What Is Deionized Water? Definition, Uses, Risks


What Is Deionized Water
Deionized water is water that has nearly all of its dissolved minerals removed by ion exchange resins.

Deionized water is water that is almost completely free of dissolved minerals. These minerals exist as cations (e.g., sodium, calcium, copper, iron, magnesium, lead) and anions (e.g., chloride, fluoride, sulfate, nitrate). Deionized water has a low electrical conductivity and does not form scale or other deposits on surfaces. On its own, deionization does not remove organic compounds, most pathogens, dissolved gases, or other uncharged contaminants. But, following other purification processes, the resulting deionized water is very pure.

Other names for deionized water are demineralized watered, DIW, or DI water. Technically, demineralized water means something slightly different, as it is nearly free of all minerals, including silica and other neutral compounds, such as iron hydroxides.

How Water Deionization Works

Because deionization only removes dissolved minerals, it follows other techniques. Typically, these include filtration, usually with activated carbon, and either distillation or reverse osmosis.

Deionization uses ion exchange resins. There are separate resins for removing cations and anions. Cations exchange with hydrogen ions on the resin, while anions exchange with hydroxide ions on the resin. The hydrogen (H+) and hydroxide (OH) ions then combine and form water. In many cases, the ion-exchange column is recharged or regenerations, while it is simply replaced for most home systems. While deionization does not categorically remove pathogens, special strong base anion resins bind Gram-negative bacteria. Some ion exchange resins leak sodium because the ion does not readily exchange with H+, but including a final run through a column specifically targeting sodium will remove it, if needed.

There are three ion exchange resin set-ups:

  • Co-current deionization: Co-current deionization is a downflow process where water and regeneration chemicals enter at the top of the ion-exchange column and exit at its bottom. The disadvantage is that regenerating chemicals become diluted by the time they reach the bottom of the column.
  • Counter-current deionization: Counter-current deionization involves water flowing up from the bottom of the column, but regenerants added to the top of the column. Or, water enters from the top and regenerants enter from the bottom. Advantages are less expensive operation (because it uses less regenerant) and a higher purity product.
  • Mixed bed deionization: In both co-current and counter-current deionization, the cation and anion exchange columns are separate. The cation column is first, yields acidic water that then flows into the anion exchange column. Mixed bed deionization uses cation and anion exchange resins within a single column. The disadvantage is the complexity of regeneration. The advantage is the superiority of the highly purified deionized water.

Can You Drink Deionized Water? Health Risks

Drinking a small amount of deionized water is not dangerous, but it’s not a good choice for a few reasons.

  • It demineralizes your teeth. Water naturally contains H+ and OH ions, which readily attract minerals, including calcium in teeth.
  • Deionized water that travels through plumbing fixtures picks up metals, potentially including toxic lead from metal fixtures and toxic plasticizers from PVC (polyvinyl chloride). Mineralized water can still leach chemicals, but does not actively attract it from plumbing.
  • It lacks flavor and tastes “flat” or strange. While not a health risk in itself, it’s harder getting enough water to stay hydrated if the water doesn’t taste good.
  • Drinking deionized water means you aren’t drinking mineralized water. The calcium and magnesium in mineralized water, in particular, are beneficial. However, this is not a big concern because people generally get the minerals the body needs from food.
  • Drinking water purified only using deionization is risky. Deionization, on its own, does not remove pathogens, organic compounds like pesticides, radon, or chlorine by-products. For this reason, is a final step after other purification methods, such as filtration, distillation, or reverse osmosis.

How Does Deionized Water Taste?

Because it lacks ions, deionized water is highly aggressive. When you drink it, it dilutes saliva. So, even though pure water doesn’t bind to taste receptors, it changes the concentration of chemicals already in your mouth, causing a perception of flavor. While the flavor is sort of bland, it’s really the sensation that distinguishes freshly deionized water from drinking water. It slightly tingles or burns. It’s bad for your teeth and probably doesn’t do your taste buds any favors, either.

Deionized Water Uses

Deionized water has several uses:

  • It is superior to other water in analytical techniques because it contains very few impurities. Usually, this is distilled deionized water.
  • Drug manufacturing requires deionized water. The lack of impurities prevents undesirable chemical reactions.
  • As a final rinse in cleaning glassware, deionized water prevents water spots.
  • Both distilled and deionized water find use in lead-acid batteries, as the low level of impurities protects against cell corrosion. Deionized water is a superior option for this purpose. For the same reason, coolants systems also use it.
  • Because of its low conductivity, deionized water can extinguish electrical fires.
  • Deionized water is the water of choice for cleaning electronics and immersion cooling.
  • It finds use in detergents, cosmetics, and beverages.

References

  • Dardel, François; Arden, Thomas V. (2008). “Ion Exchangers” in Ullmann’s Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a14_393.pub2
  • Helfferich, Friedrich G. (1962). Ion Exchange. Courier Dover Publications. ISBN 978-0-486-68784-1.
  • Kozisek, F. (2005). “Health risks from drinking demineralised water“. Nutrients in Drinking Water. World Health Organization. pp. 148–63. ISBN 92-4-159398-9.