Chroma: pure water is colorless and transparent. There are soil, organic matter, plankton and inorganic minerals in natural water, which make it present certain color. Industrial wastewater contains dyes, biological pigments, colored suspended solids and so on, which is the main source of environmental water coloring. Colored water can weaken the transparency of water, affect the growth of aquatic organisms and reduce the ornamental value of water. The color of water can be divided into true color and surface color. True color refers to the color of water after removing suspended solids, that is, chromaticity; The color of water without removing suspended solids is called surface color. For clean or low turbidity water, its true color and surface color are similar; For industrial wastewater with deep coloring, the difference between them is great. The color of water is usually determined by the following methods. （1） Platinum cobalt standard colorimetry In this method, potassium chloroplatinate and cobalt chloride are dissolved in water to form standard color column (gb11903-89), and then the color of water sample is determined by visual colorimetry. It is stipulated that the color of 1mg platinum and 0.5mg cobalt per liter of water is 1 degree. As a standard color unit, this standard solution is stable and can be stored for a long time. However, potassium chloroplatinate is more expensive. It can be made into standard series by dissolving potassium dichromate and cobalt sulfate in water, but it cannot be preserved for a long time. If the water sample is turbid, it should be placed for clarification. Centrifugation or pore size 0.45 can also be used μ M membrane filtration to remove suspended solids, but not filter paper filtration. The method is suitable for the determination of natural water and drinking water with yellow color. If there is soil or other fine dispersed suspended solids in the water sample, and it is still opaque after being treated by clarification, centrifugation and other methods, the "surface color" shall be determined. If the water sample is not yellow, it can not be compared with the platinum cobalt color column, only the appropriate words can be used to describe its color and chromaticity. （2） Dilution multiple method The method is suitable for the determination of the color of surface water and industrial wastewater polluted by industrial wastewater. During the determination, the type and depth of the color of the water sample should be described in words, such as dark blue, brown yellow, dark black, etc. Then take a certain amount of pretreated water sample and dilute it with distilled water until the color just can't be seen. The chromaticity of the water sample is expressed according to the dilution ratio (gb11903-89), and the unit is times. The water sample should be free of leaves, dead branches and other debris; After sampling, it should be determined as soon as possible, otherwise it should be refrigerated. In addition, CIE has also formulated the standard for determining the chromaticity of water samples by spectrophotometer.