What is resistivity and why should it be measured?

The resistivity of a solution describes how strongly it resists an electrical current; in other words, it’s the inverse of conductivity. Another common application for measuring resistivity is when making ultrapure water. Ultrapure water has a high resistivity (>18.18 MΩ·cm at 25° C) and therefore very low levels of conductivity (0.055 µS/cm at 25° C), which can only be accurately measured with a conductivity probe and meter to achieve confidence in its inability to conduct electricity.

This is an important parameter to measure when working with, or making, purified water, such as deionized, distilled, or reverse-osmosis water. Depending on the application, purified water may also be known as reagent water, reagent grade water, clinical lab reagent water, or Type I water. Other terms may apply depending on the purity. Ultrapure water has a high resistivity (>18.18 MΩ/cm at 25°C) and therefore very low levels of conductivity (0.055 μS/cm at 25˚C). Ultrapure water is often used for laboratory, pharmaceutical, semiconductor, or boiler applications.