Using hot dilute suphuric acid and H2O2 I cleaned some sintered glass filters that had grey residues on the top surface. I assumed the residues where
powdered metal. After cleaning they were white so I left them to dry.
When I checked them dry some time later there was still some gray residues on them but much lighter. So I repeated the cleaning until white but again
when dry some grey residue was now visible again.
Out of curiosity I put one of the just cleaned white filters out in the sun for an hour (I suspected the residue to be a silver compound) and when I
checked the filter it was now a uniform darkish grey.
So the whiteness of the sintered glass fooled me in to thinking the residue had been dissolved when it had only been oxidized to a white compound that
turned grey when exposed to sunlight. Perhaps silver sulfate or a platinum compound.
I guess I need aqua regia perhaps followed by hypo. Perhaps a nitrate with HCl will work,
A pic of a just cleaned filter top and one exposed to sun light. The color balance on the pic appears wrong to me. Its more grey colored in daylight
though it does have a brown color under the light from my desk lamp Sorry for the big file I can not edit the pic on this pc.
unionised - 7-7-2019 at 12:57
Ammonia solution will dissolve many silver compounds.
Re-acidifying the filtrate with HCl will give you a ppt of AgCl which you can recover silver from.wg48temp9 - 8-7-2019 at 03:16
Ammonia solution will dissolve many silver compounds.
Re-acidifying the filtrate with HCl will give you a ppt of AgCl which you can recover silver from.
and then filter ppt with filter paper or I will get trapped in a loop LOL.
You gave me an idea, i put some HCl and a few prills of iodine in to the sintered filter. Borrowed from a bleaching formula for silver prints.
After a few hours its white again. If it is now silver iodide then hypo (sodium thiosulphate) will dessolve it or as you suggest ammonia solution,
assuming that the iodide did not dessolve in the concentrated HCL solution by forming a complex with chloride ions.
I doubt there is more than a few tens of mgs of silver probably much less, but it should be instructive to attempt its recovery.
wg48temp9 - 8-7-2019 at 09:21
I decided to expose all the cleaned sintered filters to the sun for an hour or more. Not the one in the HCl and iodine.
One of them as i placed it in the sun I could see it visibly darkening. Within minutes part of the top of the filter was black. But when taken back
inside about an hour later it had turned grey. I warmed it up with a heat gun no effect so it probably is the sun light and not temperature. There is
no sun now so i can not check if it goes dark again.
One other developed brown greenish smudges on the bottom of the filter and the top is no long pure white.
From the same laboratory that the filters came from I have a boxes of microscope slides with various coating on them and test numbers. I will have to
investigate the coatings perhaps they where investigating photochromic coatings.
