Deionized (DI) water is sought after for its unique properties, particularly regarding electricity. In an ideal, theoretical state, DI water is non-conductive, or an extremely poor conductor. This is because deionized water is defined as water from which almost all mineral ions have been removed through purification. However, maintaining this state outside of a controlled laboratory environment is challenging, meaning that DI water encountered in daily use often possesses a slight degree of conductivity.
The Role of Ions in Water’s Electrical Conductivity
The ability of water to conduct an electrical current is not due to the water molecules themselves, but rather the presence of dissolved charged particles called ions. Pure water, consisting only of H₂O molecules, is an electrical insulator because the molecules are neutral and cannot effectively transport a charge. Conductivity requires these mobile charged entities to transport the electrical current.
These charged particles, known as cations (positively charged) and anions (negatively charged), originate from dissolved salts, minerals, and other compounds found in natural water sources. For example, when table salt dissolves, it splits into sodium ions (\(\text{Na}^{+}\)) and chloride ions (\(\text{Cl}^{-}\)), which act as carriers for electricity. Water with a high concentration of these dissolved solids, such as tap water or saltwater, is highly conductive.
Achieving Ultra-Purity: How Deionization Works
The process of deionization is designed to strip away the conductive ions naturally present in water. This purification method involves passing the water through specialized materials known as ion-exchange resins. These resins are tiny plastic beads, typically housed in columns, that are chemically formulated to attract and swap out the unwanted ions.
Cation-exchange resins trade their hydrogen ions (\(\text{H}^{+}\)) for positively charged contaminants like calcium and sodium. Anion-exchange resins exchange their hydroxide ions (\(\text{OH}^{-}\)) for negatively charged impurities such as chloride and sulfate. The exchanged \(\text{H}^{+}\) and \(\text{OH}^{-}\) ions then combine to form a new, extremely pure water molecule (\(\text{H}_{2}\text{O}\)).
The result is water with an extremely low concentration of dissolved conductive solids and very low electrical conductivity. This state of high purity is often measured in electrical resistivity, which is the inverse of conductivity. Ultra-pure deionized water can achieve a maximum resistivity of \(18.2\) megaohms-centimeter (\(\text{M}\Omega\cdot\text{cm}\)) at \(25^\circ\text{C}\).
The Instability of Deionized Water and Recontamination
While deionized water leaves the purification system in a nearly non-conductive state, this purity is highly unstable and difficult to maintain. The moment this ultra-pure water is exposed to the atmosphere, it immediately begins to absorb carbon dioxide (\(\text{CO}_{2}\)) gas, which is the most significant source of recontamination.
The dissolved \(\text{CO}_{2}\) reacts with the water to form carbonic acid (\(\text{H}_{2}\text{CO}_{3}\)). This weak acid quickly dissociates into hydrogen ions (\(\text{H}^{+}\)) and bicarbonate ions (\(\text{HCO}_{3}^{-}\)). The introduction of these new charged particles immediately increases the water’s ion concentration.
This process causes the water’s conductivity to rise rapidly from its ultra-pure value, sometimes by a factor of ten or more within moments of exposure. Furthermore, deionized water is so reactive that it can leach ions from the materials of its storage container, such as plastics or glass, introducing contamination. This quick recontamination explains why DI water in a sealed bottle is often still slightly conductive, making it a poor insulator in practical, real-world applications.