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