I haven't seen a reply to this, so I'll jump in!
Distilled water is prepared by boiling water to make steam, which is then
cooled ("condensed") so the resulting water theoretically does not contain
any impurities. (In actual practice, there can be traces of copper or other
metals from the pipes used in the process, plus any other goodies picked up
along the way. What it will not have is calcium, which can react with most
photographic chemicals in solution - such as sodium sulfite - to form
precipitates as the calcium salts are mostly insoluble. The "hardness" of
water comes from dissolved calcium carbonate and/or calcium sulfate, mostly
the former.)
Deionized water is water that is treated through an ion exchanger (like
most household water softeners) that removes calcium, iron, etc., in
exchange for sodium salts. ("Softened" or ordinary "deionized" water has a
high sodium content, not suitable if you are on a low-sodium diet! In fact,
the calcium and iron content may be good for your health, the sodium
definitely is not!)
Filtered water can be one of several types: common household filters (and
some used for commercial bottled water) remove dirt particles and possibly
some dissolved gases, but not dissolved salts such as calcium, iron, etc.
There is also filtering by osmosis, which uses pressure or vacuum to force
the liquid through special materials which remove all particles over some
infinitestimal size (such as .001 microns or even smaller); this is used
for making beer that doesn't have to be pasteurized as the filter removes
particles as small as bacteria. There are other esoteric types of
filtration (electrostatic, for example) used for special purposes, but most
are too expensive for our purposes. (If anyone reading this knows of any
practical filtering process other than the ones mentioned, I'd like to be
informed.) For routine processing, I use a filter funnel every time I use a
solution; this removes the "rocks" which can be nasty spot producers.
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net
Visit http://www.psa-photo.org/