The present invention relates to a device for measuring the total nitrogen which the total COD. The device comprises: a plurality of two-way valves (12) of which one end is connected to a buffering tube (14); the buffering tube (14) of which one end is connected to one of the two-way valves (12), the other end is connected to a three-way valve block (16), and a diameter is greater than that of a line; the three-way valve block (16) of which a first end is connected to the buffering tube (12), a second terminal is connected to a sample and reagent supply unit (15), and a third terminal is connected to a heating reactor (18); and the heating reactor (18) including an upper cap (50) formed as a conical shape inclined downwards from the edge to the center.
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