The Chroma of water is the standard when measuring the color of water. The Chroma in water quality analysis usually refers to the true color of water, that is, the color produced by soluble substances in water samples. Therefore, before determination, water samples need to be clarified, centrifuged or filtered with 0.45 um filter membrane to remove SS, but not with filter paper, because the filter paper can absorb part of the color of water.
The result of the determination of the original sample without filtration or centrifugation separation is the apparent color of water, that is, the color produced by both soluble and insoluble suspended substances. In general, the apparent color of water can not be measured and quantified by platinum-cobalt color measurement. Usually, the characters such as shade, tone and transparency can be described by words, and then determined by dilution multiple method. The results measured by platinum-cobalt color measurement and the Chroma values measured by dilution multiples are often not comparable.
The color measurement methods are platinum-cobalt color measurement method and dilution multiple method (GB 11903-1989). The two methods should be used independently, and there is generally no comparability between the measured results. The platinum-cobalt color measurement method is suitable for the determination of clean water, slightly polluted water and slightly yellow water, as well as cleaner surface water, groundwater, drinking water and reclaimed water, wastewater after advanced treatment, etc. The dilution multiple method is usually used to determine the Chroma of industrial wastewater and surface water with serious pollution.
The platinum-cobalt color measurement method is a standard unit of chromaticity, commonly known as 1 degree, when 1L water contains 1mg Pt (IV) and 2mg cobalt chloride (II) Hexahydrate. A standard Chroma unit is prepared by adding 0.491 mg K2PtCl6 and 2.00 mg CoCl2_6H 2O to 1L water, also known as the platinum-cobalt standard. Standard chromaticity units can be obtained by adding standard platinum-cobalt reagents several times. Owing to the high price of potassium cobalt chloride, K2Cr2O7 and CoSO4_7H2O are usually used to prepare standard solution of alternative Chroma according to certain proportion and operation steps. When measuring the Chroma, the Chroma of the water sample can be obtained by comparing the water sample to a series of standard liquids with different Chroma.
The dilution multiple method is to dilute the water sample with optical pure water to nearly colorless and then move it into the colorimetric tube. Under the white background, the color of the water sample is darker than that of optical pure water with the same liquid column height. If the difference is found, dilution is carried out until the color can not be detected. At this time, the dilution multiple of the water sample is the value expressing the color intensity of the water, and its unit is double.