GB/T 7477-1987 is a water quality testing standard. The total amount of calcium and mg in water is determined by the EDTA Titrimetric method. The total amount of calcium mg in water reflects the Hardness of water and has a significant impact on industrial production and daily water use. The standard regulates the operation process of the EDTA Titrimetric method. It has detailed instructions from reagent preparation, titration operation to result calculation, and can accurately determine the total amount of calcium mg. The method is suitable for various water sample testing, providing a reliable basis for water quality assessment, industrial water treatment and related research, and helping to ensure water safety and rational use.