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In fields such as laboratory analysis and electronic component manufacturing, the resistivity of ultrapure water is a core indicator for evaluating water purity. Most commercially available water purification systems are equipped with conductivity meters to monitor ion content, but users often find discrepancies between external measurements and the equipment's display. A deeper analysis of the concept, theoretical limits, and measurement methods of resistivity is necessary.
The unit of ultrapure water resistivity is MΩ·cm, which directly reflects the concentration of cations and anions in the water. The core principle is that ion concentration is inversely proportional to resistivity. The fewer impurity ions, the weaker the conductivity and the higher the resistivity; when water is contaminated, the ion concentration increases, and the resistivity decreases.
18.2 MΩ·cm is the benchmark for ultrapure water purity. At this level, the water contains almost no impurity ions, only trace amounts of H⁺ and OH⁻ from the dissociation of water molecules, meeting the stringent requirements of high-performance liquid chromatography (HPLC) and semiconductor wafer cleaning.
If ions are completely removed, can the resistivity be infinitely large? The answer is no; this is determined by the chemical properties of water. Conductivity (unit: μS/cm) is directly proportional to ion concentration, and water molecules spontaneously dissociate: H₂O⇄H⁺ + OH⁻. At 25℃, the concentrations of H⁺ and OH⁻ are both 1.0 × 10⁻⁷ mol/L, and the ion product constant K_W = 1.0 × 10⁻¹⁴.
Based on this calculation, the lowest conductivity of water at this temperature is 0.055 μS/cm. Since resistivity is the reciprocal of conductivity, the highest resistivity of ultrapure water at 25℃ is 18.2 MΩ・cm, which is the theoretical limit determined by the physicochemical properties of water and cannot be exceeded.
In actual measurements, the resistivity of ultrapure water is often lower than the value displayed on the equipment, mostly due to oversights in details:
Instrument Selection: Prioritize online conductivity meters to avoid the decrease in resistivity caused by CO₂ absorption from the water upon contact with air during offline measurements; a conductivity cell with an electrode constant of 0.01 cm⁻¹ is required to ensure detection sensitivity; the instrument should have a temperature compensation function with 0.1℃ accuracy to eliminate temperature interference.
Operating Procedures: Purge air bubbles from the conductivity cell by passing water through it to avoid affecting the detection; adjust the water flow rate to 1-2 ml per second, and record the reading after it stabilizes; simultaneously label the measurement temperature for data comparison and calibration.
18.2 MΩ・cm is not a universal standard: 1-5 MΩ・cm pure water is sufficient for ordinary glassware cleaning, while semiconductor chip manufacturing requires 18.2 MΩ・cm ultrapure water. Furthermore, the resistivity of ultrapure water decreases over storage time; for high-requirement scenarios, "preparation and use immediately" is recommended.
