Deionized water, often called DI water, is water that has been processed to remove nearly all of its mineral ions, distinguishing it from tap or even distilled water. While theoretically neutral with a pH of 7.0, deionized water almost instantly becomes slightly acidic upon exposure to the air. This change is a direct chemical consequence of its extreme purity and its interaction with the surrounding atmosphere.
Defining Deionized Water
Deionized water is created through a specialized purification technique known as ion exchange or demineralization. This process removes dissolved inorganic salts and minerals, such as positively charged cations like calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)), and negatively charged anions like chloride (\(\text{Cl}^{-}\)) and sulfate (\(\text{SO}_4^{2-}\)). The water is passed through beds of synthetic resin beads that chemically exchange these foreign ions for hydrogen ions (\(\text{H}^+\)) and hydroxide ions (\(\text{OH}^-\)), which then combine to form pure water (\(\text{H}_2\text{O}\)).
The defining characteristic of deionized water is the extremely low concentration of these charged particles. This absence of ions results in exceptionally low electrical conductivity, which is a common metric used to gauge its purity. However, the ion exchange process does not effectively remove uncharged contaminants, such as bacteria, viruses, or non-ionic organic molecules. Therefore, deionized water is chemically pure in terms of ionic content but is not necessarily sterile or free of all impurities.
The Ideal State of Neutrality
Acidity and alkalinity are measured on the pH scale, which spans from 0 to 14, with a pH of 7.0 representing perfect neutrality. This scale is logarithmic, meaning that a change of one unit represents a tenfold change in acidity or basicity. Neutrality in pure water is defined by the auto-ionization of water molecules, a process where a small fraction of water molecules spontaneously dissociate.
This dissociation produces an equal concentration of hydrogen ions (\(\text{H}^+\)), which are responsible for acidity, and hydroxide ions (\(\text{OH}^-\)), which are responsible for basicity. In chemically pure water at \(25^{\circ}\text{C}\), the concentration of both ions is \(10^{-7}\) moles per liter, resulting in a perfectly neutral pH of 7.0. This balanced state is fragile because the water contains no dissolved salts or minerals to act as a chemical buffer against external influence.
How Deionized Water Becomes Acidic
The moment deionized water is exposed to the atmosphere, its chemistry begins to change rapidly. This change is driven by the absorption of atmospheric carbon dioxide (\(\text{CO}_2\)), which is present in the air. Water, particularly highly pure water, acts as an extremely effective solvent for this gas.
Once the carbon dioxide dissolves, it immediately reacts with the water molecules to form a weak acid known as carbonic acid (\(\text{H}_2\text{CO}_3\)). This chemical reaction is represented simply as \(\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3\). The newly formed carbonic acid then partially dissociates into hydrogen ions (\(\text{H}^+\)) and bicarbonate ions (\(\text{HCO}_3^-\)).
The release of these extra hydrogen ions into the solution is what causes the pH to drop below the neutral value of 7.0. Because deionized water lacks any buffering capacity—the presence of ions that can neutralize an acid—the introduction of even a small amount of carbonic acid has a significant effect on the pH. The pH of deionized water exposed to air will typically stabilize in a slightly acidic range, usually between 5.5 and 6.5.
Practical Use and Safety
The slight acidity of deionized water has important implications across various industrial and laboratory applications. Due to its high purity and the absence of ions, it is widely used in sensitive manufacturing processes, such as electronics and semiconductor fabrication, where mineral deposits could cause defects. In laboratories, it is the standard for rinsing glassware and preparing chemical solutions to ensure that external ions do not interfere with experimental results.
Because it is highly pure and unbuffered, deionized water is an aggressive solvent that will readily attempt to dissolve ions from any material it contacts, including storage containers. While the slight acidity is not generally considered a direct health risk, drinking deionized water is not recommended as a primary source of hydration. It lacks the beneficial minerals, such as calcium and magnesium, found in natural drinking water, and its highly reactive nature means it can potentially leach some of these minerals from the body’s tissues.