The WS1501 CODcr water quality online automatic monitoring instrument adopts the potassium dichromate spectrophotometric method. A quantitative solution of potassium dichromate is added to the sample. In a strong sulfuric acid medium, silver sulfate is used as a catalyst, and mercury sulfate is added as a chloride ion masking agent. After the mixture is subjected to high-temperature and high-pressure sealed digestion at 165 ℃, hexavalent chromium is quantitatively converted to trivalent chromium, and the color of the mixture changes. The color change is measured by spectrophotometry to calculate the actual COD value. The amount of dichromate ions consumed is equivalent to the amount of oxidizable organic matter.