In recent posts about the Gomori methenamine silver (GMS) stain, we've 
touched on the problems of disposing of the chromic acid oxidant.

Janet Crookham Dapson and Richard W. Dapson's "Hazardous Materials in the 
Histopathology Laboratory" 3rd ed., Anatech 1995 (page 195) suggests:

"To reduce the volume of [chromium] waste, add sulfuric acid to dichromate 
solution [or I suppose chromic acid solution?] to get the pH down to 1.0; add 
solid sodium thiosulfate until solution turns cludy and blue. Neutralize with 
sodium carbonate. Let mixture stand overnight and filter off the precipitate. 
Discard the liquid down the drain if other ingredients permit; collect the 
precipitate (chromium trioxide) for disposal by a licensed waste firm."

Dick Dapson, could you elaborate on this rather scary procedure? Does it need 
to be done under a hood - how much sulfur dioxide and carbon dioxide does it 
generate? How do you measure the pH? - I wouldn't care to dip a pH meter in 
it. What are the chemical reactions and what are the molar quantities of the 
reactants?

Bob Richmond
Samurai Pathologist
Knoxville TN