In article ,
"Carl Ijames" wrote:

"Joseph Gwinn" wrote in message
...
In article ,
"Carl Ijames" wrote:

"Joseph Gwinn" wrote in message
...
In article ,
"Carl Ijames" wrote:

"Joseph Gwinn" wrote in message
...
I'm not sure that deionized water is in fact free of ions, because
the
beads used to fill the water treatment gadget are called ion
*exchange*
beads or media. The implication is that they have traded one kind
of
ion for another, not that all ions are removed.

They use an anion exchange resin to substitute OH- ions for whatever
anions
were originally present, like Cl- or whatever, then a cation exchange
resin
to take out positive ions like Na+ and replace them with H+, then (at
neutral pH, anyway) the H+ and OH- form water except for the residual
10-7 M
of each from the dissociation equilibrium. If one of the two beds is
saturated you get either acid or base coming out - one time in
freshman
chem
lab all our titrations came out wacky and it turned out the "neutral"
DI
water we were using to dissolve our standards was really pH 2 :-).
Oh,
it
doesn't matter which bed comes first, and in disposable cartridges the
resins are frequently mixed. Since DI water has no pH buffering
capacity
any little contaminant can shift the pH substantially. Any that has
been
exposed to air for a while will usually be down around pH 5 or so from
dissolved carbon dioxide

I always wondered just how this was supposed to work. Thanks.

I sounds like in a home system it may be difficult to achieve and
maintain neutral pH, as one or the other bed will always be a bit
ahead.

Joe Gwinn

If you wanted exactly neutral very pure water, yes it would be slightly
tricky. In practice you don't care about exactly neutral pH so it is
easy -
each resin takes out whatever is there of each charge and all that is
left
is water. Yes, a tiny amount of acid or base can shift the pH, but you
don't care precisely because the buffer capacity is so low - any tiny
amount
of anion or cation you add on purpose will be enough to completely swamp
out
any slight initial acidity or alkalinity. It's only when you run it
through
one resin and not the other (or one resin is saturated and the other one
isn't) that you have a problem. Say you start with salt water and all
the
Na+ is exchanged for H+ but the Cl- isn't exchanged for OH-: you just
made
HCl, hydrochloric acid :-).

Ahh. I was worrying about corrosion of for instance copper, and if I
understand the implications of the above, for instance a bit of salt in
the feedwater will cause the deionizer to produce hydrochloric acid,
which will dissolve the copper in no time. People are far more tolerant
of dilute HCl than copper and stainless steel, so what's good enough for
drinking may not be good enough for a boiler et al. I'm sure that there
are better deionizers, but still...

Joe Gwinn

Let me try again. There are two parts to the deionizer, one that removes
cations and one that removes anions. So long as both are working, the water
that comes out is at neutral pH. No acid or base is produced. If there is
table salt in the water, NaCl, the cation portion will remove the Na+ and
release H+, and the anion portion will absorb the Cl- and release OH-.
Since there were equal amounts of Na+ and Cl- to begin with, equal amounts
of H+ and OH- will be released and they will combine to produce water, H2O,
at neutral pH. It is only when one half malfunctions that there is a
problem, and it is easily detected by monitoring the output pH. Deionizers
are a standard way to clean up water for all kinds of applications.

I know that total ions must balance to yield zero net charge, but what
caught my eye is the following:

"It's only when you run it through one resin and not the other (or one
resin is saturated and the other one isn't) that you have a problem.
Say you start with salt water and all the Na+ is exchanged for H+ but
the Cl- isn't exchanged for OH-: you just made HCl, hydrochloric acid
:-)."

Ahh. I read the first sentence too fast. OK.

Joe Gwinn