Tom,
The procedure used in our lab for processing silver nitrate solutions
was taken from the book "Prudent Practices for Disposal of Chemicals
From Laboratories", Washington Academy Press, Washington DC, 1983, p.46.
Silver chloride is precipitated from silver stains by acidifying the
solution with nitric acid, then adding NaCl.  The precipitate can be
filtered from the solution, dried and the silver later reclaimed.  The
remaining supernatant can be disposed of into the sewer.
1. Add HNO3 slowly to used silver stain solution while stirring to bring
the pH to 2.0 (35 mls. conc. HNO3 per gallon of waste).
2. Add 76 g. NaCl and stir until dissolved.  Let stand 24 hours.
3. Pour off the supernatant and allow the precipitate to dry under a
fume hood (while pouring, use rapid filter paper to catch any suspended
precipitate).
The silver chloride precipitate may be disposed of through an X-ray
service company or the silver may be reclaimed in the laboratory.
We send our precipitate out through our hazardous chemical waste
disposal vendor.
Bob Santoianni
Emory University Hospital
Atlanta, Georgia