What is deionized water?

The deionized water is free of dissolved ions; that is, without electrically charged atoms. Water normally has a lot of ions; those that are eliminated when deionized have positive electrical charges or cations, and negative or anions. Among the positive ions removed by deionizing water are sodium, calcium, iron, and copper.

Among the anions extracted are carbonates, fluorides, chlorides and others. The deionization process takes place by passing tap, spring or distilled water through an electrically charged resin or ion exchange resin. It is worth noting that deionized water is not necessarily pure water.

Deionization does not remove uncharged organic particles (eg, most bacteria and viruses), or organic contaminants. Deionized water is often used in laboratories where the presence of dissolved ions would interfere with analyzes.

It can be drunk, but it is not advisable to do so regularly. On the one hand, because its taste and mouthfeel are not entirely pleasant; on the other hand, because it lacks minerals. Calcium and magnesium, normally found in water, have beneficial health effects.

Deionized water or DI water is reactive, so its properties begin to change as soon as it is exposed to air. Deionized water has a pH of 7 as it exits the ion exchanger.

However, when it comes into contact with carbon dioxide in the air, the dissolved CO 2 reacts to produce H (+) and HCO 3 (-), leading the water to acidification with a pH close to 5.6.

This decrease in pH makes it corrosive, so its use is inconvenient if it comes in contact with metals for a long time.

It has a very low conductivity. The specific conductivity or conductance of a substance is related to the amount of total dissolved solids (STD). This parameter is a measure of the ability to conduct electricity of an electrolyte solution.

In a deionization process, the water quality expressed with this parameter is 5.5 μS / m (micro Siemens per meter).

Uses / applications

It is used when drinking water and distilled water can negatively affect its intended use, whether mechanical or biological. In general, these are situations in which the least possible presence of dissolved salts in the water is required.

Cooling systems

Due to its low electrical conductivity, deionized water is a good coolant for equipment such as high- powered lasers .

It prevents overheating and is used in other medical devices to help control a certain level of temperature. Its use avoids possible obstructions due to the formation of mineral deposits.

Lab tests

It is used in the preparation of solvents in chemical laboratories. The use of ordinary water can cause the results to be erroneous due to the contaminants present. Deionized water is also used to clean laboratory equipment.

Industrial machinery

Regular cleaning of industrial machinery is part of basic maintenance to preserve its useful life. The use of deionized water slows down the formation of deposits of the salts present in the water, reducing corrosion.

Automobile engines

Deionized water is widely used as the best alternative to increase the life of lead-acid batteries, as well as the engine cooling system.

Impurities found in normal water significantly reduce battery life and induce corrosion in the engine. In addition, the deionized water serves to dilute the concentrated antifreeze.

Fire extinguishers

Water is not the most suitable substance to put out fires that arise around electrical equipment. Due to its low electrical conductivity, deionized water will put out the fire and will not cause as much equipment damage as normal water would.


Regular water can contain so many impurities that undesirable algae can grow in fish ponds. For this reason, the use of deionized water is usually preferred; their quality can also contribute to the overall health of the fish.


It is suitable for washing window panes or other types of glass. The deionized water used at the time of rinsing prevents the appearance of stains when drying, due to the deposit of salts. 

It is also useful in pressure cleaners for cars and buildings due to the absence of mineral deposits when cleaning.

How to get it?

The water to be deionized is passed through a bed of ion exchange resins; the ions contained in the water are adsorbed on this resin. Resins are made of synthetic material, generally they are polymer spheres on which an ion has been permanently attached.

This ion, which is fixed in the resin, cannot be removed or replaced, as it is part of the structure. To preserve the electrical neutrality of the resin, these fixed ions are neutralized by an ion that has an opposite charge. That ion has the ability to exit or enter the resin.

As the water passes through the resin, ion exchange occurs. During this, the mobile ions are replaced by an equivalent amount of ions with the same polarity from the water. That is, ions of the same sign are exchanged.

The hydronium ions H 3 O (+) are exchanged for the cations that are in water and the hydroxyl ions OH (-) for the anions dissolved in it.

Thus, all the ions present in the water remain in the resin, and the exchanged hydronium and hydroxyl ions combine to form deionized water.

Types of resins

Resins are classified into two categories according to the nature of the ions to be exchanged. If it is cation exchange, we speak of cationic resins; if it is anions that are going to be permuted, it is called an anionic resin.

It is not possible to produce a resin that exchanges cations and anions, since the permanent cations located in the resin would cancel out the permanent anions and exchange with the outside would not be possible.

Therefore, cation exchange resins and anion exchange resins must be manufactured and operated separately.

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