Getting back gold (Was: Neutralizing Silver Nitrate)

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From:"J. A. Kiernan" <> (by way of histonet)
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On Tue, 2 Nov 1999 wrote:

> 60's when (apparently) UK labs were forbidden to throw away gold chloride
> solution because we couldn't import the gold to make any more. They used to
> recover the metal from the solution with zinc. I tried it the 70's and it
> works (needs a lot of gold solution though). It is perhaps the only way
> going to get rich in our profession!

  Gold chloride has always been MUCH more expensive than the
  metal: another example of how labs are ripped off ...

  It's very easy to recycle gold chloride that has been used
  for toning silver preparations or for primary gold impregnations
  such as Ranvier's method. The recovery procedure is easy to
  do and you end up with a perfectly usable solution.
  The only pieces of equipment you may need to
  get are a separating funnel and an evaporating bowl. If you
  have 200 ml of 0.5% HAuCl4 that looks dirty, contains a
  precipitate or just doesn't work any more, you are likely to
  end up with 50-100 ml of 0.5% solution that's as good as new.
  The method of reclamation is not inefficient; the lost gold
  is in the toned or stained sections. The procedure doesn't
  use zinc (which I haven't tried), but is probably easier than
  a zinc-based method would be. It's described in Stain
  Technology 52: 245-248 (1977).

  Sometimes, because of dirt, a gold chloride solution becomes
  colourless, with a black or golden deposit in the bottom. If
  this has happened, do the recycling procedure on the deposit
  only. A solution that isn't yellow doesn't contain any gold.
  If you work really cleanly, a repeatedly re-used gold chloride
  toning solution just gets gradually paler over the years until
  it's too feeble to do its job. More commonly it goes off
  suddenly, almost certainly because of contamination with bits
  of section etc.

  Gold chloride (nickname for either sodium tetrachloroaurate or
  tetrachloroauric acid) is pretty stable stuff, which is just as
  well when you look at its outrageous price. Contrary to popular
  histomythology, it is not light-sensitive (and neither is osmium
  tetroxide, believe it or not; I could quote chapter & verse but
  won't). We who use the tetrachloroaurate anion should proudly
  continue our distinguished tradition of calling the stuff gold
  chloride, and we should always keep its beautiful solutions in
  clear glass bottles. They will be admired by (and inspire) all who
  look along the shelf. And when the golden yellow hue of one such
  bottleful is marred by slight grey-green discoloration, we will be
  alerted, and shall again perform the Ceremony of Reclamation, boldly
  defying the wiles of those who try to sell "new" gold chloride at
  ridiculous prices.

  I put "new" in quotes because nearly all the gold in the world has
  been recycled many times, despite all the current mining. If all the
  gold extracted from the ground were put in one place, it would make
  a cube with sides approximately 40 feet long. See Laist,JW 1954.
  Copper, silver, gold. Comprehensive Inorganic Chemistry, ed.
  Sneed,MC et al., Vol 2. Princeton: Van Nostrand. Older chemistry
  books are much better than modern ones when you want really useful,
  practical information!

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

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