To determine the calcium and magnesium hardness in a given Water sample separately.

Theory:

The property of water which restricts or checks the lather formation with soap is called hardness. In other words, the presence of multivalent cations, mostly calcium and magnesium ions, in water is referred to as hardness of water. Hardness is of two types: Temporary or carbonate hardness which can be removed by boiling and Permanent or Noncarbonate hardness which cannot be removed by boiling. The hardness is usually expressed in parts of CaCO3 equivalent or calcium and magnesium salts per million parts of water i.e in ppm.

The hardness of water can be determined by complexometric titration using Ethylene diammine tetra acetic acid (EDTA). EDTA in the form of its di-sodium salt forms complex with Ca2+ and Mg2+ ions of water sample. When Eriochrome Black T (EBT) indicator is added to the hard water at pH around 9–10, it gives wine red colored unstable complex with Ca2+ and Mg2+ ions of water sample. When this wine red colored complex is titrated against EDTA solution of known strength the Ca2+ and Mg2+ ions forms stable metal complex with EDTA and color changes from wine red to blue (color of EBT indicator) at the end point.

So titration at pH about 9–10 using EBT indicator gives the total amount of Ca2+ and Mg2+ ions in the water sample.

aparatus:  Conical flask, burette, pipette, and measuring cylinder. 

Chemicals:  Na2EDTA , NH4Cl-NH4OH Buffer,  Eriochrome Black T (EBT), and Calcon indicator.

Procedure:

  1. Standard Na2EDTA solution(0.01 M) and NH4Cl–NH4OH buffer solution were provided.
  1. Estimation of Ca2+ and Mg2+ in water:

                                     Pipette out 50 mL of hard water sample in a 250 mL conical flask, add 1ml of NH4Cl-NH4OH Buffer solution and 4–5 drops of EBT indicator. Titrate with standard EDTA solution till wine red color changes to clear blue. Repeat three times.

  1. Estimation of Ca2+ ion in water:
                                            Pipette out 50 mL of hard water sample in a   250 mL conical    flask, add 50 ml of distilled water & 5 ml of Diethyl amine. The solution is thoroughly shaken to precipitate all Mg2+ ion Mg(OH)2. After the completion of precipitation 8-10 drops of calcon indicator is added to the solution & titrate with standard EDTA solution till pink color changes to  blue. Repeat three times.

Observation table:

Table 1:- Estimation of Ca2+ and Mg2+ in given water sample

Entry Volume of water Burette Reading (mL) Volume of Mean volume
sample taken Initial Final EDTA of EDTA
(mL) required required (Vm)
(mL) (mL)
1. 50
2. 50
3. 50

 Table 2:- Estimation of Ca2+ ion in given sample water

Entry Volume of water Burette Reading (mL) Volume of Mean volume
sample taken Initial Final EDTA of EDTA
(mL) required required (Vm)
(mL) (mL)
1. 50
2. 50
3. 50

Calculations:

Table 1: (for Total Hardness)

EDTA forms 1:1 complex with Ca2+ and Mg2+

1000 mL of 1(M) EDTA solution = 1 mole of CaCO3 = 100 g of CaCO3

1 mL of 1(M) EDTA solution = [100 x (1/1000)] g of CaCO3

Vm mL of  p(M) EDTA solution = [100 x (1/1000) x p x Vm] g of CaCO3

= A g (say) 50  mL of water sample contain A  g of CaCO3

1mL of water sample contains    A/ 50 g of CaCO3

106  mL of water sample contain ……..ppm of CaCO3

For Table 2: (For Ca2+ ion Hardness)

EDTA forms 1:1 complex with Ca2+.

1000 mL of 1(M) EDTA solution = 1 mole of CaCO3 = 100 g of CaCO3

1 mL of 1(M) EDTA solution = [100 x (1/1000)] g of CaCO3

Vm mL of  p(M) EDTA solution = [100 x (1/1000) x p x Vm] g of CaCO3

= A g (say)

50  mL of water sample contain A  g of CaCO3

1mL of water sample contains    A/ 50 g of CaCO3

106  mL of water sample contain ……..ppm of CaCO3

Conclusion:  

i) Total hardness of water sample = ………..   ppm

ii) Ca2+ ion Hardness of water sample = ……. ppm

iii) Mg2+ ion Hardness of water sample = …… ppm

Precautions:

  • All the glass apparatus should be washed thoroughly with distilled water before use.
  • The burette and pipette should be rinsed with solution to be taken in it.
  • There should not be any leakage in the burette.
  • All the solution should be freshly prepared.
  • Same amount of indicator should be added each time.
  • pH of the solution should be maintained during titration.
  • Shaking should be proper during titration.
  • The titration flask should be placed on white paper or board to identify properly the colour change at the end point

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