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Q & A: What is deionized water?

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Most recent answer: 12/26/2014
What is dieonized water?
- Bob (age 9)
minesota international school
Deionized Water (We call it "DI water" in the chemistry labs) is just what it sounds like: Water that has the ions removed. Tap water is usually full of ions from the soil (Na+, Ca2+), from the pipes (Fe2+, Cu2+), and other sources. Water is usually deionized by using an ion exchange process.

Why de-ionize water?

Often, when you are doing chemistry experiments, the ions in water will be an interference. They can switch places with other ions you may be interested in experimenting on. You may also be interested in finding out what elements are in a small sample of material. For example, a farmer may want to know what's in his soil, or the Environmental Protection Agency wants to know what a factory's emitting into the air. Dissolving the sample in water and doing tests on the result is a common technique, and contaminants in the water will make the whole test give the wrong answers. Water with ions in it is also quite a lot more electrically conductive than water without ions in it. If you boil water with lots of ions in it until all the water's gone, you'll have a crusty salt residue in your pot.

We guess de-ionized water isn't necessarily pure water, given the usual de-ionization procedure. Non-ionic contaminants may persist. Electrically polar molecules dissolve easily in water, and some complicated molecules have polar ends and non-polar ends, which can help non-polar stuff (like oils) mix in water. Soap is an example. Soapy water may count as deionized, but most people would insist that their de-ionized water doesn't have (much) other stuff in it.

Jason and Tom

Special note for Reddit users. I noticed that somebody who linked to this answer got the impression that deionized water really has no ions. Of course it still has about 10-7 molar each of H+ and OH- ions, at room temperature. You can't stop that chemical equilibrium from occurring just by pulling out the other ions. / Mike W.

(published on 10/22/2007)

Follow-Up #1: de-ionized radiator water

...a practical application to the deionized water question: My friends and I work on our own cars. I keep hearing about using distilled or deionized water in the radiator to extend the life of the aluminum and solder in a car radiator. It seems that distilled water would be a better choice than deionized water. What would be the least corrosive?
- Robert Cary
Salt Lake City, Ut
De-ionized water is water that lacks ions coming from sodium, calcium etc.  It still may contain other organic junk.  Distilled water is purer.  For radiator use there is probably not much difference.  I use distilled water for topping up my carís battery. 

Lee H

(published on 10/22/2007)

Follow-Up #2: drinking deionized water

Is deionized water drinkable?
- Manuel Guerra
Utica, NY
Unless the water is specifically for human consumption, it might be a bad idea to drink it because it could contain traces of the de-ionizing resins. Iíve heard that this is a problem with some deionized water intended for industrial use.

On the other hand, many websites have warnings that deionized water is dangerous because it is ítoo pureí. These warnings are pure nonsense.

Mike W.

(published on 10/22/2007)

Follow-Up #3: cleaning water

Why my factory DI water is very dirty ! But the resistivity is more than 10 mohm .Do this will cause the silicon chip problem? Any way to measure the water cleaness ?
- Tan (age 30)
When you say that the water is dirty even though it has high resistivity (Is that 10 Meg ohm-cm?), I guess you mean it has some dust in it.
You can measure dust levels by shining a cheap little red laser into the water in a clear glass cuvette and measuring the amount of light scattered. You can judge by eye or do that more systematically using a photodiode (also cheap and sturdy) together with an optical filter matched to the laser wavelength. Dust can be filtered out with membrane filters available from several suppliers.

Mike W.

(published on 11/26/2007)

Follow-Up #4: types of pure water

I am cleaning solar panels and was told to use deionized water. Why won't distilled or reverse osmosis water work just as well?
- George Loving (age 80)
Portland,OR USA
 Properly distilled water or reverse-osmosis purified water should generally be cleaner than water that's just been run through an ion exchange resin. I hate to give technical advice on something where I have no direct experience, but I can't think of any reason why you should avoid distilled or reverse-osmosis water. Is de-ionized water cheaper?

Mike W.

