PURPOSE: To obtain a titled vessel into which a reagent can be injected just after water to be tested is taken into the main body of the vessel, by providing a water supply pipe leading the water to be tested into the main body of the tubular vessel via a valve, a discharge pipe discharging this water through a three-way valve, and an injection pipe injecting a color-forming reagent solution into the main body of the vessel.

CONSTITUTION: The main body 1 of a tubular vessel consists of a water supply pipe 3 connected thereto through the intermediary of a valve 2 at the lower end, and of a discharge pipe 5 and an injection pipe 6 which are connected thereto through the intermediary of a three-way valve 4 at the upper end of the main body 1 of the above vessel, respectively. One end of the water supply pipe 3 is communicated with water to be tested, while the injection pipe 6 is marked with scales for measuring a color-forming reagent solution. The measurement of dissolved oxygen is performed in the following processes. First the inlet of the injection pipe 6 is closed by rotating the three-way valve 4, and the color-forming reagent solution is poured into the injection pipe 6. Then, the valve 2 is opened to pour the water to be tested into the main body 1 of the vessel, and it is made to overflow from the discharge pipe 5 via the three-way valve 4. Next, the absence of air bubbles in the main body 1 being confirmed, the three-way valve 4 is rotated, and while the valve 2 is adjusted, the color-forming reagent solution in the injection pipe 6 is injected into the main body 1. Lastly, the vessel is fully shaken to mix the water to be tested with the reagent, the water thus mixed is compared wih a tint standard solution, and thereby the concentration of the dissolved oxygen residue is determined.