Analytical Sciences

Repetitive Determination of Chemical Oxygen Demand by Cyclic Flow Injection Analysis Using On-line Regeneration of Consumed Permanganate

Michio ZENKI, Satoru FUJIWARA, and Takashi YOKOYAMA

Department of Chemistry, Okayama University of Science, 1-1, Ridai-cho, Okayama 700-0005, Japan

A cyclic flow injection analysis (cyclic FIA) for the repetitive determination of chemical oxygen demand (COD) was developed. The acidic KMnO₄ method was carried out by adopting a single-line circulating flow system. The oxidant (KMnO₄) consumed by the oxidation of organic substances was regenerated and reused repeatedly, resulting in an extreme reduction of hazardous wastes. Only 50 ml of the reagent carrier solution containing 0.2 mM KMnO₄ and 1 mM HIO₄ in 0.8 M H₂SO₄ solution was continuously circulated through the system. The KMnO₄ could play two roles: acting as an oxidant of the organic substances and/or a spectrophotometric reagent. The co-existing HIO₄ acted as a regenerator of KMnO₄, which made it possible to recycle the system repeatedly. Under two different digestions (70 and 130°C), 50 repetitive determinations of standard sodium oxalate (6.5 mg COD L^-1) and D-glucose (7.2 mg COD L^-1) were skillfully carried out with a slightly decreased baseline. The analytical frequency was 30 samples per hour for COD determination. The proposed method saved consumption of the used reagents, KMnO₄ and H₂SO₄, and thus these wastes were extremely reduced. The obtained COD values with the proposed method were co-related with those provided by the manual standard method, but were fairly low owing to the insufficient digestion step.

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