How to determine the total hardness of water by EDTA method? What indicators are used? What's the reaction?

The determination of total hardness of water by EDTA method is to determine the total content of calcium and magnesium ions in water, which can be determined by EDTA coordination titration:

Before titration: M+EBT M-EBT (red)

Main reaction: M+Y MY

Ending: M-EBT+Y MY+EBT (red), (blue)

When the solution changes from red to blue, titration is the end point.

During titration, interfering ions such as Fe3+ and Al3+ can be masked by triethanolamine. Heavy metal ions such as Cu2+, Pb2+ and Zn2+ can be masked by KCN, Na2S or mercaptoacetic acid.

Both Ca2+ ions and Mg2+ ions in water are regarded as Ca2+ ions, and their mass is converted into the mass of CaO. 10 mg of Cao in 1L water is called 1 degree.

The specific determination method is as follows:

Use a 20 ml pipette to suck the water sample into a 250 ml conical flask, add 6 ml ammonia buffer solution, 3 ml triethanolamine solution 1: 1 and 3~4 drops of EBT indicator, and titrate with EDTA standard solution until the solution turns from purple to blue. Parallel determination for 3 times.