A HIGH-TEMPERATURE CATALYTIC-OXIDATION METHOD FOR THE DETERMINATION OF DISSOLVED ORGANIC-CARBON IN SEAWATER - ANALYSIS AND IMPROVEMENT

The high-temperature catalytic oxidation (HTCO) method for the determination of dissolved organic carbon in seawater, reported by SUGIMURA and SUZUKI (1988, Marine Chemistry, 24, 105-131) has been improved to provide more valid data on the levels of dissolved organic carbon (DOC). The improvements are as follows. (1) An open-close valve, made from an alloy of iron, molybdenum, nickel and chromium and without a silicone septum, is used as the injection port. (2) Copper oxide and sulfix (mixture of AgO and CoO) are packed with a 3% platinum (Pt) catalyst into the combustion tube for the complete elimination of halogenated and sulfur compounds. (3) The top of the catalyst is covered with Pt gauze to prevent the Pt catalyst from flying upwards. (4) A glass water-trap, containing phosphoric acid and silver nitrate in solution, is connected just below the outlet of the combustion tube. (5) A high-sensitivity, infra-red gas analyser (Beckman model 880) is used. The blank value for the system is checked using water prepared by passage through the improved HTCO system. The total blank value (system blank plus water blank) ranges from 15 to 30-mu-M C. Of the total blank volume, the blank value for the system which was examined using water collected from the HTCO system. was less than 3-mu-M C. The greatest contributions to the blank value in this machine are made by the organic matter originally present in deionized water. The oxidation efficiency of the catalyst with different levels of Pt was examined for measurements of DOC. Although there is little difference in detected concentrations of DOC from the surface to deep waters with levels of 1.5, 3 and 5% Pt in the catalyst, the alumina impregnated with higher levels of Pt is much better for the complete oxidation of the DOC in seawater. The size of the combustion tube, the quality of the catalyst and the uniform distribution of the furnace temperature are kev elements in determining the oxidative capacity of the system. It is also important to prevent backflushing just after the injection of the sample and to prevent the catalyst from moving in the combustion tube for the accurate measurement of DOC in seawater.