The Chemistry Behind Deionized Water


Water is one of nature’s most effective solvents. Over time, it is capable of dissolving many elements and aiding in the breakdown of complex molecules. This property has a negative side-effect: when found in its natural state, water often contains numerous impurities. These can include dissolved metals like copper and iron, salts such as sodium, gasses such as nitrogen, and many other minerals. Although water can appear to be clear, it may carry electrically charged ions of everyday elements. Purifying the water necessitates the removal of these ions. Therefore, water is exposed to a chemical process known as deionization to yield DI water.  This process ultimately yields water with lowered levels of mineral ions. Understanding the process can lead to a better appreciation of the end result. So, what is deionized water?

Charged Ions

Chemistry deionization of water

Table salt or other matter dissolves when it is added to water. After dissolution, particles can be found floating freely in the water itself. These particles carry charge, which is based off of both the ratio of protons to electrons as well as the number of valance electrons. Because the molecules have been separated into individual atoms, those atoms retain an electric charge. An abundance of electrons in one atom’s outermost electron cloud can lead to a negative charge. Likewise, the loss or donation of electrons can lead to a positive charge for other atoms. This creates the presence of charged particles, which are called cations and anions.

A Salty Example

Consider table salt, which is composed of a sodium atom and a chlorine atom. Together, these atoms bond with each other because they have opposite charges and electron clouds that are not filled. In water, however, these bonds break down and the sodium atoms and the chlorine atoms float freely. They are able to react with additional elements.

Deionization: An Interplay Between Charged Particles

The process of deionization takes advantage of the presence of charged particles by adding charged resins that are likely to interact with particles. Resins can include a variety of materials. A popular resin is charcoal, a substance that is mostly composed of carbon. Carbon is an excellent element to use for reactions because each carbon atom has the ability to create up to four bonds with other atoms. As carbon chains bond with other materials, the surface area increases. Greater surface area leads to reduced water solubility, which means that the materials become either greasy or solids. Both of these products can be extracted from the water through use of filters or other means.

Repeated Use of Deionization

When the water is deionized and filtered, the total concentration of mineral ions is reduced. This does not guarantee that all ions are removed, however. Depending upon the nature of the water, some mineral ions can and will remain. Performing the process on single water sample two or more times can yield even purer water and this is how DI water is made.

One comment on “The Chemistry Behind Deionized Water

  1. Paul Reeve on said:

    This is a nice and simplified introduction. However, I am uncomfortable with calling carbon a resin. As useful as carbon is for removing some dissolved salts from water, I don’t think that it is very efficient for removing sodium, calcium, or potassium ions, for example, and certainly does not work by itself to indicate the quality of water that has passed through it. One needs to add a colored resin film or bead or granule, I suspect, to do that.

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