Bob,
You raise an interesting point, and one that I've heard from various
sources myself over the years. We have a DI water tap in the lab and it
gives us water that has extremely high electrical resistivity, which is the
usual way of determining the ion content of water. Ions are typically
removed from water by means of a molecular sieve type of ion scavenging
medium. In times past, the standard method was by distilling; hence the
usual reference to "distilled" water, regardless of the method used to
reduce the ion level. I use the DI water for making coffee in the lab
because, as compared with the very hard tap water, it keeps the coffee
maker clean. One of our students recently questioned the wisdom of using
"de-ionized water" for drinking. When queried as to why he thought it was
bad for drinking, he offered a similar explanation of this water leaching
the chemicals out of our body. But, on reflection, he decided this really
didn't make much sense.
Now, I'm only a lowly physicist, but your electrochemist's concern doesn't
make much sense to me in the radiator context either. Pure water is a good
insulator. This is desirable when having dissimilar metals in contact so as
to discourage electrolysis. We all know that electrolytic corrosion is
worse in salt water than in fresh water, right? So, too much salt (ions) is
bad. So, what about too little? I guess the belief here is that at
extremely low ion content, water turns vicious (not viscous) and will
dissolve anything it comes in contact with. Even if this were true, which I
doubt, then after dissolving a small amount material it would then become
like any other "distilled" water with low, but finite conductivity.
As to the difference between de-ionized and distilled water, we are really
considering two different concepts. De-ionized refers to the end result;
i.e., no (or extremely low) ion content. Distilled, on the other hand,
refers to the method for obtaining the de-ionized water. Triple-distilled
water is one of the most thorough means of purifying and de-ionizing water.
Almost the same result can be obtained by passing water through a modern
de-ionizing medium. I believe that both methods lead both qualitatively and
quantitatively to the same result.
I'd be interested to hear more details about this subject from your
electrochemist. Maybe give him my e-mail address so we can discuss this
directly In the meantime, I'm counting on the two bottles of rust inhibitor
to do their job. ;-)
TTFN,
Bob
At 07:58 AM 8/8/99 -0400, pamelam@connix.com wrote:
> Then, filled the
> >system with two bottles of water pump lubricant/rust inhibitor and
> >de-ionized water.
>
>Bob:
>
>I was going to take some DI water home from work to fill my system, but I
>was strongly cautioned by our electrochemist that DI water can be corrosive
>in some circumstances. Since it has no cations or anions, it will readily
>pick them up from the metals that the water contacts. Copper is a great
>donor and could be leached from the radiator.
>
>I used distilled water.
>
>Bob Melusky
Robert L. Palmer
UCSD, Dept. of AMES
619-822-1037 (o)
760-599-9927 (h)
rpalmer@ucsd.edu
rpalmer@cts.com
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