The hardness of natural water mainly refers to Ca2 +, Mg2 + and other metal ions, and the alkalinity of water mainly refers to the alkalinity of bicarbonate HCO3 -. The main ions in water are Ca2 +, Mg2 +, Na +, K +, HCO3 -, SO42 -, Cl -, etc. The relationship between hardness and hardness in water can be divided into three cases.   

(1) Alkalinity > hardness (in mol / L) HCO3 - > (Ca2 + + Mg2 +) hardness in water (Ca2 + and Mg2 +) become bicarbonate, and there are also bicarbonate of Na + and K +, but there is no non carbonate hardness. At this time, the difference obtained by subtracting hardness from alkalinity is equal to bicarbonate of Na + and K +. The alkalinity of bicarbonate of Na + and K + in this part is the so-called excess alkalinity, also known as negative hardness

(2) Alkalinity = hardness (in mol / L), i.e. HCO3 - = (Ca2 + + Mg2 +). At this time, only the hardness of Ca2 +, Mg2 + and its bicarbonate alkalinity are available, neither non carbonate hardness nor Na +, K + bicarbonate.

(3) Alkalinity 3-2 + + Mg2 +). At this time, there are two cases,

One is calcium hard water with Ca2 + > HCO3 -. At this time, there are non carbonate hardness CaSO4 and MgSO4 in the water, but there is no carbonate hardness Mg (HCO3) of magnesium

2。 The other is magnesium hard water with Ca2 + > HCO3 -. In the water, there is the carbonate hardness Mg (HCO3) 2 of magnesium, but there is no non carbonate hardness of calcium, and there is the non carbonate hardness MgSO4 of magnesium. However, in either case, there is the hardness of non carbonate in the water, but there is no bicarbonate of Na + and K +.