Experiment 21Determination of the hardness of water

Purposes1. 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(Ca2+)/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 thathard water, upon heating, precipitate calciumcarbonate, 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 csum of the water sample solution?

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.

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.

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.

• 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)

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

1000)EDTA()EDTA((mmol/L) water of Hardnesssamplewater

V

Vc

Procedure

1.Preparation of 0.01mol·L-1 EDTA standard solution

Weigh about 1.5 g of EDTA with theplatform balance, put it into a clean small beaker, dissolve the solid with distilledwater, transfer the solution to a 500mL beaker,dilute to about 400 mL, mix the solution.

2 ． Standardization of EDTA standard solution

Prepare a standard MgCO3 solution as follows: Weigh accurately 0.20~0.22g of MgCO3 whichhas 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 solutiondropwise. After the solid has dissolved, add 50mLof distilled water (or deionized water). Transfer itquantitatively to a 250mL volumetric flask and dilute to the mark, mix the solution throughout.

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 o

f 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)= (EDTA))(MgCO

100000.250

00.20)(MgCO

3

3

VM

m

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 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 solutionExperiment No. 1 2 3m (Mass of MgCO3)/gIndicator The change of color at the end point of titrationV(MgCO3)/mLV Initial(EDTA)/mL V Final (EDTA)/mLVconsume(EDTA)/mLc(EDTA)/mol·L-1

cAverage (EDTA) /mol·L-1

Recording and treating data

Table 2 Determination of hardness of water

Experiment No. 1 2 3IndicatorThe change of color at the end point of titrationV(water sample)/mL 50.00 50.00 50.00V Initial(EDTA)/mLV Final (EDTA)/mLVconsume(EDTA)/mLc (Total hardness )/mol·L-1

cAverage (Total hardness)/mol·L-1