The underestimation of concentrations of dissolved organic carbon in freshwaters

We analysed a potassium biphthalate standard, a certified reference material and ten sets of freshwater samples (3.1–45.0 mg 1−1) for dissolved organic carbon (DOC) using six methods: two high temperature catalytic oxidation; three UV-promoted persulfate oxidation; and the Walkley-Black dichromate digestion. All techniques produced comparable results when determining standards. However, DOC concentrations determined by the persulfate and Walkley-Black methods were significantly lower for the freshwater samples (paired-difference t-test, α = 0.05) compared with values measured using the high temperature methods. Our results demonstrate that some commonly used techniques for the determination of DOC underestimate concentrations by 20–24%.