This isn't a question about the difference between Deionized and Distilled water, my hope is that for once it's actually a chemistry question related to this topic.

Distilled water is created by repeatedly boiling water then re-condensing it so that all of the "stuff" dissolved in it should be, in theory, dropped out of it.

Deionized water, as is my understanding, is created by running water through an electric current to pull various metal ions out.

So (and forgive me if I'm thinking aloud) does distilled water still have ions? I just did some googling to double-check and while a watermolecule is, in theory, "neutral" google returns that there is still a slight positivity (+) to the H sides of the molecule and a slight negativity (-) to the O which results in Ions(-) still being attracted to the Hydrogens(+).

Part of me wants to say that there is some possibility that distilled water could retain some ions through the distillation process despite the bond being fairly weak. However, my brain wants to lean toward that being unlikely, that Distilled water ought to also be, Deionized.

As for deionized water, my gut is that while it ought not have heavy metal ions and salts and the like in it anymore due to the deionization process, plenty of other "stuff" such as organic material and microbes ought to be present.

So... After having done my best to answer my own question...

Is distilled water also deionized? and does deionized water have "stuff" in it?


3 Answers 3


Any water contains ions due water self-ionization and usually extra ones due dissolved aerial carbon dioxide ( being a weak acid ).

Distilled water can still contain volatiles(like chloroform from water chlorination), dissolved gases/trace vapours from air (problem for trace analysis if the lab works with solvents) and trace ions from glass where it is stored.

It may also contain traces of salts accidentally travelling in water aerosol formed during distillation. (Neither sea salt stays all in sea in spite of not being volatile.)

Deionized and more generally purified water is produced by number of methods and their combination, like

capacitive deionization, reverse osmosis, carbon filtering, microfiltration, ultrafiltration, ultraviolet oxidation, or electrodeionization.

I would include also ion exchange resins, popular in tap-water filters and among equarists.

At the reverse osmosis, ions partially pass due microscopic membrane ruptures or too big holes, originally present or formed during membrane aging. The osmosis is not particularly effective for organic matter separation. Additionally, there can be ongoing microbial activity on device surfaces behind the membrane.

Electromigration obviously does not address neutral molecules and bigger particles.

These methods are often used combined, like reverse osmosis + electrodeionization.


Distilled water will always contain some ions no matter how pure

There is a simple sense in which even the purest distilled or "deionised" water will always have ions in it. Water self ionises to create small amounts of $\ce{H3O+}$ and $\ce{HO-}$ which is unavoidable whatever the purity.

But the situation is more complicated than that in practice because the purity of different types of distilled water depend on how it is made and what purpose it serves. Not all "distilled" or "deionised" water is the same. And how it is stored can make a difference.

All types of distilled water, for example, will pull some gases from the atmosphere. If they absorb carbon dioxide, for example, there will be some carbonic acid in the water which adds ions.

More importantly, the degree of purity will depend on the chemists need of water that doesn't have certain contaminants in it. Normal lab distilled water just has to be free of gross contamination by many of the things often found it tap water like magnesium and calcium carbonates or sodium chloride in some areas. But storage in glass may lead to trace contamination with ions leached from glass (potassium and sodium, for example).

More specialist "pure" water may need special manufacture and handling to avoid tiny traces of other contaminants when only tiny amounts might interfere with a sensitive reaction or process. The electronics industry, for example, often uses water that has to have contaminant levels of many components below 1 part per billion or better because the processes of making chips is exquisitely sensitive to tiny amounts of some compounds. This water is very expensive.

So most "deionised water" will have small amounts of ionic contaminants plus the ions from self-ionisation. But some specialist types of pure water will have very few ions other than those from self-ionisation. But the number will depend on the type of water and even the purest will still have a very very small amount of non-water ions in it.


Distilled water main impurity is due to the dissolution of atmospheric $\ce{CO2}$ which produces some acidity due to the reaction $\ce{CO2 + H2O -> H3O+ + HCO3^-}$. Usually it is sufficient to produce a $\mathrm{pH}$ not far from $5.5$. This value changes from one place to another one.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.