Deionized (DI) water is a form of highly purified water from which almost all dissolved mineral ions have been removed through a chemical process. This treatment results in water exceptionally free of charged particles, such as sodium, calcium, chloride, and sulfate ions, common in tap water. The absence of ionic contaminants makes DI water indispensable for precise chemical analysis and manufacturing processes. Water quality is measured by its freedom from these dissolved substances, which can interfere with sensitive experiments and industrial operations.
How Deionization Works
The deionization process relies on synthetic materials called ion exchange resins to chemically remove undesirable ions from the source water. These resins are small, porous polymer beads categorized into two main types: cation exchange resins and anion exchange resins. Cation resins carry a negative functional group and are charged with a hydrogen ion (\(\text{H}^{+}\)). As water passes through, the resin exchanges its \(\text{H}^{+}\) for positively charged contaminant ions, such as calcium (\(\text{Ca}^{2+}\)) or magnesium (\(\text{Mg}^{2+}\)).
The water then flows through the anion exchange resin, which contains a positive functional group charged with a hydroxide ion (\(\text{OH}^{-}\)). This resin captures negatively charged contaminants, including chloride (\(\text{Cl}^{-}\)) and sulfate (\(\text{SO}_{4}^{2-}\)), and releases the \(\text{OH}^{-}\) ion in exchange. The released hydrogen (\(\text{H}^{+}\)) and hydroxide (\(\text{OH}^{-}\)) ions immediately combine to form a neutral water molecule (\(\text{H}_{2}\text{O}\)). This ion-for-ion replacement mechanism is highly efficient at removing dissolved inorganic solids, resulting in water with extremely low ionic content.
Key Characteristics and Purity Levels
The quality of deionized water is primarily defined by its electrical conductivity or, inversely, its resistivity. Since pure water is a poor conductor, a low conductivity reading (measured in microSiemens per centimeter, \(\mu\text{S}/\text{cm}\)) indicates high purity due to the absence of charge-carrying ions. Ultrapure DI water can reach a resistivity of \(18.2\) megaohm-centimeters (\(\text{M}\Omega\cdot\text{cm}\)) at \(25\) degrees Celsius, corresponding to a conductivity below \(0.056\,\mu\text{S}/\text{cm}\). This electrical metric is far more accurate for assessing ionic purity than measuring total dissolved solids (TDS).
The \(\text{pH}\) of freshly produced deionized water is neutral, around \(7.0\), but it is difficult to measure accurately because the water lacks buffering capacity. Upon exposure to the atmosphere, DI water readily absorbs carbon dioxide (\(\text{CO}_{2}\)), which dissolves to form carbonic acid (\(\text{H}_{2}\text{CO}_{3}\)). This reaction introduces ions that cause the \(\text{pH}\) to drop to a slightly acidic range, often between \(5.5\) and \(6.5\). Despite this change, the water is still considered high-purity, as the acidity is derived from absorbed gas rather than mineral contamination.
Primary Uses in Laboratory and Industry
The low ionic content of deionized water makes it indispensable for applications requiring a non-reactive solvent. In the laboratory, it is used to prepare precise standard solutions and reagents for chemical analysis. The absence of interfering ions ensures that only the dissolved substances of interest contribute to the measurement, maintaining the accuracy and reliability of experimental results, such as in high-performance liquid chromatography or spectroscopy.
Deionized water is also widely employed for the final rinsing of sensitive laboratory glassware and equipment. Using it prevents the deposition of trace mineral residues, which could contaminate subsequent experiments or cause spotting on surfaces. Industrially, its use is widespread in the manufacturing of electronics, particularly semiconductors and microchips, where even minute traces of conductive ions could cause electrical shorts or defects.
DI water is the preferred choice for use in cooling systems and boiler feed water. In these applications, its lack of dissolved minerals prevents the formation of scale and deposits on internal equipment surfaces, which extends the lifespan and maintains the efficiency of the machinery. It is also regularly used to fill lead-acid batteries, as the absence of ions prevents internal corrosion and premature failure of the battery cells.