"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 :-).

-----
Regards,
Carl Ijames