CLARK'S METHOD OR SOAP TITRATION METHOD: This method depends upon the fact that when soluble soap is added to hard water, constituent of hard water. Ca, Mg and other heavy metals react with to form insoluble compounds. The lather is only produced when all the constituents causing hardness are precipitated by soap.
2C2H35COONa + Ca (HCO3)2 = (C17H35COO) 2 Ca + 2NaHCO3
2C17H35COONa + MgCl2 = (C17H35COO) 2 Mg + 2NaCl
2C17H35COONa + CaSO4 = (C17H35COO)2 Ca + Na2SO4
When sample for analysis is taken without boiling soap analysis gives total hardness and when sample is treated after removing temporary hardness by boiling, it gives permanent hardness. In both case ends point is accepted by production of lather stable for minimum 2 min.
PREPARATION OF SOLUTION: 1g of CaCo3 dried at 105 degree c for 3 hours dissolved in 1:1 water and hydrochloric acid solution. Solution is boiled to remove any CO2. Solution is made cool and neutral with 3N NH4OH to methyl red indicator. Whole solution is transferred to 1 lit. Volumetric flask and prepared 1 lit solution.
PREPARTION AND STANDARDIZATION OF SOAP SOLUTION: 10g pure dry soap is dissolved in 800ml of alcohol mixed with 200 ml of distilled water.
STANDARDIZATION: 50ml of standard hard water is taken in conical flask and titrated with soap. The end point should be noted when lather lasts even after two minutes of continuous shaking. Titrated value X1 is recorded.
LATHER FACTOR: 50 ml of distilled water is titrated by soap solution to get lather. Titrate value is X recorded as lather factor. Total hardness is determined by repeating the above experiment for 50 ml of water sample to be analysed.
Titrate value X2 -total hardness is determined.
Take 100 ml of water sample, heat it to boil for 30 min, coil it, filter it and make up 100 ml solution with distilled water.
Take 50 ml of boiled sample and titrate with soap solution.
Titrate value is X3 which is permanent hardness.
The various hardness of water can be represented as follows:
Total hardness = X2-X/X1-X*1000 Mg CaCO3/litre.
Permanent hardness = X3-X/X1-X*1000 mg CaCO3/litre.
Temporary hardness = Total hardness-Permanent hardness
= 1000(X2-X3)/(X1-X )ppm