Pure Water Equipment Terminology Explanation: TOC and Resistivity

In the technical parameters of pure water equipment, we often see the terms TOC and resistivity. These are two standards for measuring water quality. What do they mean?

I. What is Total Organic Carbon (TOC)?

The content of organic matter in water, expressed as the amount of carbon, the main element in organic matter, is called total organic carbon. TOC measurement is similar to TOD measurement. At a high temperature of 950℃, the organic matter in the water sample is vaporized and burned to produce CO2. The amount of CO2 produced is measured using an infrared analyzer, thus determining the total organic carbon content. During the measurement process, inorganic carbon compounds in the water, such as carbonates and bicarbonates, will also produce CO2, which should be measured separately and deducted. If the water sample is filtered through a 0.2μm microporous membrane, the carbon content measured is dissolved organic carbon (DOC). TOC and DOC are commonly used water quality indicators.

II. What is Water Resistivity?

When measuring the electrical conductivity of water, it is related to the water's resistance value. A high resistance value indicates poor conductivity, while a low resistance value indicates good conductivity. According to Ohm's Law, at a constant water temperature, the resistance R of water is inversely proportional to the cross-sectional area F of the electrodes and directly proportional to the distance L between the electrodes, as shown in the following formula: R = ρ, where ρ is resistivity, or specific resistance. The unit of resistance is the ohm (Ω), or microohm (μΩ), where 1 Ω equals 10⁶ μΩ; the SI unit of resistivity is the ohmmeter. If the cross-sectional area F of the electrodes is 1 cm², the distance L between the two electrodes is 1 cm. The resistivity of water is related to the amount of salt in the water, the ion content, the charge of the ions, and the velocity of the ions. Therefore, pure water has a high resistivity, and ultrapure water has an even higher resistivity. The purer the water, the greater its resistivity.