Nitric acid question

I have a question for the more chemically inclined out there in
never never land…as most do I suppose when we need to test for
gold we will pull out the acid test equipment, rub the stone and
apply acid…the diluted ones for 10, 14, 18, etc… and
sometimes apply right to object to see what occurs…

Now, I got fooled the other day because I suppose the nitric
acid in the 14 solution ( uh how do I phrase this …I am assuming
the nitric is diluted with water to create different strengths
for testing karatage…perhaps this is not so ) I am supposing the
water evaporated and made the nitric stronger …An acquaintance
of mine who buys scrap gold tells me he only uses nitric acid
undiluted to test and then goes by the color to tell approximate

Now the question…I asked him what was the strength of the
nitric and the answer I received was that it is full strength 100
percent nitric. I haven’t been able to locate any full strength
nitric and am wondering if this is accurate ? My references
don’t say how strong the nitric is ? John Burgess probably

Terry Parresol, Central Florida USA

Hello Terry Concentrated nitric acid is 69% HNO3 by mass. The
other 31% is water. In a closed bottle of concentrated nitric
acid, especially at slightly elevated temperatures, you may
notice a faint yellow-brown gas. Be very careful not to breathe
it since it is nitrogen dioxide.

In my experience chemical grade undiluted is as good as it gets,
try a chemical warehouse. Ask for pharmaceutical

“Full-strength” nitric acid is 70%. Working solutions are
usually based on volumn dilutions of the “full-strength.” A 20%
working solution is 20 ml of HNO3 “full-strength” poured into 80
ml H2O=100ml solution. This is a pretty strong solution,
especially with heat applied. Nitric usually works better with
dilution (give the ions some elbow room), but you should stick to
the textbook dilutions for karat testing. A light bulb might
work for a heat source for spot tests.

Always pour acid into water, not vise versa (acid runs to
water). Don’t hover over your work and breath the fumes.

  My references don't say how strong the nitric  (acid) is ?
John  Burgess probably knows? Terry Parresol, 

You rang milord? G’day; There are several ways of making nitric
acid and all involve concentrating the manufactured product by
distillation. 1. is by the oxidation of ammonia. 2. is by the
heating of a nitrate with sulphuric acid. 3. is by passing air
through a plasma produced by an electric arc (The arc is drawn
into a six foot diameter disc by powerful magnets and the arc is
at a temperature over 3000C.) The resultant gases pass into
water, then the acid is concentrated by distillation Distillation
is carried out in acid resistant vessels, often of Pyrex glass.
The resulting (evil) liquid is very difficult to concentrate
further, but the ultra pure acid has a boiling point of 86C. and
as it absorbs water like mad, is almost impossible to keep pure.
A concentrated solution of 68% boils at a temperature of 120.5C.
If one attempts to heat it further it loses acid and at lower
temperatures it loses water, so this is called a constant boiling
point liquid, which is probably what laboratory pure, reagent
grade nitric acid is. The commercial grade of nitric acid is
probably less concentrated than this, but there are no criteria
for it.

There is a further grade of nitric acid called, fuming nitric
acid - as it’s name suggests it gives off a constant brown fume
which is extremely corrosive and toxic. However, it is unlikely
that jewellers will want to have much to do with it. It is used
in the preparation of certain chemicals and believe me - is a
particularly nasty substance. But then, ‘ordinary’ conc.
nitric acid isn’t very benign; it will ‘eat’ a cork inside an
hour or so and a rubber bung inside a day. It used to be
exclusively stoppered with glass stoppers, but certain plastics
are reasonably inert to it and are so used these days.

Having said all that, I don’t know much about the determination
of the purity of gold by the touchstone / acid method, (I could
never afford the gold feathers essential to the method) but
always understood that it was ordinary concentrated nitric acid
which was used In the old days, it used to be called ‘Spirit
of Nitre’ because it was the ‘spirit’ that distilled off a
mixture of sulphuric acid and sodium nitrate. (Chile saltpetre)

Can you imagine the fun when some clot drops a 2.5 litre bottle
(half a gallon) of it on the floor? The aforesaid clot is rushed
under the lab shower (and someone sent home to get him a fresh
pair or trousers… It was always a him) Then about 10 pounds
of baking soda would have to be poured onto the still fuming
acid (even ordinary nitric acid fumes like mad) and the resultant
solid swept up and disposed of with the soda-pourers/sweepers
wearing respirators… Being Lab Safety Officer, it was
always me who got the good jobs like that; one dare not trust
assistants or the students. But it wasn’t my job to try and
disguise the permanent black stain on the once beautiful floor.

