A number of years ago (9 to be more exact) I made a large batch of
phosporic acid using the standard recipe.  The acid worked well, but two
years ago it started spending most of its time in a solid state rather than
liquid.  This reqired that the acid be heated before use so that it could
be pipetted.  After a about an additional 6 month the glass container in
which the acid was stored burst in the oven (and what a mess that was). 
After so many years the acid had dissolved more than half of the glass and
rendered the container too weak.  I now assume that the problem with the
crystallization of the acid was due to the presence of a substantial amount
of Si in the acid.  I have been tolds that poly bottles will last an even
shorter lenght of time, so I quess the message here is that big batches are
fine, but don't make them so big that the shelf life approaches a decade. 
I also wonder if this is a photo induced reaction, so keeping the acid in a
dark place might help.
Regards,

Bob Ilchik
Department of Geosciences
University of Arizona
Tucson, AZ  85721
(520) 621-9792 (W)
(520) 621-2672 (FAX)
(520) 881-2002 (H)