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.

Hardness of water is to mean the total concentration of calcium and magnesium ions, which expressed as c(Ca 2+ )/mmol·L -1. Principles 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.

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. How to determine the c sum of the water sample solution?

Firstly, the pH of water sample solution was adjust to pH 10 and kept stable with the addition of NH 3 -NH 4 Cl 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 (H 2 In - ), because the chrome black T combine with a part of Mg 2+ ions to form a wine red complex.

Why does this happen? Because there is a sequence of the stability of the complex: CaY 2- ＞ MgY 2- ＞ MgIn - ＞ CaIn - so the adding chrome black T give priority to combine with a part of Mg 2+ ions to form a wine red complex. After that, use EDTA standard solution to titrate the c sum (Ca 2+ and Mg 2+ ) of the water sample solution.

EDTA will usurp the Mg 2+ ions from the complex derived from Mg 2+ ions and the chrome black T after it combined with all free Ca 2+ ions and free Mg 2+ ions in the solution because it can form a more stable colorless complex with Mg 2+ 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 Mg 2+ ions and the chrome black T at the end point of the titration.

CaY 2- > MgY 2- > MgIn - > CaIn - 1) Before EDTA was added (pH ≈10) Mg 2+ + HIn 2- → MgIn - + H + (blue) (red) 2) Among titration (pH =8 ~10) Ca 2+ + HY 3- → CaY 2- + H + Mg 2+ + HY 3- → MgY 2- + H + 3) End point (pH≈8) MgIn - + HY 3- → MgY 2- + HIn 2- (red) (colorless) (colorless) (blue)

According to the consumption of EDTA standard solution, we can calculate the c sum of the water sample solution.

Procedure 1.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 a 500mL beaker, dilute to about 400 mL, mix the solution.

2 ． Standardization of EDTA standard solution  Prepare a standard MgCO 3 solution as follows: Weigh accurately 0.20~0.22g of MgCO 3 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.

 Pipet mL MgCO 3 standard solution into a 250 mL conical flask, add 4 drops of 9 mol/L NH 3 ·H 2 O, and 8 mL NH 3 -NH 4 Cl buffer solution of pH 10 and small amount of EBT indicator. 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

c(EDTA)=

3 ． Determination of water hardness Pipet mL water sample into a conical flask, add add 4 drops of 9 mol/L NH 3 ·H 2 O, and 5 mL NH 3 -NH 4 Cl buffer solution of pH 10 and small amount of EBT indicator. 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.

Table 1 Standardization of EDTA solution Experiment No. 123 m (Mass of MgCO 3 )/g Indicator The change of color at the end point of titration V(MgCO 3 )/mL V Initial (EDTA)/mL V Final (EDTA)/mL V consume (EDTA)/mL c(EDTA)/mol·L -1 c Average (EDTA) /mol·L -1 Recording and treating data

Table 2 Determination of hardness of water Experiment No. 123 Indicator The change of color at the end point of titration V(water sample)/mL50.00 V Initial (EDTA)/mL V Final (EDTA)/mL V consume (EDTA)/mL c (Total hardness )/mol·L -1 c Average (Total hardness)/mol·L -1