On Wed, 16 Apr 2003 20:53:42 +0930 Angela Hammond
 writes:
> I am curious to know why it is important to cool Schiffs reagent to 
> 50degC before adding the potassium metabisulphate.  

The metabisulfite, whether potasium or sodium, is used in Schiff reagents
to generate gas.  It is the gas that decolorizes the Schiff reagent, and
the eventual failure of the gas generator (the metabisulfite and the
acid) is why the reagent eventually becomes colored as it ages.  If you
add the metabisulfite to hot solution during making the classical "hot
Schiff" solutions, you will evolve off a large quantity of your gas
almost instantly, and the shelf life of the reagent will be very short.
Al Floyd
23126 South Shore Drive
Edwardsburg, MI 49112
(269) 699-7182 phone & fax