On Wed, 26 Jul 2000, Neralie wrote:

> I know this has been asked before but what is the best chemical to use to
> remove silver nitrate stain from a (white) benchtop?
   
   The conventional answer has its origins in the darkroom.

   Farmer's reducer:

     Sodium thiosulphate crystals  5   g
     Potassium ferricyanide        2.5 g
     Water to make                37.5 ml              

   For a dark stain you need to have it much stronger
   than the above, which is for "reducing" overexposed
   black & white negatives or paper. Dissolve the solids
   in the minimum volume of water - perhaps 5 to 10 ml,
   or just crunch some of the crystals on the bench with
   a wet paper towel. The quantities are not critical.
   Farmer's reducer, even in concentrated form, acts
   pretty slowly. 

   Nitric acid (30-50%) shifts silver quickly but you'll
   want to test it on an inconspicuous bit of bench surface
   first. (Mine is epoxy paint on wood, and it resists all 
   the mineral acids very well. In fact, I've not seen it
   attacked by any chemicals at all, in nearly 30 years.)

   An old fashioned and very effective way was to use a
   paste of mercuric chloride and water, but this is
   not a Good Laboratory Practice!

 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1