Determination of water hardness - titrimetric method -2

 

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

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).

hardness-titre-table

 Calculation

Hardness as CaCO3=

V1 x S x 1000

  mg / L

V

where, 
S = mg CaCO3 equivalent to 1 mL of EDTA titrant
   = 1 mg CaCO3 

Hardness as CaCO3=

1000 V1

  = ..........mg / L

V

 

hardness-result-table