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Re: alternative to chromium-bas



In research laboratories, Chromic acid is considered an absolute last 
resort cleaner, used only when nothing else will work.  It is used to 
remove certain metal residues, but only when other acids, and scraping 
and scrubbing has failed.  A more benign acid mixture which is often used 
to clean glassware is aqua regia.  Recipie and warnings follow:


Usually, I only use rough ratios of the
concentrated acids.  However, the defined ratio is the following:

Mix 82 mL HCl to 18 mL HNO3

If you know someone who needs to make this stuff, then be aware that it
slowly produces toxic fumes (NOCl and Cl2) that can build up A LOT of
pressure.  I've seen bottles blow.  Also, plastic bottles are preferred 
for
short storage times (like a week...no longer).

Usually aqua regia baths are stored in plastic containers with loose 
fitting lids in a fume hood.  This prevents the build up of pressure and 
removes any vapors from the laboratory environment.  Glassware is usually 
left in the aqua regia bath overnight.  Also, glassware should be cleaned 
with soap and well rinsed before being placed in an aqua regia or chromic 
acid bath.

Soap, water, aqua regia, and good scrubbing should properly clean almost 
all glassware.  They could also try placing a small amount (a few drops) 
of concentrated nitric or sulfuric acid in any glassware with stubborn 
residues and rinsing it very carefully.  Unless they are using a 
procedure that consistently produces residues that resist aqua regia and 
other concentrated acids, I suggest they simply throw away the occasional 
piece of glassware that can only be cleaned with chromic acid and do away 
with the chromic acid bath.

Diana Zinkl

Tellus Institute
11 Arlington St.
Boston, MA 02116
tel: (617) 266-5400, fax: (617) 266-8303
dzinkl@tellus.org, http://www.tellus.org
(note change from 'tellus.com'.  '.com' is still usable)