From: Alberto Novo (alnovo@inwind.it)
Date: 02/27/03-01:17:48 AM Z
on 02/26/03-09:19:00 AM Z Christina Z. Anderson wrote:
...
> My notes: Some use a 1% solution of sulphuric acid to clear hard to
> remove residual dichromate stain, but this is very toxic and dangerous, so
> don't try this unless you know how to handle chemicals. You can also use a
> solution of sodium sulfite (0.5%) with some sulfuric acid added (0.1%). The
> clearing time is about ten minutes, then wash 5 minutes in tap water, then
> 5 minutes in 0.5% sol. of sodium carbonate (to be sure of neutralizng the
> sulphuric acid), then two rinses in tap water.
Christina, this treatment looks very similar to mine, but I must point out that your proportion between sodium sulphite and sulphuric acid (5:1) is almost equivalent to the use of sodium metabisulphite plus.
5 g of sulphite= 39.7 millimoles,
1 g of sulph. acid= 10.2 millimoles.
If that 0.1% is 1 ml of concentrated sulph,. acid (96%, density= 1.835) then
1 ml of H2SO4 conc= 20.5 millimoles.
Instead, an equal amount (by weight) of sulphite and sulphuric acid leaves free sulphuric acid and sulphurous acid. This last reduces Cr(VI) to Cr(III) in a while, and the sulphuric acid helps to dissolve the residual chromium (III) stain in a very short time.
Alberto
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