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
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, –
/ / Johnb@ts.co.nz
(_______) Long retired in sunny temperate Mapua, NZ but still uses
conc nitric acid on occasion.