What is Deionized Water?

Posted by
John Woodard on March 20, 2024

Deionized water (DI water) is water with almost all its dissolved ion and mineral content removed. It is one of the purest forms of water, and therefore lends itself to applications in the medical, pharmaceutical, and laboratory fields, but its usefulness doesn’t stop there. From aquariums to food processing facilities, to car washes, you have most likely encountered deionized water. In this article, you can learn about deionized water, why it is important, and its many uses.

What is deionization of water?

Deionization of water is the removal of ions, such as minerals and salts, from water through a process called ion exchange. To better understand deionization, it is helpful to first understand what ions are. Ions are electrically charged atoms or molecules that either have a net negative or a net positive charge. Ions with a positive charge are called cations and ions with a negative charge are called anions. For example, calcium and iron are cations and chloride and nitrates are anions.

In deionization, water flows through a bed of resin beads that have positive or negative charges. Positively charged resin attracts and traps negative anions, while negatively charged resin attracts and traps positive cations. The resin then releases hydrogen (H+) ions in exchange for the cations and hydroxide (OH-) ions in exchange for the anions. Lastly, the hydrogen and hydroxide ions combine to form water (H+ + OH- = HOH or H2O). All that remains is water molecules, hence deionized water’s reputation for being one of the purest forms of water.

What does deionization remove from water?

Deionization removes ions from water, including the following dissolved salts and minerals.

    • Calcium
    • Magnesium
    • Iron
    • Sodium
    • Chloride
    • Sulfates
    • Nitrates
    • Carbonates
    • Silica
    • Iodine

How is deionized water measured?

Deionized water is measured by its electrical conductivity and resistivity. Conductivity refers to a material’s ability to conduct electricity, while resistivity refers to how strongly a material resists an electrical current. Ions conduct electricity, so the number of ions in water will determine its conductivity and resistivity. DI water contains few if any ions, and therefore has low conductivity and high resistivity. The lower the conductivity and the higher the resistivity, the higher quality of deionized water.

Conductivity is measured in microsiemens/cm (µS/cm) and resistivity is measured in megohm/cm (mΩ/cm). Deionized water typically has a conductivity of 0.055 µS/cm and a resistivity of 18.2 mΩ/cm.

What is a deionization water filter?

A deionization water treatment system is an arrangement of tanks that deionizes water through the ion exchange process. Water is forced through a resin bed (either in a pressure tank or in a drop-in cartridge inserted into a tank), where mineral particles attach to resin beads with the corresponding electrical charges. Non-water ions remain trapped in the resin, while pure water continues to flow through. Deionized water systems can be a simple one-tank setup, or a complex multi-tank setup with additional filtration systems, such as reverse osmosis or ultraviolet systems, to further polish the water.

Learn More: What is a reverse osmosis system? | What is a UV water purifier? 

What is deionized water used for?

Deionized water is predominantly used industrially but can also be used around your home. Below are the most common uses of DI water.

  • Medicine
  • Pharmacies
  • Laboratory testing
  • Electronics manufacturing
  • Engine cooling systems
  • Food processing
  • Cosmetics
  • Aquariums
  • Hydroponics
  • Car washes
  • Household appliances
  • Cleaning solutions
Water in Lab

Why is deionized water important?

Deionized water is important because of its purity. It is a blank state of water, assuming the chemistry of other substances added to it. This characteristic is critical in the medical and pharmaceutical industries because water is a component of most medicines and medical solutions. If ions are left in the water, they can react to other substances and change the chemical composition of the final product. Medicine is a very precise science, and DI water is an important part of providing safe, reliable medicines and care. DI water’s purity and reliability also prevent unwanted reactions and allow for accuracy in lab tests and experiments.

In industrial settings, deionized water is important because it won’t react to other materials or leave behind mineral deposits and scaly residue. By using deionized water in car washes and in other cleaning services and products, you can ensure a spot-free rinse with no unwanted water stains left behind.

Can you drink deionized water?

Yes, you can drink deionized water, but it may not always be the best choice. Remember that deionization only removes ions, so if you are going to drink it, ensure that the water has also been treated to remove bacteria, viruses, and other harmful contaminants. DI water does not contain any minerals or electrolytes, so it may have a flat taste and be unable to quench your thirst.

Three types of deionization

There are three types of deionization determined by the type of resin used in the ion exchange process: weak base deionization, strong base deionization, and mixed bed deionization.

1. Weak base deionization

Weak base deionization uses a resin composed of strong acid cations and weak base anions. It specializes in demineralization and reducing hardness in water. Compared to strong base deionization and mixed bed deionization, it produces the lowest quality of deionized water, but is often used for spot-free car wash rinses and window cleaning.

2. Strong base deionization

Strong base deionization uses a resin composed primarily of strong base anions, as well as strong acid cations to create a mid-range quality of deionized water. Like weak base deionization, strong base deionization demineralizes and de-alkalizes water, but is also apt at removing silica. Strong base deionization is commonly used in manufacturing, as it is less acidic and gentle on metals.

3. Mixed bed deionization

Mixed bed deionization uses a resin composed of a 40:60 ratio of strong acid cations to strong base anions to produce the purest form of deionized water. It essentially delivers the benefits of both weak base and strong base deionization, by efficiently removing all ions from water. Mixed bed deionization is used in many applications, including medical uses, pharmaceuticals, lab tests, and electronics.

Deionized vs distilled water

Deionized water and distilled water are similar in that they both produce highly purified water, but they use two distinct processes. Deionization removes ions from water through the ion exchange process, while distillation is a process that mimics the hydrologic cycle. Water distillers convert water into steam, eliminating a myriad of contaminants that do not evaporate with water. When the water returns to its liquid form, it is free of contaminants.

Depending on the original water source, distilled water may be more pure than deionized water, as distillation can remove bacteria, viruses, and other organic material from water that deionization does not. Deionization only removes ions, charged non-organic materials such as minerals and salts, from water. If water is pre-filtered by a reverse osmosis system to remove organic materials, the deionization will be more efficient and produce a higher quality pure water.

Deionization is a quicker and more efficient process than distillation. Since deionization is a chemical process, energy is only typically required to move the water through the system and to monitor the process. In contrast, distillation is a lengthy physical process that requires an energy source to heat the water, especially when large amounts of water need to be boiled, cooled, and collected.

Learn More: What is distilled water and is it safe to drink?

Deionization with reverse osmosis

In many cases, deionization is used in conjunction with reverse osmosis systems. Reverse osmosis reduces dissolved and suspended contaminants in water by using pressure to push unfiltered water through a semipermeable membrane. The membrane has small pores that block contaminants but allow clean water to flow through.

Placing a reverse osmosis system before a deionization system will increase the deionization system’s efficiency and capacity while reducing cost. A reverse osmosis system removes the bulk of total dissolved solids from water, and deionization polishes the water by removing any remaining ions without interference from other contaminants. In fact, reverse osmosis in tandem with deionization results in a high-quality deionized water that costs 10 to 20 times less per gallon than water through deionization alone. When both systems are used together, the cost per gallon is about two to four cents, but if you rely only on deionization, the cost can exceed 50 cents per gallon.

Learn more: 3 of the best reverse osmosis systems


If you have any additional questions, please do not hesitate to contact us.

No comments yet.
Leave a comment