Re: Chromalum

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From: Alberto Novo (alnovo@inwind.it)
Date: 12/08/01-02:59:49 PM Z


Dear Judy and all,
though the answer is directed to Ted, maybe I could reply quicker.
Many salts include some molecules of water in their crystal reticule to
reach a more stable structure. Chemists use to write these formula like
salt.nH2O -for example copper sulphate, CuSO4.5H2O. The dot means the
crystalization water, which is not part of the "salt" chemical compount, but
it is needed by the crystalized salt.
Well, as the n molecules of water are part of the crystal itself, the salt
is then called mono- di- try- tetra- ... and so on... "hydrate".

The "alum" salts are all with the formula KMe(SO4)2.12H2O, where K is
potassium and Me is a trivalent metal, usually Aluminium (and this is why
"alum"), Iron, and Chromium.
While KAl(SO4)2.12H2O is usually called simply "Alum", the other are called
"Iron Alum" and "Chromium Alum".
So chromium isn't in all the "alums", but only in the "chromium alum", and
the toxicity of the salt depends on the toxicity of the metal.

Ciao
Alberto

-----Messaggio originale-----
Da: Judy Seigel [mailto:jseigel@panix.com]
Inviato: sabato 8 dicembre 2001 20.40
A: alt-photo-process-l@sask.usask.ca
Oggetto: Re: Chromalum

On Sat, 8 Dec 2001, Ted Edkins wrote:

> Judy, I stand admonished for mentioning "that" book and writer. A good
> example of what you say is that I wrote the mail from home and when I got
> to work I looked up my chemical catalogues and found that chrome alum is
> not usually sold as the dihydrate but as the dodecahydrate ie not 2H20 byt
> 12H2O. This makes you wonder about other typos and mistakes. I promise
> never to mention that work again.
> Ted
>

Ted, you are obviously a gentleman & a scholar... now if you can come
up with a test for glyoxal...

But perhaps you'd be willing to help a lay person understand. What is a
dihydrate? Does that mean two parts of.... what? And a dodecahydrate???
Do you mean he's got the formulas themselves wrong? I don't find any
mention of those words in Scopick.

OK, you made me go to the actual book (ugh!), where I found formula for
chrome alum:

CrK(SO4)2 12H0

and at the end of the description: "Warning: See potassium alum."

Under postassium alum, was formula AlK(SO4)2 12H20 (as everybody
knows, imagine all numbers dropped a half line). It said under that:

"Warning all chromium compounds are extremely dangerous... etc."

My question is, does the Potassium alum have as much chrome as the chrome
alum? If so, why is it considered less lethal?

Judy


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