Principle: A mixture of water sample, potassium dichromate digestion solution, silver sulfate solution (silver sulfate as a catalyst can more effectively oxidize linear fatty compounds), and mercury sulfate solution is heated to 165 ℃. After the organic matter in the solution is oxidized by dichromate ions, the color will change. The analyzer detects this color change and converts it into COD value output. The amount of dichromate ions consumed is equivalent to the amount of oxidizable organic matter.