(published on 04/27/2011)

Follow-Up #5: deionized water for electronics

In response to Mike W.'s question, the likely reason for using deionized water is to avoid the effect of contaminant ions on the electrical parameters of the panel. Quoting the textbook from my IC Engineering classes [Introduction to Integrated Circuit Engineering by D. K. Reinhard], "One improperly cleaned beaker or inadvertent contact with tap water can send threshold voltages into the stratosphere." [I can't tell if this is getting through or not because there is no message if it did or didn't - it just comes back to the same page with a new challenge. How about adding a message "It didn't work - try again."
- Randall Fisher, Ph.D. (age 46)
Grand Haven, MI, USA
Certainly the water needs to be nearly free of ions, as you say. The question, however, was why distilled water or water purified by reverse osmosis, each also free of ions, wouldn't work. Our guess was that they are more expensive.

Mike W.

(published on 06/08/2012)

Follow-Up #6: deionized water for welding

In our Laser Welder, the manual says to use only DI Water in the cooling system. The reason being is that the cooling water passes over the high voltage contacts on both ends of the stimulation lamp while the unit is firing and therefore must be as non-conducting as possible. I hope this helps. =8o)
- Paul (age 35)
Arnprior, ON, Canada
That makes sense. Any form of deionized water (distilled, ion-exchange-resin, reverse osmosis) should work for this.

Mike W.

(published on 05/07/2013)

Follow-Up #7: de-ionized water for steam irons

Is it a good idea to use de-ionised water in steam generator irons? Will such use prevent or reduce mineral build up in the appliance?
- Jack Rodaway (age 79)
London England

Yes, it should work well for that.

Mike W

(published on 05/19/2013)

Follow-Up #8: residue in de-ionized water

When I put a small droplet of deionized water onto an ultra clean silicon wafer and view through a microscope I noticed the following: As evaporation progresses, I see hundreds of tiny bubbles moving around then eventually pulling to the edge of the drop (the coffee ring effect I think). Then it eventually dries into tiny circles (not sure if its a non-ionic residue or micro droplets - they're typically 1 to 10 microns in diameter). If I put the wafer in an oven at 80C for an hour they don't change. Similarly if I put the wafer in a vacuum oven at 80C for an hour. That makes me think the circles are dried out residue as I'd have thought pure water should evaporate away to nothing. What are these small circles or why doesn't the DI water completely disappear?
- Mark Taylor (age 47)

It sounds like you've done just the tests needed to show that these spots are from some residue. De-ionized water has usually been run through an ion-exchange resin that pulls out most anions and cations. They're replaced with H+ and OH-, which recombine to form water. That can still leave traces of other gunk, and I guess that's what you're seeing. Sorry I can't be more specific. Perhaps with distilled water or reverse-osmosis purified water you'd not get those residues.

Mike W.

(published on 06/07/2013)

Follow-Up #9: making lots of de-ionized water

How can we make de- ionised water on site for our MeCanolav degreasing facility and what checks can we perfom on site to authenticate its purity and that it is 100% de-ionised. Best regards Tom Brothwood
- Tom Brothwood (age 54)

In the lab when we wanted de-ionized water on demand, we'd usually use columns of de-ionizing resin, available commercially. The output is not 100% de-ionized, but then nothing really is. Occasionally you have to replace the columns. Alternatives are old-fashioned stills (not very convenient or energy efficient) or reverse-osmosis systems. Industrial users seem to generally prefer the exchange resins, I suppose for price reasons. You can monitor the output with a conductivity meter. Depending on how pure you really need the water to be, you may be able to use one as cheap as under $200.

The Wikipedia article on this topic looks pretty good to me:

Mike W.

(published on 11/25/2013)

Follow-Up #10: practical water purification

I work at G.E. Water and Process technologies. We make EDI or E-Cell units (Electrodeionization), as well as other water processing modules and systems. The EDI cells I think perform better than simply filtering with ion exchange resins because they don't leave any residue behind in the water from the resins themselves. If you use reverse osmosis, you can still have ions left over in the water. If you combine reverse osmosis followed by deionization, either by DI resin filters or EDI units, this is probably the best way to get water as clean as it needs to be. If you want absolutely pure water, then you can follow this up with distillation.
- Dave (age 30)
Waterloo, Ontario, Canada

Thanks for the info. I have one small quibble. For the combined resin+reverse osmosis method, wouldn't it make sense to use the resin first? That way whatever organic gunk it gave off would be removed in the second stage.

Mike W.

(published on 12/26/2014)

Follow-up on this answer.