THE UNDERESTIMATION OF CONCENTRATIONS OF DISSOLVED ORGANIC-CARBON IN FRESH-WATERS

We analysed a potassium biphthalate standard, a certified reference ma terial and ten sets of freshwater samples (3.1-45.0 mg l(-1)) for diss olved organic carbon (DOC) using six methods: two high temperature cat alytic oxidation; three UV-promoted persulfate oxidation; and the Walk ley-Black dichromate digestion. All techniques produced comparable res ults when determining standards. However, DOC concentrations determine d by the persulfate and Walkley-Black methods were significantly lower for the freshwater samples (paired-difference t-test, alpha = 0.05) c ompared with values measured using the high temperature methods. Our r esults demonstrate that some commonly used techniques for the determin ation of DOC underestimate concentrations by 20-24%.