The most widely used solvent in biochemical laboratories is "water". Tap water can not be used to prepare reagents for biochemical experiments, but purified water can only be used. The purity of water used in biochemical experiments is required to be relatively high. It can usually be considered that the higher the quality of water, the more true, reliable and accurate the experimental results will be. Therefore, the quality of experimental water must be guaranteed. Two kinds of pure water commonly used are secondary distilled water and ionless water. Higher water quality is required in ultra-pure analysis and special biochemical experiments, such as 15 ~ 18 M Ω undefinedcm high-purity ionless water, high-purity water without heat source, sterile water, sub-boiling distilled water, carbon dioxide-free distilled water, etc.