High purity water refers to water with a conductivity less than 0.1 μs/cm at 25°C and a residual salt content less than 0.3 mg/L, which has been purified by removing trace impurities such as bacteria, microorganisms, and particles that are not electrolytes. Preparation methods include distillation, membrane separation, ion exchange, and sterilization. It is primarily used in the electronics and microelectronics industries, as well as in papermaking, electronics, and nuclear industries.

The purity of water is typically expressed as the relative content of impurities in the water. However, once the purity of water reaches a certain level, the total amount of impurities is minimal, and the concentration of individual impurities is even lower, some are difficult to detect. In such cases, the resistivity of water is commonly used to indicate its purity. Since the concentration of H+ and OH- ions in pure water is both 10^-7 mol/L, its conductivity is very low, almost non-conductive. When certain impurities (such as soluble salts) are present in the water, due to the conductivity of the impurity ions, the conductivity rises rapidly. Therefore, the conductivity of water is closely related to its purity.

High-purity water is a chemical purity water, referring to water with almost all conductive media removed, as well as colloidal substances, gases, and organic matter in the water all reduced to a very low level. The impurity content is less than 0.1 mg/L, the resistivity is greater than 17.5 Ω, and the pH is between 6.8 and 7.0, with impurity content lower than 0.01 mg/L.