Aim
To
determine the total hardness of the given samples by EDTA titrimetric
method.]
Principle
Originally, the hardness of water was understood to be a measure of the
capacity of water for precipitating soap. Soap is precipitated chiefly by
the calcium and magnesium ions commonly present in water, but may also be
precipitated by ions of other polyvalent metals, such as aluminium, iron,
manganese, strontium and zinc, and by hydrogen ions. Because, all but the
first two are usually present in insignificant concentrations in natural
waters, hardness is defined as a characteristic of water, which represents
the total concentration of just the calcium and the magnesium ions
expressed as calcium carbonate. However, if present in significant amounts,
other hardness producing metallic ions should be included.
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When the hardness is numerically greater than the sum of the carbonate
alkalinity and the bicarbonate alkalinity, the amount of hardness, which is
equivalent to the total alkalinity, is called carbonate hardness; the amount
of hardness in excess of this is called non-carbonate hardness. When the
hardness is numerically equal to or less than the sum of carbonate and
bicarbonate alkalinity all of the hardness is carbonate hardness and there is
no noncarbonate hardness. The hardness may range from zero to hundreds of
milligrams per litre in terms of calcium carbonate, depending on the source
and treatment to which the water has been subjected.
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Ethylenediamine tetra-acetic acid and its
sodium salts (EDTA) form a chelated soluble complex when added to a
solution of certain metal cations. If a small amount of a dye such as
Eriochrome black T is added to an aqueous solution containing calcium and
magnesium ions at a pH of 10 ± 0.1, the solution will become wine red. If
EDTA is then added as a titrant, the calcium and magnesium will be
complexed. After sufficient EDTA has been added to complex all the
magnesium and calcium, the solution will turn from wine red to blue. This
is the end point of the titration.
Apparatus
1. Burette
2. Pipette
3. Erlenmeyer flask
4. Bottle etc.
Reagents
1. Standard EDTA titrant (0.01 M)
2. Eriochrome black T indicator
3. Ammonia buffer solution
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Procedure
1. Dilute 25 mL of sample (V) to about 50 mL with distilled water in an
Erlenmeyer flask.
2. Add 1 mL of buffer solution.
3. Add two drops of indicator solution. The solution turns wine red in
colour.
4. Add the standard EDTA titrant slowly with continuous stirring until the
last reddish tinge disappears from the solution. The colour of the solution
at the end point is blue under normal conditions.
5. Note down the volume of EDTA added (V1).
Observation
Calculation
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Hardness as CaCO3=
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V1 x S x 1000
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mg / L
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V
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where,
S = mg CaCO3 equivalent to 1 mL of EDTA titrant
= 1 mg CaCO3
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Hardness as CaCO3=
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1000 V1
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= ..........mg / L
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V
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Results
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