Methods for measuring organic carbon concentration in seawater

　One way to determine the weight concentration of DOM (dissolved organic matter) is to separate inorganic matter from seawater by chemical or thermal treatment and then weigh the residue (organic matter). Although weight measurement sounds simple, it is difficult to measure accurately because water must be excluded and the particle weight is much smaller than the filter weight. It can be quite a tedious process.

　If you want to determine DOC concentration (carbon concentration of dissolved organic matter), you can remove inorganic carbonic acid from a water sample and measure the concentration of carbon dioxide produced by combustion of the organic matter. High-temperature combustion using a platinum catalyst has improved the accuracy of DOC concentration measurements and has recently revealed the distribution of DOC in the ocean.

　Below we show how to measure carbon content in seawater with filtering and chemical treatment. Please familiarize yourself with this procedure.

【Procedures for measuring dissolved organic carbon】

①　Filters seawater to separate the particulate and dissolved forms
　Since the carbon content is measured, the filter material should be carbon-free. A quartz glass fiber filter (Glass Fiber Filter/ GF filter) is used. Since the concentration of dissolved organic carbon in seawater is low, filters are heated at 500°C before use to remove carbon contamination.

②　Add acid to the dissolved form to remove the carbonic acid component (when measuring the carbonic acid component, measure the amount of carbonic acid generated)
　Hydrochloric acid or phosphoric acid are used. When seawater becomes acidic, all carbonic acid and bicarbonate ions that are in dissociative equilibrium in seawater become carbonic acid (H₂CO₃). By bubbling high-purity nitrogen gas into the sample water, the carbonic acid is removed as carbon dioxide. When measuring the carbonic acid content of seawater, this carbon dioxide content is measured.

③　The dissolved form is heated at high speed to burn organic carbon
　Using a platinum catalyst enables high-speed combustion and high-precision measurement of dissolved organic carbon in seawater. The technology was developed by Shimadzu Corporation of Japan.

④　Measures the concentration of carbon dioxide produced by combustion