On Mon, 1 Nov 1999, HistoScientific wrote:

> I recently visited a large histology research laboratory where they mix
> deionized salt (yes, Morton's table salt) with their Silver Nitrate
> waste and dump it down the drain.

  How can you have deionized salt?

> They swear that it neutralizes the Silver Nitrate waste.
> Is this true?  Does anyone else use this
> "protocol".  I look forward to your responses.

  The chloride ions in the salt precipitate the silver ions of
  silver nitrate. Silver chloride is almost insoluble in water
  and settles out as a white sludge that goes grey when exposed
  to light. It doesn't have the skin-blackening property of
  silver nitrate. The MSDS sheet for AgCl says "slight" for all
  categories of hazard. (Most of the other stuff on the MSDS sheet
  relates to soluble silver salts or all chlorides - the usual
  irrelevant rubbish that conceals any useful information present
  in these crazy documents.) It seems reasonable to think of chloride
  precipitation as "neutralizing" the hazardous properties of a
  concentrated solution of silver nitrate

  It is wasteful to throw AgCl sludge away. I keep mine in a big
  bottle, with excess chloride (I use HCl rather than NaCl).
  When it's getting full, I decant off the supernatant, wash with
  a few changes of water (decanting every day or 2, to replace the
  HCl) and then add zinc powder to the sludge. This slowly reduces
  the AgCl to metallic silver. After a week or so, I filter in a
  big funnel and wash through with dilute hydrochloric acid to
  dissolve out the remaining zinc (effervescence of hydrogen). When
  HCl causes no more fizzing,  wash well with distilled water,
  leave the deposit to dry on the filter paper and then collect it
  into a glass jar.

  I have tried making silver nitrate by dissolving reclaimed
  silver powder in nitric acid, but could never get a transparent
  solution. Probably the powder contains residual chloride ions,
  or other impurities. It should be well suited to taking to a
  refiner, but I've never got round to doing that. I'm sure a
  refiner would take untreated silver chloride sludge, without
  your having to bother with trying to recover the metal.
  Industrial purification is done by electrolysis.  This is
  a significant business for used photographic fixer, which
  contains much less silver than AgCl sludge.

  So don't waste your used silver nitrate. Recycle it.

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