlorine gas and nitric oxide (NO).
Nitric acid
auto-oxidizes into nitrogen dioxide (NO2):
2NOCl (g) → 2NO (g) + Cl2 (g)
2NO (g) + O2 (g) → 2NO2(g)
Nitric acid (HNO3),
hydrochloric acid (HCl), and aqua regia are strong acids.
Chlorine (Cl2),
nitric oxide (NO), and nitrogen dioxide (NO2) are toxic.
Aqua Regia Safety
Aqua regia
preparation involves mixing strong acids.
The reaction produces
heat and evolves poisonous vapors, so it's important to follow safety protocols
when making and using this solution:
Make and use aqua
regia solution inside a fume hood, with the sash down as much as is practical
to contain the vapors and protect against injury in case of splashing or
glassware breakage.
Prepare the minimum
volume needed for your application.
Make sure your
glassware is clean. In particular, you don't want any organic contaminants
because they can produce a vigorous or violent reaction.
Avoid using any
glassware that may be contaminated with a chemical containing a C-H bond.
Do not use the
finished solution on any material containing an organic.
Wear safety goggles.
Wear a lab coat.
Wear gloves.
If you get drops of
any of the strong acids on your skin, wipe them off immediately and rinse with
lots of water.
If you spill acid on
clothing, remove it immediately.
In the case of
inhalation, move immediately to fresh air.
Use the eyewash and
seek emergency medical attention in case of eye contact.
In the case of
ingestion, rinse the mouth with water and do not induce vomiting.
Neutralize any spills
with sodium bicarbonate or similar compound.
Remember, it's best
to neutralize a strong acid with a weak base and not a strong base.
Prepare Aqua Regia
Solution
The usual molar ratio
between concentrated hydrochloric acid and concentrated nitric acid is HCl:HNO3
of 3:1.
Keep in mind,
concentrated HCl is about 35%, while concentrated HNO3 is about 65%, so the
volume ratio is usually 4 parts concentrated hydrochloric acid to 1 part
concentrated nitric acid.
A typical total final
volume for most applications is only 10 milliliters. It's unusual to mix up a
large volume of aqua regia.
Add the nitric acid
to the hydrochloric acid. Do not add hydrochloric to nitric!
The resulting
solution with be a fuming red or yellow liquid.
It will smell
strongly of chlorine (although your fume hood should protect you from this).
Dispose of leftover
aqua regia by pouring it over a large amount of ice.
This mixture may be
neutralized with a saturated sodium bicarbonate solution or 10% sodium
hydroxide.
The neutralized
solution may then be safely poured down the drain. The exception is used
solution that contains heavy metals.
A heavy
metal-contaminated solution needs to be disposed of according to your local
regulations.
Once you have prepared
aqua regia, it should be used when it's fresh. Keep the solution in a cool
location.
Do not store the
solution for an extended length of time because it becomes unstable.
Never store stoppered
aqua regia because pressure build-up could break the container.
Another potent acid
solution is called "chemical piranha."
If aqua regia isn't
suitable for your needs, piranha solution may be what you need.
Anne
Marie Helmenstine, Ph.D.
Chemistry
Expert
Education
Ph.D.,
Biomedical Sciences, University of Tennessee at Knoxville
B.A.,
Physics and Mathematics, Hastings College
Introduction
Ph.D. in
biomedical sciences from the University of Tennessee at Knoxville - Oak Ridge
National Laboratory.
Science
educator with experience teaching chemistry, biology, astronomy, and physics at
the high school, college, and graduate levels.
ThoughtCo
and About Education chemistry expert since 2001.
Widely-published
graphic artist, responsible for printable periodic tables and other
illustrations used in science.
Experience
Anne
Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education
since 2001, and other sciences since 2013. She taught chemistry, biology,
astronomy, and physics at the high school, college, and graduate levels. She
has worked as a research scientist and also abstracting and indexing diverse
scientific literature for the Department of Energy.
In
addition to her work as a science writer, Dr. Helmenstine currently serves as a
scientific consultant, specializing in problems requiring an interdisciplinary
approach. Previously, she worked as a research scientist and college professor.
Education
Dr.
Helmenstine holds a Ph.D. in biomedical sciences from the University of
Tennessee at Knoxville and a B.A. in physics and mathematics with a minor in
chemistry from Hastings College. In her doctoral work, Dr. Helmenstine
developed ultra-sensitive chemical detection and medical diagnostic tests.
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https://www.thoughtco.com/making-aqua-regia-acid-solution-603641
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