Estimation of Hardness of water : Hardness of water is conveniently expressed in terms of dissolved salts of calcium, magnesium and iron—calculated as parts of CaCO3 equivalent per million parts of water (ppm). If a sample of water contains Ts parts of calcium carbonate equivalent per 10,00,000 parts, it possesses n degree of hardness. One degree of hardness Is therefore, the hardness caused by one part of CaCO3 (or its equivalent) in one million parts of water.
Water hardness is of two kinds Temporary hardness (carbonate hardness) and Permanent Hardness (non-carbonate hardness). Total hardness of water is the sum of the temporary and permanent hardness.
The degree of hardness for soft water: 0-10°., Medium tiara water: 10-20°, Hard water: 20-300, and very hard water -above 30.
Permute water treatment process gives water of almost zero hardness.
Ca(HCO3)2+2HCl = CaC12+2H20+CO3
i.e., 1 ml. of 0.1N HCl = 0.005 gm of CaCO3
The solutions required are:
Procedure: 50 nil of water under test is taken in a stoppered bottle and to it is added standard soap solution by burette 0.2 ml at a time. The mixture is shaken vigorously after eacb addition until lather formation starts. Then the addition of soap. solution is made at the rate of 0.1 ml at a time till a lather is obtainedwhich persists for 5 minutes when the bottle is kept aside. The total soap solution added is read from the burette. The hardness as CaCO3 is then found out from a standard table.
For the determination of permanent hardness the water is_ boiled and then treated with a measured excess of standard Na2CO2 solution, and the filtrate treated with a standard acid using Methyl Orange as indicator, and the volume of Na2CO3. originally added, minus that left over, is the volume actually consumed in the precipitation of alkaline earth salts, and its-CaCO3 equivalent gives the permanent hardness:
CaSO4 + Na2CO3 = CaCO3 + Na2SO4
MgC12 + Na2CO3 = MgCO3+2NaCl
i.e., 1 ml. of 0. 1N. Na2CO3 = 0.005 gm CaCO3
Total hardness is conveniently determined by titration of EDTA (the disodium salt of ethylene diamine tetra acetic acid available as Versene).
EDTA is formulated as: (HOOC.CH2)2 – N—CH2—CH2 — N ( CH2-COOH)2
Versense, represented as Na2H2V, forms soluble complex ions with Ca++ and Mg ++ ions:
H2Ve+Cal-F (CaV) + 2H+
The sample of water is made alkaline to a pH of 10 with NH4Cl and NH4OH and titrated for the total calcium and magnesium with a standard solution or Versene (often standardized against a CaCl2 solution of known strength, using Erichrome Black T as an indicator)—the color of the indicator changes from red to blue at the end point. Na2H2V.2H20 may In. directly used as a primary standard.