Having told you far more about nitric acid than you ever wanted
to know, be aware that the subject isn’t exhausted - not by a
long way. Phew!! and cheers, –

       / /
      / /
     / /__|\
    (_______)   Long retired in sunny temperate Mapua, NZ but still uses

conc nitric acid on occasion.

Terry, you might consider giving Bryant Laboratory a call,
1-800-367-3141. They are a lab glassware and chemical supplier
in Berkeley, CA, and they are very artist-friendly. (They cater
to bronze sculpture/patina people.) What you want is Nitric Acid,
ACS Reagent grade. Usually reagent grade acids won’t be “100%”,
but they will be as strong as is commercially available.

Hope this helps.
Rene Roberts

P.S. Don’t know your experience with acids, but if you ever do intend to
dilute it, always add acid to water, never water to acid.

Now the question…I asked him what was the strength of the
nitric and the answer I received was that it is full strength 100
percent nitric. I haven’t been able to locate any full strength
nitric and am wondering if this is accurate ? My references
don’t say how strong the nitric is

Nitric acid is a solution of NO (a gas) in water. You can’t
have something like that and call it full strength. But you can
specify a specific gravity/density, or a molar value, among other
things. As the acid concentration gets higher, the degree to
which the gas comes back out of solution increases, so stronger
acid will tend to “fume”. A waste of good air…

Generally, the strength you get depends on the “grade” you buy.
Reagent grade, industrial grade, etc. The reagent or electronics
grade acid is generally the strongest you will find sold. That
is probably what your friend was referring to as 100%.

If I have nitric acid at 74% and want to etch sterling silver,
what % of water would I use to the acid? I was experimenting with
nitric acid several years ago, on my formica, white, kitchen
counter (not knowing the correct ratio, water to acid—just
guessing to see how fast it etched) and although I tried to be
very careful not to make a mess, some drops fell on the counter
when I moved the object from the acid to the baking soda
solution. In a very short time they showed up violet, in color,
on the counter top. Ok, I thought, I’ll just bleach them out.
The bleach seemed to do it for a while and then there they were
again!! I’ve been bleaching them for all these years and just
like an apparition they keep reappearing!!! That was the end of
that experiment and the remaining acid is still sitting on a
shelf in my studio. Before I try again, I want to know what I’m
doing. I have a great “respect” for 74% nitric acid. Dolores New

Don’t forget to request MSDA sheet from supplier when purchasing
nitric acid (and other chemicals we use)



Let me tell you just how foolish I am sometimes. …I should
tell you about my chemistry adventure 40 years ago. I decided
to make nitric acid because being just a boy (14 I think),
no-one wanted to sell me a bottle. I needed some to experiment
with nitrocellulose, guncotton, and maybe make “flashpaper” the
fireballs you see magicians toss in the air that vanish “in a
flash” Now I was really into this chemistry thing, read all the
books I could find and understand, knew the properties of many
elements, had all the lab apparatus, pyrex, condensers, several
types of rubber gloves, and I really wasn’t going about this
half baked. I got the pottasium nitrate and other supplies
needed and indeed made some pretty potent nitric acid.

But as I was cleaning up one day in the laundry tub, I felt the
slightest drop of acid hit my arm, right in the bend, the soft
tender skin. I had baking soda and water on it within seconds,
flushed for 15 or more minutes. Couldn’t have been but a
pinhead size drop.

Well it burned a NASTY hole in my arm the size of a nickel.
This was a festering, deep, ugly wound that took more than a
month to heal, maybe two, and for twenty years I looked at the
scar that drop left in my arm. It’s smaller now. I still work
with all types of acids and dangerous materials. But I (thank
God) haven’t made another mistake like that one (oh maybe some
but we tend to easily forget that stuff) since.

Just to let you know that your post gave me the willies because
you shouldn’t handle acids if you tend to spill a drop or two
here and there. Not a flame by any means, but you gave me an
opportunity to tell my story in the hopes that others who may
not be FULLY aware of the dangerous equipment and supplies we
work with, take heed. It can easily be taken too lightly.
Gloves? eye protection? backup thoughts in the event of a
mistake? Fifty six hundred degree torch flame anyone?

John g

Dolores, your “great respect for 75% nitric acid” is a good
place to start!

