4.1 Appearance and stability of sensory indicators: a uniform liquid without stratification and no obvious mechanical impurities; no crystallization, no discoloration, no odor, and no stratification when returned to normal temperature (20℃±5℃) after heating or cooling. . 4.2 Physical and chemical indicators The physical and chemical indicators of negative ion liquid should comply with the provisions of Table 1. Table 1 Physical and chemical indicators of negative ion liquid Item indicators Oxidation reduction potential (ORP) (25℃, 100% solution) mV ≤-20 Total dissolved solids content (TDS) (25℃, 100% solution) mg/L ≥10000 Purification performance , % pollutant purification efficiency Class I and Class II formaldehyde ≥85 ≥70 Total volatile organic compounds (TVOC) ≥80 ≥65 4.3 Hazardous substance limits No hazardous substances specified in GB18582 shall be detected.