Object: Determine the total hardness of given water samples.
Introduction:
Hardness in water is due to the presence of dissolved salts of calcium and magnesium.
It is unfit for drinking, bathing, washing and it also forms scales in
boilers.
Hence it is necessary to estimate the amount of hardness producing
substances present in the water sample.
Once it is estimated, the amount of chemicals
required for the treatment of water can be calculated.
The estimation of hardness is
based on complexometric titration.
Hardness of water is determined by titrating with
a standard solution of ethylene diamine tetra acetic acid (EDTA) which is a
complexing agent.
Since EDTA is insoluble in water, the disodium salt of EDTA is
taken for this experiment.
EDTA can form four or six coordination bonds with a
metal ion.
Two type of hardness is present in water
first is temporary hardness and
second is permanent hardness.
Temporary hardness is due to the presence of
bicarbonates of calcium and magnesium ions.
It can be easily removed by boiling.
Permanent hardness is due to the presence of chlorides and sulphates of calcium and
magnesium ions.
This type of hardness cannot be removed by boiling.
Requirements:
Water sample
Burette 25-30ml
Glass funnel
Pipette 1ml
Flask
Dropper
Measuring cylinder
Reagents: EDTA, Eriochrome Black-T, NH2CL, Ammonia Buffer, Magnesium
Carbonate, 90% ethyl alcohol, Distilled water.
Reagent preparation:
1. EDTA solution: 4gm EDTA and 0.1gm magnesium bicarbonate dissolve in
800 ml distilled water.
2. Eriochrome Black-T: 0.4gm Ericrome Black T, 4.5 gm hydroxylmine
hydrochloride add in 100ml 95% ethyl alcohol.
3. Ammonia Buffer: Stock A: 16.9gm of NH4CL in 143ml of conc. NH4OH,
Stock B: 1.25gm magnesium salt of EDTA dissolve in 50 ml distilled water.
Mix both stock solutions and dilute to 250ml with DDW. Dilute 10ml of the
solution to 100ml with DDW.
Procedure:
1. The burette is filled with standard EDTA solution to the zero level.
2. Take 50ml sample water in flask. If sample having high Calcium content then
take smaller volume and dilute to 50ml.
3. Add 1ml Ammonia buffer.
4. Add 5 to 6 drop of Ericrome black – T indicator. The solution turns into wine
red colour.
5. Note the initial reading.
6. Titrate the content against EDTA solution. At the end point colour change
from wine red to blue colour.
7. Note the final reading and record it. Repeat the process till we get concordant
value.
8. Take 50ml sample in another flask and boiled it. (Add distilled water to get
final volume of water.)
9. Repeat step 3-7.
Calculations:
S.No. Water sample (50ml)
Initial Value
Final Value Ml of EDTA
1
2
3
Total hardness of water mg/L (CaCO3 Scale) = ml of EDTA used (unboiled) *103
/ml of sample
Permanent hardness of water mg/L (CaCO3 Scale) = ml of EDTA used (boiled)
*103 /ml of sample
Temporary hardness of water mg/L (CaCO3 Scale) = Total hardness of water -
Permanent hardness of water
Observation:
The colour of soluble distilled water and R.O water
instantly changed into blue while tap water and pond
water turned wine red when Ericrome black T was added
and therefore after turned blue when titrated against
EDTA solution.
Result:
The collected water sample contains Total hardness = _____ppm
Permanent hardness = _____ppm
Temporary hardness = _____ppm
