[Histonet] Cleaning Acid (Long)

[John A. Kiernan]

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 nitric:hydrochloric 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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