Hardness of Water (exp:7) - assignment - Q-A

Determination of Hardness of Water

(1)Q: What is hardness and how it is caused? Discuss the environmental significance of “hardness”.
Ans: Hard water are generally considered to be those waters that require considerable amounts of soap to produce a foam or leather and that also produce scale in hot water pipes, heaters, boilers and other units in which the temperature of water is increased substantially. Such property of water is called Hardness.
Hardness is caused by multivalent metallic cations. Such cations are capable of reacting with soap to form precipitates and with certain anions present in water to form scale. The principal hardness causing cations are the divalent calcium, magnesium, strontium, ferrous iron and manganous ions.
These cations and the important anions with they are associated are shown in the following table in the order of their relative abundance in natural waters. Aluminum and ferric ions are sometimes considered as contributing to the hardness of water. However, their solubility is so limited at PH values of natural waters that ionic concentrations are negligible. The hardness of water is derived largely from contact with the soil and rock formation.
Table: Principal cations causing hardness and major anions associated with them
Cations causing hardness Anions
Ca2+ HCO3-
Mg2+ SO42-
Sr2+ Cl-
Fe2+ NO3-
Mn2+ SiO32-
Environmental significance:
Hard waters are as satisfactory for human consumption as soft waters. Because of their adverse action with soap, however, their use for cleaning purpose is quite unsatisfactory, unless soap costs are disregarded. Soap consumption by hard waters represents an economic loss to the water user. Sodium soaps react with multivalent metallic cations to form a precipitate, thereby losing their surfactant properties. In recent years these problems have been largely alleviated by the development of soaps and detergents that do not react with hardness.
Boiler scale, the result of the carbonate hardness precipitation, may xause considerable economic loss through fouling water heater and hot water pipes. A change is pH in the water distribution systems may also result in deposits of precipitates.

(2)Q: A water has the following analysis:
Na+ : 20 mg/L Cl- : 40 mg/L
K+ : 30 mg/L HCO3- : 67 mg/L
Ca2+ : 5 mg/L CO32- : 0 mg/L
Mg2+ : 10 mg/L SO42- : 5 mg/L
Sr2+ : 2 mg/L NO3- : 10 mg/L

What is the total hardness; carbonate hardness and noncarbonate hardness in mg/L as CaCO3?
Ans: Ca2+, Mg2+ and Sr2+ are hardness producing agents
Hardness as mg/l CaCO3 = M2+(mg/l) X 50 / (equivalent weight of M2+)
Divalent
Cation Equivalent weight of M2+ mg/l of divalent cation (M2+) Hardness as
mg/l of CaCO3
Ca2+
Mg2+
Sr2+ 20
12
43.8 5
10
2 12.5
41.67
2.283
Total hardness as mg/l CaCO3 = 56.45
The anions only HC03- and C03- are alkalinity-producing agents.
Carbonate alkalinity mg/l as CaCO3 =(mg/l C03- )/ .6 = 0/. 6 = 0
Bicarbonate alkalinity mg/l as CaCO3 =(mg/l HC03- )/ 1.22
= 67/1.22 = 54.92
Total alkalinity mg/l as CaCO3 = 54.92
As alkalinity< total hardness
Carbonate hardness (mg/l as CaCO3) =alkalinity (mg/l as CaCO3)
= 54.92 mg/l as CaCO3
Noncarbonate hardness (NCH) =total hardness _ Carbonate hardness
= (56.45 – 54.92) mg/l as CaCO3
= 1.53 mg/l as CaCO3
Total hardness as mg/l CaCO3 = 56.45
Carbonate hardness = 54.92 mg/l as CaCO3
Noncarbonate hardness (NCH) = 1.53 mg/l as CaCO3

(3) Q: Calculate hydroxide, bicarbonate and carbonate alkalinities of a water sample with a total alkalinity of 210 mg/L as CaCO3 and a pH of 7.7. Which type of alkalinity dominates the total alkalinity? Also calculate the concentrations of carbonate (CO32-) and bicarbonate (HCO3-) ions.
Ans: If alkalinity< total hardness
Carbonate hardness (mg/l) =alkalinity (mg/l)
If alkalinity> total hardness
Carbonate hardness (mg/l) = total hardness (mg/l)
alkalinity of our sample = 260 mg/l as CaCO3
Total hardness of our sample = 315 mg/l as CaCO3
As alkalinity< total hardness
Carbonate hardness (mg/l) =alkalinity (mg/l)
= 260 mg/l as CaCO3
Noncarbonate hardness (NCH) =total hardness _ Carbonate hardness
= (315 – 260 ) mg/l as CaCO3
=55 mg/l as CaCO3
Carbonate hardness (mg/l) = 260 mg/l as CaCO3
Noncarbonate hardness (NCH) = 55 mg/l as CaCO3

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