[Histonet] Cleaning Acid (Long)... Aqua Regia Correction?

John A. Kiernan jkiernan <@t> uwo.ca
Tue Apr 26 16:12:59 CDT 2005

Sorry, my mistake. Well spotted. Aqua regia
is, of course, 3 volumes of hydrochloric and
one volume of nitric acid.
(I have corrected it also in the quoted email
below, just for posterity.)
John A. Kiernan
Department of Anatomy and Cell Biology
The University of Western Ontario
London,   Canada   N6A 5C1
"Due, Brice" wrote:
> Hello John, as always, thank you for sharing your knowledge. below you list aqua
> regia as 3:1 nitric:hydrochloric. I have more often seen it as 1:3
> nitric:hydrochloric, and this is what I've used before. Was this a typo or can
> it go both ways?
> Thanks,
> -brice
> Neuropathology Lab
> Brigham & Women's Hospital,
> Boston
> -----Original Message-----
> From: histonet-bounces <@t> lists.utsouthwestern.edu
> [mailto:histonet-bounces <@t> lists.utsouthwestern.edu]On Behalf Of John A.
> Kiernan
> Sent: Tuesday, April 26, 2005 12:29 PM
> To: Scott, Allison D
> Cc: 'histonet <@t> lists.utsouthwestern.edu'
> Subject: Re: [Histonet] Cleaning Acid (Long)
> An answer to a question asked last week about
> cleaning glassware for a silver staining method.
> Skip to the bottom line if you don't want to
> know the reasons.
> Glassware that has been used for silver methods can
> collect traces of metallic silver - sometimes enough
> to see as a mirror or grey marks. If the glassware
> is used again for silver staining, these traces serve
> as nuclei for deposition of more silver, and they
> are bigger than the desired nucleation sites in
> the sections. The staining method may simply fail
> or the chemical reaction may go wild, with nonspecific
> deposition of silver all over the place.
> Silver is soluble in nitric acid. My cleaning technique
> is to put a little concentrated HNO3 in the vessel
> (Coplin jar or larger tank) and carefully move it over
> all the inside surface, over a sink with running water
> so that any spilled drops of acid are quickly diluted.
> Visible silver (look in the corners) disappears
> instantly, so smaller amounts must also be removed.
> Pour the used nitric acid into a beaker containing
> tap water (for later neutralization and disposal). If
> the tap water becomes opalescent you have removed a
> significant amount of silver from the glass.
> Next - and this is important - Fill the vessel with
> PURE (eg distilled) water and empty it; do this twice
> so that the concentration of silver ions in the water
> adhering to the sides is infinitessimal. Tap water
> must not be used for these washes because it always
> contains anions (chloride, bicarbonate, others?) that
> form insoluble silver salts. Any colloidal silver
> chloride particles that stay on the glass will be
> partly reduced to silver by exposure to light and
> can be expected to provide nucleation sites in
> later silver staining methods. Finally dry the
> glassware by letting it drain and store the vessels
> upside-down in a closed cupboard.
> Deposited silver is not the only kind of dirt that
> can spoil silver staining. Any kind of organic
> chemical deposit (such as a fragment of a section)
> or even residue from evaporated tap water will
> work in the same way. Concentrated nitric acid
> quickly oxidizes and dissolves pretty well everything,
> with one notable exception.
> The exception is metallic gold. This can replace
> deposited silver in glassware used for gold-toning,
> a procedure often used to improve contrast in
> silvered preparations. Gold on glass may appear
> only as a light purple discoloration. Any colour
> that resists nitric acid is probably gold. It can be
> removed with aqua regia, which is a 3:1 mixture of
> concentrated hydrochloric:nitric acids. Make and
> use aqua regia in a fume hood because it emits
> fumes of chlorine and nitrogen oxides. I have
> resorted to aqua regia 3 or 4 times (in >30 years)
> to get rid of gold on glass. The obvious way to
> prevent contamination is to reserve certain jars
> and dishes for gold-toning and nothing else. This
> is not very practical if we do many different
> methods and do not have a cupboard big enough for
> all jars that might carry catalytic contaminants.
> Some people use Farmer's reducer (a solution
> containing potassium ferricyanide and sodium
> thiosulphate). This is an altogether gentler
> liquid than nitric acid and it can dissolve silver
> from black & white photographs. The action of
> Farmer's reducer on visibly discoloured glass is
> very slow, and this mixture is not going to
> destroy insoluble organic forms of dirt such
> as bits of tissue.
> Bottom line: Nitric acid, followed by pure (not
> tap) water.
> John Kiernan
> London, Canada.
> ________________________________
> "Scott, Allison D" wrote:
> >
> > Hello to all in histoland.  I need help in locating a cleaning acid solution
> > for cleaning glassware.  We are having a problem with our GMS stain.  I
> > think it has something to do with the glassware.  Thanks in advance
> > Allison Scott
> -------------------------

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