10.A] Explain the determination of hardness of water by EDTA method.
Answer:
Determination of hardness of water using EDTA titration:
Complexometric Titration Method using EDTA:
Principle:
Hardness of water is due to the presence of calcium & magnesium salts in water. Since Ca2+ & Mg2+
forms complex with Ethylene diamine tetra acetic acid. It is possible to determine the total hardness
of water using EDTA reagent and Eriochrome black – T indicator. The reaction is favorable in basic
media & involves liberation of H+ ion, a buffer (NH3-NH4Cl) has to be used to maintain pH 10.
Ethylenediamine tetra acetic acid (EDTA) is a reagent, which reacts with metal ions like Ca2+,
Mg2+ forming complex compounds. Therefore, this reagent can be used to determine the concentration
of hardness-causing substances.
Reactions:
PROCEDURE:
Part A: Preparation of standard solution of disodium salt of EDTA.
About 1g of disodium salt of EDTA is transferred in to a 250 cm3 volumetric flask.
Dissolve the salt in ion exchange water, add few drops of ammonia to increase the solubility dilute up
to the mark and mix well. Calculate the molarity of EDTA.
Part B: Determination of total hardness of water
Pipette out 25cm3 of the given water sample into a clean conical flask. Add 2 to 3cm3
of NH3-NH4Cl buffer (To maintain pH 10) and a pinch of Eriochrome black – T indicator. Titrate
against Standard EDTA solution till the color of the solution changes from wine red to clear blue. Note
the volume of EDTA solution consumed. Repeat for concordant values. Calculate the total hardness
of the water sample.
Calculations:
RESULT: – The Total hardness of the given water sample =_______ ppm of CaCO3.
• Note: Temporary Hardness = Total Hardness-Permanent Hardness