Re: Dichromate Hazards

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From: stan johnson (sjohnson@gwi.net)
Date: 04/27/00-08:49:03 PM Z


** Reply to note from kasim@juno.com Thu, 27 Apr 2000 18:58:13
-0700
> What the text above implies is that you can remove Cr and Ag from
> solution by mixing in some lye and common table salt. It appears
> that this is an alternative to simply mixing some sodium thiosulfate
> or sodium sulfite into the dichromate solution to precipitate Cr, as
> Cr2O3?
 
If you mean what I think you do, no. The dichromate (the
orange/yellow hexavalent form Cr(VI)) needs reduction by the sulfite or
thiosulfite [or some other chemical reductant] to convert it to the much
less soluble trivalent [green] form Cr(III). Alkali alone won't do it. I
expect the author meant the alkali to *follow* the [thio]sulfite.
 
The alkali is to reduce solubility, I presume, but I don't recall the details
of trivalent chromium chemistry. I may be that Cr(III) sulfate/sulfite is
quite soluble. The hydroxide I expect decomposes to the 'insoluble'
oxide, as with silver, e.g.. Perhaps someone else will correct any errors
here.

stan

-- 
Stan Johnson 
sjohnson@gwi.net
04/27/00 10:45pm


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