Deionize Water Misconceptions
Date: January 2007
I obtained a gallon of Deionized water from an
aquarium shop to mix 1:1 with propylene glycol to use as a safer
type of antifreeze. I am now being told that deionized water is
not only "Killer Water" if one was to drink it, (The fish are
living in it) but will be very corrosive to the metals within my
engine and cooling system.
Automotive antifreeze needs at least three "components".
You've only assembled two of them, but forgotten one.
It needs a glycol for high boiling point,
perhaps some water to make it cheaper and reduce the boiling point to just
over what the engine runs at,
and it needs corrosion inhibitor: dissolved minerals that dissuade the
dissolving metal from the engine block.
This metal can only be safe from water if it's blanketed by a layer of
and maybe a softer organic blanket-layer over that to protect the thin
oxide layer from disturbances.
An optional fourth component might be an oily liquid to keep hoses and
seals pliable and non-brittle.
Sorry I'm not sure right now what they use for corrosion inhibitor.
Start reading labels on antifreezes.
I think something with silicon in it, such as silicates or silicone, is
probably the main one.
A pH-neutral buffer, maybe a phosphate, might also be in there.
If it's an old car so you can afford the risk of an experiment,
good, but at least find one corrosion-inhibiting additive and try using it
from the beginning.
If your car then stays OK for a long time, you've done really well.
Whatever you do, don't add any chlorine or chlorides to your antifreeze.
Chloride ion makes metals corrode even when the liquid has protective
PS- DI water isn't "killer water". It has no minerals, so its pure that
way, and pure is OK.
What's cautionary about it is: all germs may not have been removed.
In industrial DI water loops they usually don't do any _biological_
along with the repetitive mineral purification.
So it could have germs in it, and the facilities people will tell you so.
It takes pointy-headed attention to make sure water is germ-free,
and for industrial water, nobody is paid to watch that.
So it might hurt when drunk, or might not.
Contrary to your friend's belief, using deionized water will not rust your
car, nor will it kill you--that is unless you drink very large amounts all
of the time. Deionized water is simply water with all of the ions
removed. These ions include regular table salt, (sodium and chloride ions),
chalk (calcium carbonate), gypsum (magnesium sulfate) and also metal ions
like copper and iron. There are also other ions present in most tap water,
but these account for most of them. While absolutely pure water has a
theoretical pH of 7.0, which is completely neutral, it is near impossible to
get water absolutely pure. For this reason, the water has either a slightly
acidic or slightly basic pH. In my personal experience, the water has
always been slightly acidic (pH 4-6) when I have tested it.
This pH range is nowhere near acidic enough to cause corrosion to metal, so
you do not have to worry about that. In fact, using deionized water is
better because it contains fewer salts, which may not be soluble in the
resulting antifreeze mixture. If these ions "salt out", they will deposit
themselves on the walls of your engine block, thermostat and coolant
hoses. This resulting layer insulates the water from the metal that it is
supposed to be cooling, which might eventually result in overheating your
That being said, unless you use really hard water (contains a LOT of salts
and ions) and change your coolant more frequently than recommended, this is
rarely a problem.
Propylene glycol is a much safer alternative for both fish and household
pets compared to ethylene glycol, which is much more toxic and has a
pleasant sugary taste to dogs and cats. Your pets thank you!
As far as the toxicity of DI water goes, the only harm that it can cause is
due to the lack of essential minerals and ions that your body needs. You
can easily get these ions and minerals from other sources with even a
slighly healthy diet. If you are drinking a lot of DI water, then you will
essentially be diluting out the minerals and ions in your body, which might
be cause for concern if you drink lots (and by lots, I mean excessive
amounts) of DI water every day. I'm not sure if there have been studies out
there that have looked at how much DI water you would have to drink to hurt
your body, but certainly no normal amount of water would do this. In fact
there have been many studies that do show that more pure water hydrates you
better and helps athletic performance by a mild amount, but this does not
address electrolyte replacement (like Gatorade would do), which is
important, especially during long periods of excersize.
So go ahead, use DI water freely in your antifreeze and for your drinking
water--just don't drink your antifreeze!
"Deionized water" is no more or less than the term "de-ionized" implies.
Most ions, positively and negatively charged atoms and molecules, have been
removed -- usually by passing the water through beds of polymer beads
designed to remove those ions.
Using it with propylene glycol for a "safer" antifreeze is probably not
a good idea. Antifreeze for auto radiators is a formulated product
containing various additives to protect the radiator and associated plumbing
from corrosion and other functions. That is where the advice about its
corrosiveness comes from. The little you save by making your own antifreeze
probably isn't worth the risk of radiator and engine corrosion that might
result from the absence of these additives.
Deionized water is useful for preventing deposits in appliances such as
clothes irons where water is evaporated concentrating any non-volatile ions
like calcium, magnesium, or iron.
However, deionized water may not be biologically pure. It can still be
contaminated by various microbes. So while water "purifiers" can improve the
flavor of tea and coffee, etc. Don't assume that it is biologically pure.
Even distilled water, water collected from the water vapor of boiled water,
can be biologically contaminated if left sitting around open to the air. But
it is an exaggeration to call it "killer water". It may taste a little
"flat" compared to bottled water, or even tap water but it is not poisonous
It is used in fish tanks because some fish are sensitive to low levels
of some salts that may be present in some tap water.
Various myths are invented about things we eat and drink. The toxicity
of deionized water is no exception.
Tap water, fresh water (from rivers, wells, and such), ocean/sea water, and
even rain water all have some ions dissolved in them to some degree. Some of
the most common ions are calcium, magnesium, sodium, chlorine and
carbonates. Deionized water is essentially water that has been passed
through some filter (like those advertised attachments for your faucet) that
are designed to remove these ions.
The only reason I can imagine that someone would -mistakenly- think of
deionized water as "killer water" is that some people want to think that you
must not only hydrate yourself with water but also replenish lost minerals
when you sweat (this whole isotonic drink craze, or those sports drinks that
advertise the fact that their water has the same mineral content as sweat).
If regaining lost minerals in sweat is high on your list of importance when
you hydrate/drink water, than it might be a good idea to drink regular or
mineral water and not deionized water.
But if preventing corrosion is your objective than deionized water is the
better choice. Since it is these ions that allow water to conduct
electricity (and corrosion is a galvanic/electrical process), then tap water
is in fact more corrosive to metals than deionized water. The ions in
regular water facilitate the movement of electrons that cause corrosion.
While we're on the subject, you might also be considering distilled water.
While the distillation process extracts water and leaves some of the ions
behind, depending on the vigor of the distillation, ions still get
transported in the distillation process. What distillation does remove are
dissolved organic substances (coming from bacteria, fungi and such). So
distilled water still has a higher capacity for corroding metal than
deionized water, but not as high as tap water, and definitely not anywhere
near that of salt water.
Greg (Roberto Gregorius)
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Update: June 2012