Experiment 21

1. Experiment 21 Determination of the hardness of water Purposes 1. To know the basic principle of complexometric titration. 2. Grasp the condition and the method of operation to determine the hardness of water.

2. Principles Hardness of water is to mean the total concentration of calcium and magnesium ions, which expressed as c(Ca2+)/mmol·L-1. The determination of hardness is a useful analytical process for measuring the quality of water for household and industrial uses. The importance for the latter is due to the fact that hard water, upon heating, precipitate calcium carbonate, which then clogs boilers and pipes.

3. How to determine the csum of the water sample solution? Water hardness is ordinarily determined by the EDTA titration after the sample has been buffered to a pH of 10, and Eriochrome black T serves as the indicator.

4. Firstly, the pH of water sample solution was adjust to pH 10 and kept stable with the addition of NH3-NH4Cl buffer solution in order to guarantee the titration was carried out in a proper pH environment. Then the solution is wine red with the addition of chrome black T indicator (H2In-), because the chrome black T combine with a part of Mg2+ ions to form a wine red complex.

5. Why does this happen? Because there is a sequence of the stability of the complex: CaY2- ＞ MgY2- ＞ MgIn-＞ CaIn- so the adding chrome black T give priority to combine with a part of Mg2+ ions to form a wine red complex. After that, use EDTA standard solution to titrate the csum (Ca2+ and Mg2+ ) of the water sample solution.

6. EDTA will usurp the Mg2+ ions from the complex derived from Mg2+ ions and the chrome black T after it combined with all free Ca2+ ions and free Mg2+ ions in the solution because it can form a more stable colorless complex with Mg2+ ions than the chrome black T. Finally, the color of the solution turn blue with all free chrome black T released from the red complex derived from Mg2+ ions and the chrome black T at the end point of the titration.

7. CaY2- > MgY2- > MgIn- > CaIn- • 1)Before EDTA was added (pH ≈10) Mg2+ + HIn2- → MgIn-+ H+ (blue)(red) 2) Among titration (pH =8 ~10) Ca2+ + HY3- → CaY2- + H+ Mg2+ + HY3- → MgY2- + H+ 3) End point (pH≈8) MgIn- + HY3- → MgY2- + HIn2- (red) (colorless) (colorless) (blue)

8. According to the consumption of EDTA standard solution, we can calculate the csum of the water sample solution.

9. Procedure • Preparation of 0.01mol·L-1 EDTA • standard solution Weigh about 1.5 g of EDTA with the platform balance, put it into a clean small beaker, dissolve the solid with distilled water, transfer the solution to a500mL beaker, dilute to about 400 mL, mix the solution.

10. 2．Standardization of EDTA standard solution • Prepare a standard MgCO3 solution as follows: Weigh accurately 0.20~0.22g of MgCO3 which has been previously dried at 110℃ into a clean beaker, add 5 drops of distilled water to moisten the solid, add about 3mL 3mol·L-1 HCl solution dropwise. After the solid has dissolved, add 50mL of distilled water (or deionized water). Transfer it quantitatively to a 250mL volumetric flask and dilute to the mark, mix the solution throughout.

11. Pipet 20.00 mL MgCO3 standard solution into a 250 mL conical flask, add 4 drops of 9 mol/L NH3·H2O, and 8 mL NH3-NH4Cl buffer solution of pH 10 and small amount of EBTindicator. Titrate with EDTA standard solution to the point where the color changes from wine-red to pure blue. Repeat the titration twice and calculate the concentration of EDTA solution

12. c(EDTA)=

13. 3．Determination of water hardness Pipet 50.00 mL water sample into a conical flask, add add 4 drops of 9 mol/L NH3·H2O, and 5 mL NH3-NH4Cl buffer solution of pH 10 and small amount of EBTindicator. Titrate with EDTA solution until the color changes from wine-red to pure blue. Repeat the titration twice and according to the consumed volumn of EDTA solution to calculate the total hardness of water.

14. Recording and treating data Table 1 Standardization of EDTA solution

15. Table 2 Determination of hardness of water