High-Purity Water High-Purity Water may be prepared by deionizing previously distilled water and then filtering it through a 0.45-μm rated membrane. This water must have an in-line conductivity of NMT 0.15 μS/cm (NLT 6.67 megohm-cm) at 25°C. If the water of this purity contacts the atmosphere even briefly as it is being used or drawn from its purification system, its conductivity will immediately increase by as much as about 1.0 μS/cm at 25°C as atmospheric carbon dioxide dissolves in the water and equilibrates to hydrogen and bicarbonate ions. Therefore, if the analytical use requires that water conductivity remains as low as possible or the bicarbonate/carbon dioxide levels be as low as possible, the water should be protected from atmospheric exposure. High-Purity Water is used as a reagent, as a solvent for reagent preparation, and for test apparatus cleaning where less stringent water specifications would not be considered acceptable. However, if a user’s routinely available Purified Water is filtered and meets or exceeds the conductivity specifications of High-Purity Water, it could be used in lieu of High-Purity Water.