■ How does the wet oxidation method oxidize? An oxidizing agent is added to the sample, chemically oxidizing organic matter to carbon dioxide. For example, sodium peroxodisulfate is used as an oxidizing agent, and some systems combine heating or ultraviolet irradiation to assist the reaction. ■ Why is wet oxidation suitable for ultra-pure water? In combustion methods, there is a limit to the amount of sample that can be injected at one time. In contrast, wet oxidation allows for a milder reaction than combustion, enabling the oxidation of a larger amount of sample at once, making it more effective for measuring low concentrations. However, its oxidizing power does not match that of combustion, and the recovery rate tends to decrease for particulate organic matter or substances that are difficult to decompose. Therefore, it is suitable for managing clean water such as ultra-pure water and pharmaceutical water. ■ How is carbon dioxide measured? 1. Infrared absorption: Utilizing the property that carbon dioxide absorbs specific infrared wavelengths. It has high selectivity and is less affected by coexisting components. 2. Conductivity: Organic matter is decomposed using ultraviolet light, and the change in conductivity accompanying the decomposition is measured. ■ Combination of oxidation and detection Typical methods include combinations such as combustion oxidation with infrared absorption, wet oxidation with infrared absorption, and ultraviolet oxidation with conductivity.

■What are the points to note when measuring with conductivity?! The method of determining TOC from the difference in conductivity before and after decomposition, without separating carbon dioxide from the sample, can be affected positively or negatively by ionic organic substances or organic substances containing nitrogen, sulfur, phosphorus, or halogens. ■What should be used for calibration? In the water supply testing method, 2.125g of potassium hydrogen phthalate is dissolved in purified water to make a 1L solution, which is used as the standard stock solution. This solution contains 1mg of carbon per 1mL. Calculating from the molecular weight of potassium hydrogen phthalate (C8H5KO4), the carbon in 2.125g is approximately 1.000g. This is then diluted stepwise to create a calibration curve. ■Things to check before choosing a method 1. Type of sample: Is it wastewater or environmental water, or clean water like ultra-pure water? 2. Suspended matter: How much particulate organic matter needs to be measured? 3. Concentration range: How low of a concentration needs to be measured? ■Rental of measuring devices Our company offers fixed-rate rentals of COD, total nitrogen, and total phosphorus measuring devices, as well as consultations for the maintenance of automatic measuring devices. Please feel free to contact us first.