To etch silver with nitric acid, you need about a 10% - 25%
solution, by volume. I suggest starting on the weak side until
you see what the stuff does. To dilute, always add acid slowly to
water, never add water to acid. This is very important! (When you
dilute a strong acid, a lot of heat is generated. If you add the
water to the acid, it doesn’t really want to flow into the acid,
and a lot of heat will be generated very quickly at the interface
of the two solutions. It could easily boil or spatter.) Have
baking soda on hand to neutralize any spills. Wear rubber gloves,
rubber apron, and above all, eye protection. Work outside or
under positive ventilation, because as your piece etches, the
nitric will give off nitrous oxide which is very nasty stuff.
Your etch time will be around a half hour, depending on strength
of acid, depth of bite, etc. Neutralize your silver in baking
soda. Just take it slowly.

Good luck (and don’t forget to add the acid to the water)!

Rene Roberts

A good discussion of the uses of nitric acid, but I haven’t seen
anyone say what happens when you’re done with it. I’m sure you
don’t pour it down the drain, but what’s the correct method of
disposal? If you do a lot of silver etching, of course, the
silver is easily reclaimed, so you can probably find a reclaimer
to take it.


     When using Nitric Acid to etch silver... Work outside or
under positive ventilation, because as your piece etches, the
nitric will give off nitrous oxide which is very nasty stuff. 

I’m not trying to nit pick, but this could be a safty issue.
The gas given off by this process is not Nitrous Oxide (laughing
gas), but Nitrogen Dioxide(Icky gas that will make you sick).
Just want to make sure misdoesn’t turn into an


Good point, Michael . Thanks for the correction. You’re right,
one is harmless, one is deadly.

Well Al, I was going to mention about disposal but decided not
to because my views on proper disposal of these chemicals is
somewhat different from most other Orchid members, given the
responses to “disposal” issues of pickle and nitric acid in the
last few months.

Some people here feel there is little in these solutions that’s
really “harmful” when diluted, and the sewer treatment facilities
will remove it anyway. About nitric acid, someone here suggested
it could be diluted to about 10% and used in the garden to water
acid-loving plants. I suppose it could.

My personal opinion is that these chemicals, and the metals in
them, don’t belong on the ground or in the sewer system. (I also
am an organic gardener, so that might help explain my bias.)
Also, I’m not on a city sewer system, so “pouring it down the
drain” here means it goes into the groundwater or the stream
below me via the septic system. I am less concerned about the
acid than I am the metals. And even sewerage treatment systems
don’t remove 100% of the metals. Also consider that the effluent
needs to go somewhere. Often it’s used to water crops, or it’s
dumped into rivers. Or the ocean. (So much for the soap box.)

I take everything to the toxic waste disposal site that is run
by the local sanitation company. I dilute nitric acid down to
below 10% and then neutralize it with baking soda. Yeah, it’s a
mess, but the volume is really pretty small, so I just put it in
a large container like a bucket so I won’t get fizz all over
everything, take it outside and gradually dump the baking soda
into it. Just a little at a time, waiting each time for the froth
to die down. I use litmus paper to test the pH. When it’s
neutralized, I bottle the waste in a gallon plastic bottle with a
sturdy screwtop lid. It really is no big deal to do this, just a
little inconvenient.

As I said, there are others here who completely disagree about
the necessity of doing this (and have even gotten a little
sarcastic about it in the past). I think we will just have to
agree to disagree.

Rene Roberts

you neutralize it with baking soda, until there are no more
bubbles and the solution is blue. Then it is safe to pour down
the drain.

Blue? The products should be sodium nitrate and silver nitrate,
which aren’t blue. Are you adding a Ph indicator?

Anyway, if the nitric has been used for etching, the solution
will contain heavy metals, not suitable for pouring down the


Blue? The products should be sodium nitrate and silver nitrate,
which aren’t blue. However the copper nitrate is blue, remember
that Sterling Silver is 7 1/2 percent copper

Anyway, if the nitric has been used for etching, the solution
will contain heavy metals, not suitable for pouring down the
drain. A good point the silver is considered as a hazardous
material as some silver compounds are bio toxins. While the
silver could be precipitated out by the addition of sodium
chloride (table salt) I am not sure that the resultant liquid
would be dumpable in your local. I would suggest that you
discuss with your local waste water utility how much and the
nature of the solutions you are thinking of poring down the drain.
They should know whether there system would dandle the proposed