Measurement of Dissolved Organic Carbon in Seawater: A Comparison between the High-Temperature Catalytic Oxidation Method and the Persulfate Oxidation Method

Abstract

A high-temperature catalytic oxidation method (HTCO) has been developed to measure dissolved organic carbon (DOC) in natural seawater. The method employs Pt/Al2O3 as a catalyst to oxidize DOC at 680¢XC into CO2 which is detected by a nondispersive IR detector. The detection limit of the method was estimated to be 29.5 uMC on the basis of a sample volume of 66 ul. The precision was generally better than 7% with respect to selected organics added to natural seawater. The detection limit and precision are expected to be lower and greater, respectively, if the injection volume is increased. The HTCO method is faster, more reliable, but higher in analytical cost than the persulfate oxidation method for the analysis of DOC in seawater samples. To obtain reliable values of DOC in a land laboratory, the seawater sample should be filtered, acidified and stored at 4¢XC immediately after sampling. The HTCO method measured the higher values of DOC than did the persulfate method in shelf waters off northeastern and southwestern Taiwan, indicating the incomplete oxidation of the persulfate method. The persulfate method, however, is still recommended for use along with the HTCO method because each method for analyzing DOC provides different geochemical information. The DOC value increases as salinity decreases in surface water off northeastern Taiwan reflecting the influence of terrestrial sources on the distribution of DOC. A vertical decrease of DOC with depth was also found at a station off southwestern Taiwan, indicating that there may be a relationship between DOC distribution and biological turn over of organic carbon. The priorities extended from this study will focus on the spatial and temporal distributions of DOC and their geochemical significance reflected from the use of both methods for marginal seawater under the influence of the Kuroshio.

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