The term “hardness” of water refers to salts, mainly of Ca2+ and Mg2+ with the ions Cl–, SO42– and H — Physical Chemistry — Kinetics Chemistry Question

The term “hardness” of water refers to salts, mainly of Ca²⁺ and Mg²⁺ with the ions Cl–, SO₄₂– and HCO₃–.
By boiling water, the soluble salts of HCO₃– are transformed to insoluble salts of CO₃₂–:
2 HCO₃– H₂O + CO₂ (g) + CO₃₂–
Ca²⁺ + CO₃₂– CaCO₃ (insoluble)
The hardness that due to such salts disappears by boiling the water is called as non-permanent hardness.
The hardness which is due to Cl– and SO₄₂– anions, is permanent because it does not disappear by boiling the water.
Total hardness is the sum of permanent and non–permanent hardness.
The hardness of water is expressed in mg CaO / 100 cm₃ H₂O (Deutch degrees, D°), in mg CaCO₃ / 100 cm₃ H₂O (French degrees, F°) and in mg CaCO₃ / 1000 cm₃ H₂O (American way of expressing hardness).

Procedure: Determination of the total hardness of water. Titration with EDTA.
1. 50 cm₃ of H₂O are measured accurately. Instead of “hard water” a solution containing a quantity of Mg²⁺ and Ca²⁺ ions may be used. 2 - 3 cm₃ of buffer solution NH₃ / NH₄Cl having pH = 10 are added and 3 - 4 drops of a solution (in alcohol) of melan eriochrome T, 0.5% (indicator). The solution turns red–violet because of the formation of a complex between Mg²⁺ and the indicator.
2. A solution of the dissodium salt of EDTA of known concentration (0.01 F) which is equivalent to 1 mg CaCO₃ / cm₃ EDTA (question 1) is added with stirring.
The ions Ca²⁺ and Mg²⁺ form 1 : 1 complexes with EDTA. EDTA will form complexes with all the free Ca²⁺ and Mg²⁺ ions and finally it will also bind with the ions Mg²⁺ which are to the indicator, replacing it. At this point, which is the titration end point, the colour of the solution turns from red to blue because of the liberation of the indicator. At pH = 10, the colour change is clear and the determination of the equivalence point more accurate.
If V is the volume of the EDTA solution consumed and c its concentration in mg CaCO₃ equivalent to 1 cm₃ of EDTA solution and the volume of the water sample used is 50 cm₃, the hardness of water in mg CaCO₃ / 100 cm₃ H₂O is given by: Fo = 2 V c.
If c = 0.01 F ~ 1 mg CaCO₃ / 1 cm₃ EDTA then Fo = 2 V.