hardness of water

EXP.NO:03

11-10-12

DETERMINATION OF HARDNESS OF WATER 

AIM

To determine the hardness of the given sample of water. REAGENTS

HCl, (N/50) Na2CO3, EDTA, Methyl orange indicator, Phenolphthalein indicator, Erichrome Black-T indicator, MgSO4, Buffer solution. THEORY

Hardness is caused by the presence of chlorides, sulphates and bicarbonates of calcium and magnesium. Hardness caused by calcium and magnesium bicarbonates is called temporary since it can be destroyed by boiling. Ca(HCO3)2

 —› CaCO3 +

H20 + CO2

Calcium Carbonate precipitates out and so hardness is removed. This form of hardness can be determined by simple titration with standard acid. Permanent hardness is caused by sulphates and chlorides of calcium and magnesium and is not affected by boiling. Total hardness is represented by titration with EDTA. Permanent hardness is determined as the difference between the total and temporary hardness. For the purpose of estimating total hardness, the most widely used is the disodium salt of  EDTA. EDTA forms stable water soluble complexes with certain metal ions such as Ca and Mg. Erichrome Black-T solution is used as indicator and ammonium chloride solutions are added as buffer solutions to maintain the necessa ry pH.

OBSERVATIONS

Determination of Temporary Hardness

S.No

Vol. of water  Sample (ml)

Burette Reading (ml) Initial Final

Volume of HCl (ml), V1

1

10

0

9.8

9.8

2

10

0

9.8

9.8

Determination of Total Hardness

S.No

Vol. of water  Sample (ml)

1

10

Burette Reading (ml) Initial Final 0 15.4

2

10

0

Volume of EDTA (ml), V3

15.4

15.4 15.4

Standardization of HCl

S.No

Vol. of water  Sample (ml)

1

20

2

20

Burette Reading (ml) Initial

Volume of HCl (ml), V4

0

Final 19.9

19.9

0

19.9

19.9

The equations for calculating hardness are: 1. Temporary hardness of water sample = (V 2-V1) × N2 × 50 × 1000 100 Where, N2 = Normality of HCL V1 = Vol. of HCl required for disappearance of pink color. V2 = Vol. of HCl required for changing yellow to orange red. 2. Total hardness of water sample = V3 × N4 × 50 × 1000 100 Where, N4 = Normality of EDTA V3 = Volume of EDTA

PROCEDURE

1. Temporary Hardness Take 100ml of water under test into a clean conical flask after rinsing the flask with the same water. Add one or two drops of phenolphthalein indicator and titrate against approximately 0.01 N HCl. When the colour just disappears, note the titre value (V 1, ml). Add methyl orange indicator and titration is continued till golden yellow just turns to orange red (V 2, ml).

2. Total Hardness Measure out 100ml water under test into a conical flask. Add 3ml of buffer solution followed by 4 to 5 drops of Erichrome Black-T indicator. Mix thoroughly. The solution will become wine red in colour. It is then titrated against approximately 0.01  N EDTA. Contents of the flask are kept stirred throughout the titration. End point is the change of purple to blue colour persistent for about 15 seconds. Titre value is noted as V3 ml. 3

Standardization of HCl  Na2CO3 can be used to standardize HCl. Prepare an approximately equal normal solution of Na2CO3 and titrate against HCl using methyl orange indicator.

RESULT

i)

Temporary hardness of water sample = 49.24 ppm of CaCO 3

ii)

Permanent hardness of water sample = 27.755 ppm of CaCO3

3. Permanent hardness of water sample = Total hardness – Temporary hardness

CALCULATIONS

1) Standardisation of HCl  Normality of Na2C03 ×Volume of Na2CO3 = Normality of HCl × Volume of HCl 0.01 × 10 = NHCl × 19.9  NHCl =0.0105 N

2) Temporary hardness of water sample = (V 2 – V1) × NHCl × 50 × 1000 ppm 100 = 49.24 ppm of CaCO 3

3) Total hardness of water sample = V 3 × N4 × 50 × 1000 ppm 100 = 77 ppm of CaCO3

4) Permanent hardness of water sample = Total hardness – Temporary hardness =27.25 ppm of CaCO 3