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 from non-ionic media. 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 present. However, once the purity of water reaches a certain level, the total amount of impurities is already minimal, and the concentration of individual impurities is even lower, with some being difficult to detect. In such cases, the resistivity of water is commonly used to indicate its purity. Since pure water has a concentration of H+ and OH- ions of 10^-7 mol/L, its conductivity is very low, nearly non-conductive. When water contains certain impurities (such as soluble salts), the conductivity increases rapidly due to the conductive nature of the impurity ions. Therefore, the conductivity of water is closely related to its purity.

High-purity water is a type of chemical-grade water, which refers to water with almost all conductive media removed, as well as colloidal substances, gases, and organic matter reduced to a very low degree. The impurity content is less than 0.1 mg/L, with a resistivity greater than 17.5Ω, and a pH range of 6.8 to 7.0, with impurities below 0.01 mg/L.