> I have been etching boards with a sodium persulphate based etchant. Leaving
> the solution in the tank over night in the garage (2/3 degrees C) 'ice'
> crystals appeared in the bottom of the tank. On heating they don't 'melt'
> and on taking out they don't either. Don't appear to disolve in water
> either. They don't really stick together either. Any idea as to what they
> are?
> 
> Wayne

Copper persulphate?

Whatever it is, you now have to dispose of it.  If you only have a
little, you might get away with trashing it.  Otherwise, contact local
waste disposal authorities.  The copper is something they prefer not
to get into the ground water.

Mike H.
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