Introduction:
Hard waters are generally considered to be those waters that require considerable amounts of soap to produce foam or leather and that also produce scale in hot-water pipes, boilers, and other units in which the temperature of water is increased substantially. The hardness of water varies considerably from place to place. In general, surface water is softer than groundwater. The hardness of water reflects the nature of the geological formations with which it has been in contact.
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 ion and manganaous ions. These cations and the important anions with they are associated with 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 formations.
Table-7.1: Principal hardness causing cations and the major anions associated with them
Cations causing Hardness
Associated Anions
Ca2+HCO3- Mg2+ SO42- Sr2+ Cl- Fe2+ NO3- Mn2+ SiO32-
Hardness caused by each cation can be calculated as follows:
Hardness (mg/l as CaCO3) 7.1
Where, M2+ = concentration of divalent metal cation (mg/l) and 50 is the equivalent weight of CaCO3.
Total hardness 7.2
7.3
Environmental significance:
Hard water is as satisfactory for human consumption as soft waters. Because of their action with soap, however, their use for cleansing 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 developments of soaps and detergents that do not react with hardness.
Boiler scale, the result of the carbonate hardness precipitation, may cause considerable economic loss through fouling of water heater and hot water pipes. Change in pH in the water distribution systems may also result in deposits of precipitates. Bicarbonates begin to convert to the less soluble carbonates at pH values above 9.0.
Magnesium hardness, particularly associated with the sulfate ion has a laxative effect on persons unaccustomed to it. Magnesium concentrations of less than 50 mg/l are desirable in potable waters, although many public water supplies exceed the amount. Calcium hardness presents no public health problem. In fact, hard water is apparently beneficial to the human cardiovascular system. Water can be generally classified in terms of the degree of hardness as follows:
Table 7.2: Water quality with respect to hardness
Water Quality
Hardness (mg/l as CaCO3)
Soft
<50
Moderately hard
50-150
Hard
150-300
Very hard
>300
Theory on Experimental Method
Hardness is usually expressed in terms of CaCO3. Perhaps the most accurate method of determining hardness is by a calculation based upon the divalent ions found through the complete cation analysis (Equation 7.1 – 7.3). This method is preferred where complete analysis are available.
However, complete analysis of metal ions is not always available and in laboratory total hardness is usually determined by the EDTA Titrimetric method. This method yields very precise and accurate results and is the methods of choice in most laboratories.
The EDTA titrimetric method involves the use of solutions of ethylene-diamine-tetra-acidic acid (EDTA) or its sodium salt as the titrating agents. These compounds are chelating agents (A chemical compound in the form of a heterocyclic ring, containing a metal ion attached by coordinate bonds to at least two non-metal ions) and thus form extremely stable complexes with Ca2+, Mg2+ and other divalent cations causing hardens, as shown in the following equation.
M2+ + EDTA = [M.EDTA]complex 7.4
The successful use of EDTA for determining hardness depends upon having an indicator present to show when EDTA is present in excess, or when all the ions causing hardness have been complexed.
The Erichome Black T dye serves as an excellent indicator to show when all the hardness ions having form complex with EDTA. When small amount of Erichrome Black T is added to hard water with a pH of about 10.0, it combines with a few of the Ca2+ and Mg2+ ions to form a weak complex ions which produce wine red color in the solution.
M2+ + Erichrome Black T = (M. Erichrome Black T)complex 7.5
During titration with EDTA, all free hardness ions are complexed according to euqtion7.2. Finally, the EDTA disrupts the weak wine red complex compounds (M. Erichrome Black T) and forms more stable complex with the divalent ions. The action frees the Erichrome Black T indicator, and the wine red colour changes to a distinct blue colour.
(M. Erichrome Black T)complex+ EDTA = (M.EDTA)complex + Erichrome Black T 7.6
(blue color)
Reagents:
Buffer solution (for attaining a pH close to 10.0)
Erichrome Black T dye
Standard EDTA solution
Apparatus:
- Beaker
- Measuring cylinder
- Dropper
- Stirrer
- Auto-titration device
Procedure:
Take 50 ml sample in a 150 ml beaker.
Add one ml of standard buffer solution (supplied by HACH) to raise the pH of water sample to about 10.0.
Add one packet of Erichome Black T dye(supplied by HACH) indicator to the beaker. The sample would turn wine red (if hardness is present).
Fit the cartridge containing standard EDTA solution to the titrator device (supplied by HACH).
Turn the flow control knob of the device until the solution starts to come out of the tube fitted to the cartridge. Take initial reading of the counter.
Immerse the tube fitted to the cartridge into the water sample and start titrating (under constant stirring) by turning the flow control knob of the auto-titrator. Continue until the wine red colour of the sample changes to blue. Take final reading of the counter.
Calculation:
Total hardness (mg/L as CaCO3)
= Multiplying Factor (MF) x (initial counter reading – final counter reading)
Where, M.F.
(Normality of EDTA titrant used in the lab is 0.002N)
Therefore for 100ml sample,
Total hardness (mg/L as CaCO3) = final counter reading – initial counter reading
And for 50ml sample,
Total hardness (mg/L as CaCO3) = 2× (final counter reading – initial counter read
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