Ask A Scientist© Chemistry Archive

Name  Bob
Status  Educator

Question - What essential is the difference between distilled and deionized
water?  We are building a new high school and we have the option of one of
the other with plumbing to each room.  Does anyone have experience with
this situation?

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Bob,

So far as quality goes, distilled is better than deionized. However, 
having had
extensive experience with both, I suggest  you simply purchase distilled water
from a reputable supplier. The costs and aggravations associated with
maintaining deionizers and stills is considerable. Unless you have funds to
hire a maintenance technician, you will be spending  a lot of your time 
looking
after whichever system you select.

Regards,
ProfHoff
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Either distillation or deionization (reverse osmosis?) will produce fairly
pure water.  The industrial R.O. system I monitored at my last position
produced water that conformed to USP specifications, but we also distilled
the R.O. water for lab work.  Some analyses require freshly distilled water
(hasn't drawn carbon dioxide from the air).  A Nanopure(TM), or similar, ion
trapping system can further help if you need ultrapure water.
Either way you go, there will be some system maintenance.  You will be
removing impurities, which have to end up somewhere (in your apparatus!).
I'm sure you have heard the sizing recommendations, based on total and
periodic usage, from the various vendors.  For the initial costs, energy
costs, upkeep costs and down-time for maintenance, would you rather change
cartridges, (perhaps backflush and/or sanitize cartridges) or clean a
boiling pot?  Use of softened water also eases maintenance considerably, as
sodium rinses out much more easily than calcium.
A quick way to detect problems in your line (either system) is to draw a
sample and add a little barium chloride.  Formation of insoluble salts
signals the presence of anions, sure to be accompanied by cations.  While
the water will be measurably pure, some extra bells and whistles may be
needed to ensure compliance with the Safe Drinking Water Act (1991?) if some
is intended to be diverted to the cofeemaker(s).
Perhaps others will better address your question of chemical differences.
For most uses, R.O. is sufficient, provided the system is properly
maintained.  Above certain volumes, R.O. is much cheaper than distilled.
Does this help?

Tim Spry
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I'm not knowledgeable about the engineering aspects such as the cost of one
compared to the other. However, as the names imply deionized water is water
that has been passed through a column or membrane to remove ions present. If
it is of the type used in homes, it is not truly a de-ionizer, removing all
ions, but rather an ion exchange column that exchanges polyvalent ions such
as Mg++ and Ca++ for Na+ ions. A de-ionizing column will not remove nonionic
organic substances from the water.

In contrast, distilled water is actually boiled in a still and the
condensate collected and distributed. Distillation removes both ionic and
nonionic organic contaminants.

Either method will require periodic regeneration, in the case of a
deionizer, or cleaning in the case of a still. I think you will have to
weigh the cost of maintenance and the required capacity needed before making
your decision.

Vince Calder
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