hardness of water
Water Technology
Hardness of Water: Natural waters containing large quantities of dissolved salts of Calcium (Ca) & Magnesium (Mg) is called hard water. It is a characteristic of preventing lather formation of water with soap. Ca2+ & Mg2+ ions react with soaps which are salts of fatty acids (stearic or palmitic acid) to give insoluble scums or precipitates of calcium or magnesium stearate or palmitate.
2C17H35COO- + Ca2+ → (C17H35COO)2 Ca (scum)
2C17H35COO- + Mg2+ → (C17H35COO)2 Mg(scum)
This property or tendency is called hardness of water. The cause of hardness is the precipitation of soap and hence prevents lathering.
Hardness of water is of two types:
1. Temporary/Alkaline/Carbonate hardness is due to dissolved bicarbonates of Ca2+ & Mg2+ in water. It is so called since it can be easily removed simply by boiling with bicarbonates readily getting precipitated.
Ca(HCO3)2 à CaCO3 + H2O + CO2
Mg(HCO3)2 à MgCO3 + H2O + CO2
Ca(HCO3)2 /Mg(HCO3)2 + 2C17H35COONa à (C17H35COO)2 Ca/Mg + 2 NaHCO3
CaCl2/MgCl2 + 2C17H35COONa à (C17H35COO)2Ca/Mg + 2NaCl
CaSO4/MgSO4 + 2C17H35COONa à (C17H35COO)2 Ca/Mg + Na2SO4
2. Permanent Hardness/Non-alkaline/Non-carbonate is due to the presence of dissolved sulfates & chlorides of Ca, Mg, Fe, etc in water. Permanent hardness cannot be removed easily on boiling, but requires special chemical treatment. Temporary & Permanent hardness added together gives Total hardness.
Determination of Hardness of water by EDTA Titration: It is for determining total amount of Ca2+ & Mg2+ ions in water. The procedure involves titrating sample with standard EDTA solution using an organic dye indicator EBT (Eriochrome Black T). EDTA is a weak acid with the structure given below:
(a) Structure of ethylenediaminetetraacetic acid, EDTA. (b) Structure of tetracarboxylate ion, [EDTA]4–, formed by dissociation of EDTA.
In an aqueous solution buffered at pH 10, EDTA dissociates to form a maximum amount of tetracarboxylate
Hardness of Water: Natural waters containing large quantities of dissolved salts of Calcium (Ca) & Magnesium (Mg) is called hard water. It is a characteristic of preventing lather formation of water with soap. Ca2+ & Mg2+ ions react with soaps which are salts of fatty acids (stearic or palmitic acid) to give insoluble scums or precipitates of calcium or magnesium stearate or palmitate.
2C17H35COO- + Ca2+ → (C17H35COO)2 Ca (scum)
2C17H35COO- + Mg2+ → (C17H35COO)2 Mg(scum)
This property or tendency is called hardness of water. The cause of hardness is the precipitation of soap and hence prevents lathering.
Hardness of water is of two types:
1. Temporary/Alkaline/Carbonate hardness is due to dissolved bicarbonates of Ca2+ & Mg2+ in water. It is so called since it can be easily removed simply by boiling with bicarbonates readily getting precipitated.
Ca(HCO3)2 à CaCO3 + H2O + CO2
Mg(HCO3)2 à MgCO3 + H2O + CO2
Ca(HCO3)2 /Mg(HCO3)2 + 2C17H35COONa à (C17H35COO)2 Ca/Mg + 2 NaHCO3
CaCl2/MgCl2 + 2C17H35COONa à (C17H35COO)2Ca/Mg + 2NaCl
CaSO4/MgSO4 + 2C17H35COONa à (C17H35COO)2 Ca/Mg + Na2SO4
2. Permanent Hardness/Non-alkaline/Non-carbonate is due to the presence of dissolved sulfates & chlorides of Ca, Mg, Fe, etc in water. Permanent hardness cannot be removed easily on boiling, but requires special chemical treatment. Temporary & Permanent hardness added together gives Total hardness.
Determination of Hardness of water by EDTA Titration: It is for determining total amount of Ca2+ & Mg2+ ions in water. The procedure involves titrating sample with standard EDTA solution using an organic dye indicator EBT (Eriochrome Black T). EDTA is a weak acid with the structure given below:
(a) Structure of ethylenediaminetetraacetic acid, EDTA. (b) Structure of tetracarboxylate ion, [EDTA]4–, formed by dissociation of EDTA.
In an aqueous solution buffered at pH 10, EDTA dissociates to form a maximum amount of tetracarboxylate