Bjorn:

Most labs, and it sounds like yours, probably use the recipe for making
phosphoric acid found in Bowen (1966), in which a "spatula tip" of CrO3 is
added to the acid.  In our lab, we add a like amount of potassium
dichromate, K2Cr2O7, instead, which gives the acid an even more attractive
green color.  Potassium dichromate, however, is no less a carcinogenic
hazard, as it too is a chromium (IV) compound.

As to why it is added to the acid, I must confess I'm not entirely sure,
especially as McCrea (1950) didn't add it to his phosphoric acid, as far as
I can tell.  Old phosphoric acid turns a brownish color and I always
thought the chromium was added to facilitate determining when it was time
to make a new batch of acid.  Likewise, I assumed the addition of a few
milliliters of hydrogen peroxide was to oxidize any organic matter in the
acid, but maybe this too is folklore.

Steve


>Dear isogeochemists
>Since the dawn of stable isotope geochemistry it has been good
>behaviour to spike the phosphoric acid for carbonate reactions with
>CrO3. Now my technician tells me that this colourfull compound is
>carcenogeneous and she wants (understandably) to avoid it.
>Does anybody know why we add it and what would happen if we leave it
>out?
>Do anybody add it at all or is it just us being old fashioned?
>Replies will be appreciated
>Bjorn
>


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