Titration analysis method-coordination titration method (4)

5.

(1) Hardness

Generally, the hardness of natural surface water is relatively small.

(2) The representation method of hardness

1)mmol/L

2) mg/L ( calculated as CaCO ₃ )

3) German degree (abbreviated to degree)

1 degree=10mg/L (calculated as CaO)

1mmol/L(CaO)=56.

1 degree=100.

(3) Analysis method

Ammonia at a pH = 10 buffer solution, chrome black T indicator and EDTA can and of Mg ²⁺ , of Ca ²⁺ form complexes, and the complexes were the order of the stability CaY ^2- > MgY ^2- > MgIn ^- > CALn ^-

After the start of the titration, EDTA first coordinates with the free Mg ²⁺ and Ca ^{2+ in the} sample to form stable and colorless MgY ^2- and CaY ^2- complexes:

When the free Ca ²⁺ and Mg ²⁺ are completely coordinated with EDTA, the stability of the CaY ^2- and MgY ^2- complexes is much greater than that of the Caln ^- and MgIn ^- complexes.

According to the concentration of EDTA standard solution and the amount of titration, the total hardness can be calculated:

In the formula, C _{EDTA —} the concentration of EDTA standard solution, mol/L;

V _EDTA ——Volume of _EDTA standard solution, ml;

V——The volume of the original water sample, ml

It can be seen from the above reaction that H+ is produced in each step of the measurement process.
In order to control the titration condition to pH=10, so that EDTA can form a stable complex with Ca ²⁺ and Mg ²⁺ , it is necessary to use ammonia The buffer solution stabilizes the pH of the solution
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