Hardness is defined as calcium and magnesium ion content. Hard water contains bicarbonate, chlorides and sulphates of calcium and magnesium. The water having low level or absence of calcium and magnesium ion refers as soft water. Water hardness is usually noticed by difficulty in lathering soap because it gets precipitated in the form of insoluble salts of calcium and magnesium. Hardness of water is a measure of the total concentration of the calcium and magnesium ions expressed as calcium carbonate. When water passes through or over deposits such as limestone, the levels of Ca2+, Mg2+, and HCO3 – ions present in the water can greatly increase and cause the water to be classified as hard water. Hardness of water is one of the important water qualities measuring parameter but it does not consider as pollution parameter. Hardness usually does not harm the human health of in major way; however soft and very hard water may create the problems in human health.
There are two types of hardness
1. Temporary hardness
Temporary Hardness is due to the presence of bicarbonates of calcium and magnesium. It can be easily removed by boiling.
2. Permanent hardness
Permanent Hardness is due to the presence of chlorides and sulphates of calcium and magnesium. This type of hardness cannot be removed by boiling.
Hardness is usually reported as parts per million (ppm) of calcium carbonate (by weight). A water supply with a hardness of 100 ppm contains the equivalent of 100 g of CaCO3 in 1 million g of water or 0.1 g in 1 L of water (or 1000 g of water since the density of water is about 1 g/mL).
The maximum level
of hardness for potable water by WHO is 500mg/L and recommended hardness is
moderately hard.
PRINCIPLE
The total hardness
is determined by titration with ethylenediamine-tetraacetic acid, disodium salt
dihydrate (Na2EDTA). A water sample is buffered to pH 10.1 and taken in to a
conical flask. On addition (erichrome black T) EBT indicator dye in solution
containing Calcium and Magnesium ions turns to wine red due to complex
formation between calcium and magnesium ion with EBT at pH 10.1. EDTA, the
titrant, removes the magnesium and calcium ions complexes with the indicator.
When all the Mg and Ca are complexes with EDTA, the indicator will turn blue.
This is the end point of the titration.
Materials required
Apparatus required
1. Burette
2. Pipettes
3. Conical flask
5. 100 ml graduated cylinders
6. Beaker
Chemicals required
1. Ammonium Buffer pH 10.1
2. EDTA (Disodium Salt of EDTA)
-0.02N
4. Erichrome Black T
Procedure:
Take 50 ml of sample in a clean 250mL
conical flask. Add 2mL of Ammonia buffer solution to the water sample so that
the pH will be maintained between 10 and 11. Add few drops (100mg) of EBT
indicator to the conical flask and the sample turns to wine red in color. Titrate
the sample against the EDTA solution in the burette till the blue colour
appear. This is the end point of the titration. Note down the burette reading.
Repeat the titration for concordant values.
Observation
Preparation of chemicals:
1. EDTA solution (0.02N =0.01M):
Take 3.723g Na2EDTA and Dissolve
completely in 1 liter distilled water completely.
2. Ammonium buffer pH 10.1:
i. Dissolve 16.9g ammonium chloride
in 143 ml of concentrated ammonium hydroxide.
ii. Take 1.179g of Na2EDTA and 0.78g
of MgSO4 .7H2O
in 50 ml distilled water and dissolve completely.
Mix both solution and maintain volume
to 250 ml.
3. Erichrome Black T (EBT) indicator:
Take 0.4g EBT and mix with 100g NaCl
and grind finely.
Significance of Hardness:
1. Scales are formed as inner coating
of the pipelines prevents corrosion.
2. Absolutely soft waters are
corrosive and dissolve the metals.
3. More cases of cardio vascular
diseases are reported in soft water areas.
4. Hard water is useful to growth of
children due to the presence of calcium.
5. Hard waters cause excessive consumption
of soap used for cleaning purpose.
Bibliography:
1. APHA Standard Methods for the
Examination of Water and Wastewater - 20th Edition.
Method 2340 - C.
2. Methods for Chemical Analysis of
Water and Wastes, EPA-600/4-79-020, USEPA, Method 130.2.
